Plumeria Care

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Leafhopper – Pest

Leafhopper – Pest

Leafhoppers feed on many different fruits, vegetables, flowers, and woody ornamental hosts. Most species of leafhoppers feed on only one or several closely related plant species. Adults mostly are slender, wedge-shaped, and less than or about equal to 1/4-inch long. Leafhoppers generally are varying shades of green, yellow, or brown, and are often mottled. Some species are brightly colored, while others blend with their host plant. Leafhoppers are active insects; they crawl rapidly sideways or readily jump when disturbed. Adults and nymphs and their pale cast skins are usually found on the underside of leaves.

Potato Leafhopper on Alfalfa | Ohioline

Identification

Leafhoppers may sometimes be confused with aphids or Lygus bugs. Look for leafhoppers or their cast skins on the undersides of affected leaves. Look at their actions; they are faster than aphids and run sideways and jump. Lygus bug nymphs are light green and also move much faster than aphids. They can be identified by their red-tipped antennae. Aphids can be distinguished by two tubelike structures, called cornicles, protruding from the hind end. One or more long rows of spines on the hind legs of leafhoppers and characters on their head distinguish leafhoppers from most other insects they resemble.

Some common leafhopper species in gardens and landscapes. The common leafhopper found on plumeria is the potato leafhopper.

Life cycle

All five nymphal instars and adult potato leafhopper, left. | Download Scientific DiagramLeafhoppers go through incomplete metamorphosis in their development. Female leafhoppers insert tiny eggs in tender plant tissue, causing pimple-like injuries. Overwintered eggs begin to hatch in mid-April. Wingless nymphs emerge and molt four or five times before maturing in about 2 to 7 weeks. Nymphs resemble adults except that they lack wings; later-stage nymphs have small wing pads. There is no pupal stage. Leafhoppers overwinter as eggs on twigs or as adults in protected places such as bark crevices. In cold-winter climates, leafhoppers may die during winter and in spring migrate back in from warmer regions. Most species have two or more generations each year.

Damage

The potato leafhopper has a sucking mouthpart that it uses to pierce into the vascular tissue of the plant and feed on the plant juice, sapping it of nutrients.

However, according to Groves, the damage is most commonly seen on the plumeria as a result of a condition called hopper-burn, which is the plant’s physiological response to the leafhopper’s saliva. The saliva causes the vascular system to be disrupted, and the leaves that have been fed on curl up and can become necrotic.

Leafhopper feeding causes leaves to appear stippled, pale, or brown, and shoots may curl and die. Black spots of excrement and cast skins may be present on leaves. Some leafhopper species transmit plant diseases, but this is troublesome mostly among herbaceous crop plants.

This is a must read article written by plumeira experts from Southern California. Link to pdf file  leaf-hopper

Control for LeafhoppersBon-Neem

for Leafhoppers

Bon-Neem

Specially formulated to kill mites, aphids, whitefly and more on contact.

Tip: To improve the effectiveness of insecticidal soap, mix 1 tablespoon of isopropyl alcohol with 1 quart of the spray. It helps the soap penetrate the insects’ outer shells.

Growth Responses

Growth Responses

A plant’s sensory response to external stimuli relies on hormones, which are simply chemical messengers. Plant hormones affect all aspects of plant life, from flowering to fruit setting and maturation, and from phototropism to leaf fall. Potentially, every cell in a plant can produce plant hormones. The hormones can act in their cell of origin or be transported to other portions of the plant body, with many plant responses involving the synergistic or antagonistic interaction of two or more hormones. In contrast, animal hormones are produced in specific glands and transported to a distant site for action, acting alone.

Plant hormones are a group of unrelated chemical substances that affect plant morphogenesis. Five major plant hormones are traditionally described: auxins, cytokinins, gibberellins, ethylene, and abscisic acid. In addition, other nutrients and environmental conditions can be characterized as growth factors. The first three plant hormones largely affect plant growth, as described below.

Auxins

The term auxin is derived from the Greek word auxein, which means “to grow. ” Auxins are the main hormones responsible for cell elongation in phototropism and gravitropism. They also control the differentiation of meristem into vascular tissue and promote leaf development and arrangement. While many synthetic auxins are used as herbicides, indole acetic acid (IAA) is the only naturally occurring auxin that shows physiological activity. Apical dominance (the inhibition of lateral bud formation) is triggered by auxins produced in the apical meristem. Flowering, fruit setting and ripening, and inhibition of abscission (leaf falling) are other plant responses under the direct or indirect control of auxins. Auxins also act as a relay for the effects of the blue light and red/far-red responses.

Commercial use of auxins is widespread in plant nurseries and for crop production. IAA is used as a rooting hormone to promote the growth of adventitious roots on cuttings and detached leaves. Applying synthetic auxins to tomato plants in greenhouses promotes normal fruit development. Outdoor application of auxin promotes synchronization of fruit setting and dropping, which coordinates the harvesting season. Fruits such as seedless cucumbers can be induced to set fruit by treating unfertilized plant flowers with auxins.

Cytokinins

The effect of cytokinins was first reported when it was found that adding the liquid endosperm of coconuts to developing plant embryos in culture stimulated their growth. The stimulating growth factor was found to be cytokinin, a hormone that promotes cytokinesis (cell division). Almost 200 naturally occurring or synthetic cytokinins are known, to date. Cytokinins are most abundant in growing tissues, such as roots, embryos, and fruits, where cell division is occurring. Cytokinins are known to delay senescence in leaf tissues, promote mitosis, and stimulate differentiation of the meristem in shoots and roots. Many effects on plant development are under the influence of cytokinins, either in conjunction with auxin or another hormone. For example, apical dominance seems to result from a balance between auxins that inhibit lateral buds and cytokinins that promote bushier growth.

Gibberellins

Gibberellins (GAs) are a group of about 125 closely related plant hormones that stimulate shoot elongation, seed germination, and fruit and flower maturation. GAs are synthesized in the root and stem apical meristems, young leaves, and seed embryos. In urban areas, GA antagonists are sometimes applied to trees under power lines to control growth and reduce the frequency of pruning.

GAs break dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure to cold or light to germinate. Abscisic acid is a strong antagonist of GA action. Other effects of GAs include gender expression, seedless fruit development, and the delay of senescence in leaves and fruit. Seedless grapes are obtained through standard breeding methods; they contain inconspicuous seeds that fail to develop. Because GAs are produced by the seeds and because fruit development and stem elongation are under GA control, these varieties of grapes would normally produce small fruit in compact clusters. Maturing grapes are routinely treated with GA to promote larger fruit size, as well as looser bunches (longer stems), which reduces the incidence of mildew infection.

Beneficial Insects

Beneficial Insects

Beneficial Insects are used in gardens and landscapes and even small organic production. Take a look at the age old organics list of beneficial insects promoting Integrated Pest Management.

Beneficial Nematodes– These nematodes work very well in controlling hundreds of types of garden pests. For example, grubs, slugs, fungus gnats, fruit flies, beetles, and cockroaches. You can buy nematodes at your local garden center. There are two genera of nematodes and depending on your garden pest, you will want to know which one to use. As always, read the labels on all things going into your garden and landscape. Apply them by mixing with water and using your favorite fertilizer application tools such as a watering can, a backpack sprayer, or a hose-end sprayer. Make sure the holes are big enough to allow nematodes to be applied. It is best to keep the nematode application area moist so they get a chance to work themselves into the soil. Also, nematodes don’t like bright light so applying at dusk is ideal.

Fun Fact- Nematodes “swim” through the soil or sit and wait for their host. They can find their host by the bug’s breath or the CO2 they give off when breathing.

Green Lacewings– Green Lacewing larvae after hatching are also known as “aphid lions.” Now that’s a beneficial insect you want near your Brassicas and Roses! They are extreme predators of soft-bodied insects like aphids, thrips, mealybugs, whiteflies, and mites which if you have encountered them are ‘numerous’ to say the least. Lacewing larvae eat by injecting venom through tube-like structures from their jaws and then suck out their fluids. One lacewing can consume a couple hundred of these guys in a week. When applying Lacewing larvae, do so early morning or early evening as they don’t prefer extreme heat. Adult Lacewings will stay in your yard if you have some pollen, honeydew, and nectar to stick around for. Please note- it is great if you know what the life stages of these good bugs look like.

Click on the University of California’s Integrated Pest Management link to find images of the four stages of the Green Lacewing’s life cycle to be sure you don’t remove this buddy from your garden. http://www.ipm.ucdavis.edu/PMG/NE/green_lacewing.html

Ladybugs- We all know or should know by now that Ladybugs are delightful friends to have in the garden. There are about 4,300 different kinds of ladybugs in the world. Sometimes they are called Lady Beetles or Ladybird Beetles.

They are largely known for controlling aphid populations but also attack soft-bodied insects much like that of Lacewing larvae. Ladybugs can be many different colors such as white, pink, black, yellow, orange, and of course red. They usually have spots but they do fade with age. Did you know that the French call ladybugs, les betes du bon Dieu, which translates to ‘creatures of God’. They truly are a fantastic garden friend. And good eaters! One ladybug can eat up to 5,000 aphids in its lifetime.

You can buy ladybugs from your garden center. Be aware that most will fly away. Don’t be discouraged, if you welcome ladybugs into your yard (with herbs like fennel, cilantro, and dill, and flowers like geraniums and dandelions) you will be sure to see the benefit of your purchase. Increase your odds by watering the area of release first so they can have a drink. Release them early morning and not in the hot sun in mid-day.

Earthworms– One of the best things you can do for your soil is to make sure earthworms inhabit it. Earthworms act as a catalyst in decomposition. While they feed off of organic matter, breaking it down, all while mixing the layers of soil inside your garden. Charles Darwin has referred to earthworms as ‘nature’s ploughs’ in his last book published in 1881 before he died, entitled, The Formation of Vegetable Mold through the Action of Earthworms. He thought earthworms so fantastic that he studied them for 40 years. Earthworm castings are a rich source of available N, P, K, enzymes, and beneficial bacteria. You can buy Earthworms and you can also buy Age-Old earthworm castings from us.

Bumble Bees and Honey Bees– It should be well known worldwide that it is imperative to protect our bees. They are natural pollinators of our flowers and without them, we would have to find our own way of pollinating for fruit and vegetable production. Sometimes people become perplexed why their squash had so many blossoms and never produced one squash. It is very possible that the squash blossoms have not been visited by any bees.

Bumble Bees and Honey Bees have many different characteristics. For instance, Bumble Bees byways of ‘buzz pollination’ only have to visit a flower once to pollinate while honey bees have to visit several times. Their big furry bodies can hold a lot of pollen to make the job easier. One benefit Honey Bees have over Bumble Bees is their ability to produce a surplus of honey, which we love to eat. It is very effective on our immune systems if consumed raw and regularly. Watch out for the ultra-filtered grocery store knockoffs.  Bees work hard for us on a worldwide scale. They are responsible for most things beautiful.

Praying Mantis– Females are green while Males are brown. Their heads can turn almost a full 360 degrees. These alien looking creatures prey on almost all bugs so one can argue that they are a good thing to have in the garden. (Just as long as they’re not eating your beneficial bugs). Praying Mantes are carnivores and can even be cannibalistic. They play a major role in insect control and natural balance. The strange thing is (as if there is only one) about praying mantes is that they even attack and eat hummingbirds, lizards, and even small mammals.

Ants– Have you ever seen an anthill? Does it not look like the most aerated soil you have ever seen? Ants are also very effective in the decomposition of dead carcasses of animals and converting them into usable organic matter. There is also the issue of ants farming aphids for the sweet substance that aphids secrete while sucking the plant. The ants paralyze the aphids making them unable to move or fly away. Since the ants don’t actually eat the aphids (like our dear ladybugs and lacewings) it is yet to be determined whether or not the ants keep the aphids around for easy access to food or if the ants are actually helping control the aphid population. You can read more about it here:

About Potting Soil for Plumeria

Posted on by PlumeriaDB

About Potting Soil for Plumeria

Low-quality or substandard potting media is a major source of problems with plumeria grown in containers (and, really, any container-grown plants). Plumerias that are potted in substandard or old decomposed potting soil are weaker than healthy plants, so they’re more likely to drop leaves, turn yellow, attract pests and just generally die.

Potting media, of course, is just one of the factors that contribute to healthy plumeria. But because most people only re-pot when they have to, it’s actually the hardest single factor to quickly control. Other factors that are much easier to control include moisture levels, pH, humidity, temperature, and light levels.

Because potting soil is so important and relatively hard to change, it’s extra important to pay careful attention to your potting media and start off with the best possible mix. Your plumeria will thank you for it, and ultimately, you’ll end up with healthier plumeria and more blooms.

It’s important to remember that all potting soil will be depleted of its nutrients by the plants in the pot, including your plumeria and weeds.

A good potting soil (media) should:

  • Provide support for the growing plant
  • Provide adequate drainage, very important for Plumeria
  • Provide adequate available nutrients (usually through fertilizer and/or organic matter added to the soil)
  • Provide adequate aeration around the roots

You can easily find many good potting mixes. Most are based on sphagnum peat moss, with additives like compost, humus, perlite, vermiculite, and pumice. Some have water-retention crystals, these are not good for plumeria. You can also find soils with fertilizer added, pine bark, and other ingredients like coir, seaweed, and worm castings. I prefer to use a non fertilized soil mix, that way I know what I’m starting with.

For plumeria it is important the soil you select allows for adequate drainage, has air pockets around the root zone and critically, how long does it last before the inevitable decay sets in (all organic things decay) and starts to lose its desirable qualities?

Well prepared garden soil is great for growing Plumeria in the ground but when it comes to growing Plumeria in containers, it needs to be changed improved, or changed as the roots grow and use up the nutrients.

Soils for containers need to be well aerated and well drained while still being able to retain enough moisture for plant growth.

When choosing what to use to fill containers, never use garden soil by itself no matter how good it looks or how well things grow in it out in the garden. When put into a container both drainage and aeration are severely impeded, and the results are that plumeria grows poorly or not at all.

Soils for containers need to be modified in some way to ensure proper drainage and aeration. Container soils are often referred to as soilless or artificial media because they contain no soil at all. They are often composed of various things such as peat, perlite, vermiculite, bark, coir fiber (ground coconut hulls) in a variety of recipes depending on the manufacture and the type of plant material being grown. They can be found under a variety of trade names and in sizes ranging from a few quarts to bales that are many cubic feet in size. The choice of growing media for plumeria is well drained and doesn’t retain a lot of moisture over a long period of time. You should choose media that’s courser in texture containing more bark, perlite, or sand.

When soil mixes are used, they should be moistened slightly before planting. Fill a tub with the media, add water and lightly fluff the media to dampen it.

Some garden soils can be used as a container media but it needs to be modified or amended. An acceptable soil based mix can be made by using one part garden soil, one part peat moss, and one part perlite or coarse builders sand. Don’t use fine beach sand or play sand.

Advantages and Disadvantages

The disadvantage is that the soil you are using may contain insects, weed seeds, and disease organisms. Soilless media are generally free of these things. Soil-based media are heavier. This may be an advantage though especially if the containers are used in a very windy location and the extra weight will help keep the pot upright. Soil-based media are also a bit more forgiving when it comes to water and fertility. They tend not to dry out as fast, and they also tend to hold on to nutrients longer. Because soilless media can be a bit more costly, you could mix 25 percent soil with the soilless media to stretch your soilless media for a few more pots.

Can soilless media be reused from year to year? If the plants in the containers were healthy during the growing season with no major disease issues, you could remove the plant material at the end of the season and reuse the media next season. A good way to make sure you soil disease free by putting the soil in a black plastic bag and keep in full sun for a week or so, rotating the bag every few days. You may also have to add some additional new media to fill the container. There will come a time when the media in the container will need to be replaced. Over the course of time, the organic materials that the soilless media is made from a breakdown and decompose to the point where you will lose a lot of nutrients, drainage, and aeration properties that are inherent in soilless container media. When that happens, discard the media to the compost pile or to the garden and refill the container with fresh media.

There are also bagged products labeled as topsoil that tend to be largely sedge peat. While they are inexpensive and look very good, once put into a pot they are poorly drained and poorly aerated. They can be used in combination with soilless media products or amended much the same way as if you were using garden soil.

When filling containers with media, don’t fill the pot to the top. Leave about a one-inch space between the top of the soil and the rim of the pot. This will help make watering the pot easier as it provides a place to “put water” and not have it run over the edge.

Filling very large containers can be costly especially when using commercially prepared media. To reduce the cost and also the weight of the container consider adding “filler” to the bottom of the container to take up space. Many things can be used, but the chipped mulch will decompose and add nutrients to your pot over the course of the growing season.

Soil and Soil Mixes

Posted on by PlumeriaDB

Soil and Soil Mixes

Garden soil is great for growing Plumeria in the ground but when it comes to growing Plumeria in containers, it needs to be amended to improve drainage and adjust its ability to maintain moisture. Also, you will need to make sure the pH is correct for plumeria. You will still need to change the soil in your pots as your plumeria use up the nutrients. 

Soils for containers need to be well aerated and drain well while still being able to retain enough moisture for plant growth.

When choosing what to use to fill containers, never use garden soil by itself no matter how good it looks or how well things grow in it out in the garden. When put into a container both drainage and aeration are severely impeded, and the results are that plumeria grows poorly or not at all.

Soils for containers need to be modified in some way to ensure proper drainage and aeration. Container soils are often referred to as soilless or artificial media because they contain no soil at all. They are often composed of various things such as peat, perlite, vermiculite, bark, coir fiber (ground coconut hulls) in a variety of recipes depending on the manufacture and the type of plant material being grown. They can be found under a variety of trade names and in sizes ranging from a few quarts to bales that are many cubic feet in size. When choosing a growing media for plumeria it is important to make sure it drains well and doesn’t retain too much moisture over a long period of time. You should choose media that’s courser in texture containing more bark, perlite, or sand.

When soil mixes are used, they should be moistened slightly before planting. Fill a tub with the media, add water and lightly fluff the media to dampen it.

Some garden soils can be used as a container media but it needs to be modified or amended.

An acceptable soil mix can be made by using:

  • one part garden soil
  • one part peat moss
  • one part decomposed mulch and
  • one part perlite.

CAUTION: Don’t use fine beach sand or play sand. If you must use sand use coarse builders sand.

Advantages and Disadvantages

Soil or soilless mix media?
The disadvantage is that the soil you are using may contain insects, weed seeds, and disease organisms. Soilless media are generally free of these things. Soil-based media are heavier. This may be an advantage though especially if the containers are used in a very windy location and the extra weight will help keep the pot upright. Soil-based media are also a bit more forgiving when it comes to water and fertility. They tend not to dry out as fast, and they also tend to hold on to nutrients longer. Because soilless media can be a bit more costly, you could mix 25 percent soil with the soilless media to stretch your soilless media for a few more pots.

Can soilless media be reused from year to year?
If the plants in the containers were healthy during the growing season with no major disease issues, you could remove the plant material at the end of the season and reuse the media next season. A good way to make sure your soil is free of diseases is by putting the soil in a black plastic bag and keep in full sun for a week or so, rotating the bag every few days. This will sterilize the soil. You may also have to add some additional new media to fill the container. There will come a time when the media in the container will need to be replaced. Over the course of time, the organic materials that the soilless media is made from will break down and decompose to the point where you will lose a lot of nutrients, drainage, and aeration properties that are inherent in soilless container media. When that happens, discard the media to the compost pile or to the garden and refill the container with fresh media.

An acceptable soil mix using old soil using:

  • one part old soil
  • one part peat moss
  • one part decomposed mulch and
  • one part perlite.

There are also bagged products labeled as topsoil that tend to be largely sedge peat. While they are inexpensive and look very good, once put into a pot they are poorly drained and poorly aerated. They can be used in combination with soilless media products or amended much the same way as if you were using garden soil.

When filling containers with media, don’t fill the pot to the top. Leave about a half inch of space between the top of the soil and the rim of the pot. This will help make watering the pot easier as it provides a place to “put water” and not have it run over the edge.

Filling very large containers can be costly especially when using commercially prepared media. To reduce the cost and also the weight of the container consider adding “filler” to the bottom of the container to take up space. Many things can be used, but pine mulch will decompose and add organic nutrients to your pot over the course of the growing season. Do not use colored much, it may have chemicals that could harm your plumeria.

Water Uptake and Transport in Vascular Plants

Water Uptake and Transport in Vascular Plants

By: Andrew J. McElrone (U.S. Department of Agriculture, Agricultural Research Service, University of California, Davis), Brendan Choat (University of Western Sydney), Greg A. Gambetta (University of California, Davis) & Craig R. Brodersen (University of Florida) © 2013 Nature Education 
 
Citation: McElrone, A. J., Choat, B., Gambetta, G. A. & Brodersen, C. R. (2013) Water Uptake and Transport in Vascular Plants. Nature Education Knowledge 4(5):6
 
How does water move through plants to get to the top of tall trees? Here we describe the pathways and mechanisms driving water uptake and transport through plants, and causes of flow disruption.
Aa Aa Aa
 
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Why Do Plants Need So Much Water?

Water is the most limiting abiotic (non-living) factor to plant growth and productivity, and a principal determinant of vegetation distributions worldwide. Since antiquity, humans have recognized plants’ thirst for water as evidenced by the existence of irrigation systems at the beginning of recorded history. Water’s importance to plants stems from its central role in growth and photosynthesis, and the distribution of organic and inorganic molecules. Despite this dependence, plants retain less than 5% of the water absorbed by roots for cell expansion and plant growth. The remainder passes through plants directly into the atmosphere, a process referred to as transpiration. The amount of water lost via transpiration can be incredibly high; a single irrigated corn plant growing in Kansas can use 200 L of water during a typical summer, while some large rainforest trees can use nearly 1200 L of water in a single day! 
If water is so important to plant growth and survival, then why would plants waste so much of it? The answer to this question lies in another process vital to plants — photosynthesis. To make sugars, plants must absorb carbon dioxide (CO2) from the atmosphere through small pores in their leaves called stomata (Figure 1). However, when stomata open, water is lost to the atmosphere at a prolific rate relative to the small amount of CO2 absorbed; across plant species an average of 400 water molecules are lost for each CO2 molecule gained. The balance between transpiration and photosynthesis forms an essential compromise in the existence of plants; stomata must remain open to build sugars but risk dehydration in the process.

Rendering of an open stoma on the surface of a tobacco leaf.

Figure 1: Rendering of an open stoma on the surface of a tobacco leaf.
Stomata are pores found on the leaf surface that regulate the exchange of gases between the leaf’s interior and the atmosphere. Stomatal closure is a natural response to darkness or drought as a means of conserving water.
© 2013 Nature Education All rights reserved. View Terms of Use
 

From the Soil into the Plant

Essentially all of the water used by land plants is absorbed from the soil by roots. A root system consists of a complex network of individual roots that vary in age along their length. Roots grow from their tips and initially produce thin and non-woody fine roots. Fine roots are the most permeable portion of a root system, and are thought to have the greatest ability to absorb water, particularly in herbaceous (i.e., non-woody) plants (McCully 1999). Fine roots can be covered by root hairs that significantly increase the absorptive surface area and improve contact between roots and the soil (Figure 2). Some plants also improve water uptake by establishing symbiotic relationships with mycorrhizal fungi, which functionally increase the total absorptive surface area of the root system.

Root hairs often form on fine roots and improve water absorption by increasing root surface area and by improving contact with the soil.

Figure 2: Root hairs often form on fine roots and improve water absorption by increasing root surface area and by improving contact with the soil.
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Roots of woody plants form bark as they age, much like the trunks of large trees. While bark formation decreases the permeability of older roots they can still absorb considerable amounts of water (MacFall et al. 1990, Chung & Kramer 1975). This is important for trees and shrubs since woody roots can constitute ~99% of the root surface in some forests (Kramer & Bullock 1966).

Roots have the amazing ability to grow away from dry sites toward wetter patches in the soil — a phenomenon called hydrotropism. Positive hydrotropism occurs when cell elongation is inhibited on the humid side of a root, while elongation on the dry side is unaffected or slightly stimulated resulting in a curvature of the root and growth toward a moist patch (Takahashi 1994). The root cap is most likely the site of hydrosensing; while the exact mechanism of hydrotropism is not known, recent work with the plant model Arabidopsis has shed some light on the mechanism at the molecular level (see Eapen et al. 2005 for more details).

Roots of many woody species have the ability to grow extensively to explore large volumes of soil. Deep roots (>5 m) are found in most environments (Canadell et al. 1996, Schenk & Jackson 2002) allowing plants to access water from permanent water sources at substantial depth (Figure 3). Roots from the Shepard’s tree (Boscia albitrunca) have been found growing at depths 68 m in the central Kalahari, while those of other woody species can spread laterally up to 50 m on one side of the plant (Schenk & Jackson 2002). Surprisingly, most arid-land plants have very shallow root systems, and the deepest roots consistently occur in climates with strong seasonal precipitation (i.e., Mediterranean and monsoonal climates).

Tree roots at significant depths accessed via caves.

Figure 4: Tree roots at significant depths accessed via caves.
Plant scientists examine: deep roots of Juniperus asheii growing at 7m depth in a cave in Austin, TX USA (left); an extensive fine root network attached to a single ~1cm diameter tap root accessing a perennial underground stream at 20m depth in a cave in central TX, USA; and twisty roots in a cave located in southwest Western Australia below a forest dominated by Eucalyptus diversicolor — roots in this cave system are commonly found from 20-60m depth.
© 2013 Nature Education Images provided by W. T. Pockman (Univ of New Mexico), A. J. McElrone, and T. M. Bleby (Univ of Western Australia). All rights reserved. View Terms of Use
 
Through the Plant into the Atmosphere
Water flows more efficiently through some parts of the plant than others. For example, water absorbed by roots must cross several cell layers before entering the specialized water transport tissue (referred to as xylem) (Figure 4). These cell layers act as a filtration system in the root and have a much greater resistance to water flow than the xylem, where transport occurs in open tubes. Imagine the difference between pushing water through numerous coffee filters versus a garden hose. The relative ease with which water moves through a part of the plant is expressed quantitatively using the following equation:

Flow = Δψ / R,

which is analogous to electron flow in an electrical circuit described by Ohm’s law equation:

i = V / R, 

where R is the resistance, i is the current or flow of electrons, and V is the voltage. In the plant system, Vis equivalent to the water potential difference driving flow (Δψ) and i is equivalent to the flow of water through/across a plant segment. Using these plant equivalents, the Ohm’s law analogy can be used to quantify the hydraulic conductance (i.e., the inverse of hydraulic R) of individual segments (i.e., roots, stems, leaves) or the whole plant (from soil to atmosphere).

Upon absorption by the root, water first crosses the epidermis and then makes its way toward the center of the root crossing the cortex and endodermis before arriving at the xylem (Figure 4). Along the way, water travels in cell walls (apoplastic pathway) and/or through the inside of cells (cell to cell pathway, C-C) (Steudle 2001). At the endodermis, the apoplastic pathway is blocked by a gasket-like band of suberin — a waterproof substance that seals off the route of water in the apoplast forcing water to cross via the C-C pathway. Because water must cross cell membranes (e.g., in the cortex and at apoplastic barriers), transport efficiency of the C-C pathway is affected by the activity, density, and location of water-specific protein channels embedded in cell membranes (i.e., aquaporins). Much work over the last two decades has demonstrated how aquaporins alter root hydraulic resistance and respond to abiotic stress, but their exact role in bulk water transport is yet unresolved.

Representation of the water transport pathways along the soil-plant-atmosphere continuum (SPAC).

Figure 4: Representation of the water transport pathways along the soil-plant-atmosphere continuum (SPAC).
(A) Water moves from areas of high water potential (i.e. close to zero in the soil) to low water potential (i.e., air outside the leaves). Details of the Cohesion-Tension mechanism are illustrated with the inset panels (A), where tension is generated by the evaporation of water molecules during leaf transpiration (1) and is transmitted down the continuous, cohesive water columns (2) through the xylem and out the roots to the soil (3). The pathways for water movement out of the leaf veins and through the stomata (B) and across the fine roots (C) are detailed and illustrate both symplastic and apoplastic pathways.
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Once in the xylem tissue, water moves easily over long distances in these open tubes (Figure 5). There are two kinds of conducting elements (i.e., transport tubes) found in the xylem: 1) tracheids and 2) vessels (Figure 6). Tracheids are smaller than vessels in both diameter and length, and taper at each end. Vessels consist of individual cells, or “vessel elements”, stacked end-to-end to form continuous open tubes, which are also called xylem conduits. Vessels have diameters approximately that of a human hair and lengths typically measuring about 5 cm although some plant species contain vessels as long as 10 m. Xylem conduits begin as a series of living cells but as they mature the cells commit suicide (referred to as programmed cell death), undergoing an ordered deconstruction where they lose their cellular contents and form hollow tubes. Along with the water conducting tubes, xylem tissue contains fibers which provide structural support, and living metabolically-active parenchyma cells that are important for storage of carbohydrates, maintenance of flow within a conduit (see details about embolism repair below), and radial transport of water and solutes.

Three dimensional reconstructions of xylem imaged at the Ghent microCT facility.

Figure 5: Three dimensional reconstructions of xylem imaged at the Ghent microCT facility.
Differences in xylem structure and conduit distributions can be seen between Ulmus americana (left) and Fraxinus americana (right) xylem.
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When water reaches the end of a conduit or passes laterally to an adjacent one, it must cross through pits in the conduit cell walls (Figure 6). Bordered pits are cavities in the thick secondary cell walls of both vessels and tracheids that are essential components in the water-transport system of higher plants. The pit membrane, consisting of a modified primary cell wall and middle lamella, lies at the center of each pit, and allows water to pass between xylem conduits while limiting the spread of air bubbles (i.e., embolism) and xylem-dwelling pathogens. Thus, pit membranes function as safety valves in the plant water transport system. Averaged across a wide range of species, pits account for >50% of total xylem hydraulic resistance. The structure of pits varies dramatically across species, with large differences evident in the amount of conduit wall area covered by pits, and in the porosity and thickness of pit membranes (Figure 6).

Comparison of different types of wood from flowering and cone-bearing plants.

Figure 6: Comparison of different types of wood from flowering and cone-bearing plants.
This features wider conduits from flowering plants (top), a cartoon reconstruction of vessels, tracheids and their pit membranes (middle), which are also shown in SEM images (bottom).
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After traveling from the roots to stems through the xylem, water enters leaves via petiole (i.e., the leaf stalk) xylem that branches off from that in the stem. Petiole xylem leads into the mid-rib (the main thick vein in leaves), which then branch into progressively smaller veins that contain tracheids (Figure 7) and are embedded in the leaf mesophyll. In dicots, minor veins account for the vast majority of total vein length, and the bulk of transpired water is drawn out of minor veins (Sack & Holbrook 2006, Sack & Tyree 2005). Vein arrangement, density, and redundancy are important for distributing water evenly across a leaf, and may buffer the delivery system against damage (i.e., disease lesions, herbivory, air bubble spread). Once water leaves the xylem, it moves across the bundle sheath cells surrounding the veins. It is still unclear the exact path water follows once it passes out of the xylem through the bundle sheath cells and into the mesophyll cells, but is likely dominated by the apoplastic pathway during transpiration (Sack & Holbrook 2005).

An example of a venation pattern to illustrate the hydraulic pathway from petiole xylem into the leaf cells and out the stomata.

Figure 7: An example of a venation pattern to illustrate the hydraulic pathway from petiole xylem into the leaf cells and out the stomata.
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Mechanism Driving Water Movement in Plants

Unlike animals, plants lack a metabolically active pump like the heart to move fluid in their vascular system. Instead, water movement is passively driven by pressure and chemical potential gradients. The bulk of water absorbed and transported through plants is moved by negative pressure generated by the evaporation of water from the leaves (i.e., transpiration) — this process is commonly referred to as the Cohesion-Tension (C-T) mechanism. This system is able to function because water is “cohesive” — it sticks to itself through forces generated by hydrogen bonding. These hydrogen bonds allow water columns in the plant to sustain substantial tension (up to 30 MPa when water is contained in the minute capillaries found in plants), and helps explain how water can be transported to tree canopies 100 m above the soil surface. The tension part of the C-T mechanism is generated by transpiration. Evaporation inside the leaves occurs predominantly from damp cell wall surfaces surrounded by a network of air spaces. Menisci form at this air-water interface (Figure 4), where apoplastic water contained in the cell wall capillaries is exposed to the air of the sub-stomatal cavity. Driven by the sun’s energy to break the hydrogen bonds between molecules, water evaporates from menisci, and the surface tension at this interface pulls water molecules to replace those lost to evaporation. This force is transmitted along the continuous water columns down to the roots, where it causes an influx of water from the soil. Scientists call the continuous water transport pathway the Soil Plant Atmosphere Continuum (SPAC).

Stephen Hales was the first to suggest that water flow in plants is governed by the C-T mechanism; in his 1727 book Hales states “for without perspiration the [water] must stagnate, notwithstanding the sap-vessels are so curiously adapted by their exceeding fineness, to raise [water] to great heights, in a reciprocal proportion to their very minute diameters.” More recently, an evaporative flow system based on negative pressure has been reproduced in the lab for the first time by a ‘synthetic tree’ (Wheeler & Stroock 2008).

When solute movement is restricted relative to the movement of water (i.e., across semipermeable cell membranes) water moves according to its chemical potential (i.e., the energy state of water) by osmosis — the diffusion of water. Osmosis plays a central role in the movement of water between cells and various compartments within plants. In the absence of transpiration, osmotic forces dominate the movement of water into roots. This manifests as root pressure and guttation — a process commonly seen in lawn grass, where water droplets form at leaf margins in the morning after conditions of low evaporation. Root pressure results when solutes accumulate to a greater concentration in root xylem than other root tissues. The resultant chemical potential gradient drives water influx across the root and into the xylem. No root pressure exists in rapidly transpiring plants, but it has been suggested that in some species root pressure can play a central role in the refilling of non-functional xylem conduits particularly after winter (see an alternative method of refilling described below).

 

Disruption of Water Movement

Water transport can be disrupted at many points along the SPAC resulting from both biotic and abiotic factors (Figure 8). Root pathogens (both bacteria and fungi) can destroy the absorptive surface area in the soil, and similarly foliar pathogens can eliminate evaporative leaf surfaces, alter stomatal function, or disrupt the integrity of the cuticle. Other organisms (i.e., insects and nematodes) can cause similar disruption of above and below ground plant parts involved in water transport. Biotic factors responsible for ceasing flow in xylem conduits include: pathogenic organisms and their by-products that plug conduits (Figure 8); plant-derived gels and gums produced in response to pathogen invasion; and tyloses, which are outgrowths produced by living plant cells surrounding a vessel to seal it off after wounding or pathogen invasion (Figure 8).

Sources of dysfunction in the xylem.

Figure 8: Sources of dysfunction in the xylem.
Left to right: (A) xylem-dwelling pathogens like Xylella fastidiosa bacteria; (B) tyloses (plant-derived); (C and D) conduit (in blue) implosion (Brodribb and Holbrook 2005, Pine needle tracheids); and (E) embolized conduits among water filled ones in a frozen plant samples (Choat unpublished figure, Cryo SEM).
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Abiotic factors can be equally disruptive to flow at various points along the water transport pathway. During drought, roots shrink and lose contact with water adhering to soil particles — a process that can also be beneficial by limiting water loss by roots to drying soils (i.e., water can flow in reverse and leak out of roots being pulled by drying soil). Under severe plant dehydration, some pine needle conduits can actually collapse as the xylem tensions increase (Figure 8).

Water moving through plants is considered meta-stable because at a certain point the water column breaks when tension becomes excessive — a phenomenon referred to as cavitation. After cavitation occurs, a gas bubble (i.e., embolism) can form and fill the conduit, effectively blocking water movement. Both sub-zero temperatures and drought can cause embolisms. Freezing can induce embolism because air is forced out of solution when liquid water turns to ice. Drought also induces embolism because as plants become drier tension in the water column increases. There is a critical point where the tension exceeds the pressure required to pull air from an empty conduit to a filled conduit across a pit membrane — this aspiration is known as air seeding (Figure 9). An air seed creates a void in the water, and the tension causes the void to expand and break the continuous column. Air seeding thresholds are set by the maximum pore diameter found in the pit membranes of a given conduit.

Air seeding mechanism.

Figure 9: Air seeding mechanism.
Demonstrates how increasing tension in a functional water filled vessel eventually reaches a threshold where an air seed is pulled across a pit membrane from an embolized conduit. Air is seeded into the functional conduit only after the threshold pressure is reached.
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Fixing the Problem

Failure to re-establish flow in embolized conduits reduces hydraulic capacity, limits photosynthesis, and results in plant death in extreme cases. Plants can cope with emboli by diverting water around blockages via pits connecting adjacent functional conduits, and by growing new xylem to replace lost hydraulic capacity. Some plants possess the ability to repair breaks in the water columns, but the details of this process in xylem under tension have remained unclear for decades. Brodersen et al. (2010) recently visualized and quantified the refilling process in live grapevines (Vitis vinifera L.) using high resolution x-ray computed tomography (a type of CAT scan) (Figure 10). Successful vessel refilling was dependent on water influx from living cells surrounding the xylem conduits, where individual water droplets expanded over time, filled vessels, and forced the dissolution of entrapped gas. The capacity of different plants to repair compromised xylem vessels and the mechanisms controlling these repairs are currently being investigated.

Embolism repair documented in grapevines (<i>Vitis vinifera</i> L.) with X-ray micro-CT at the ALS facility at Lawrence Berkeley National Lab CA, USA.”></p> <div class=

Figure 10: Embolism repair documented in grapevines (Vitis vinifera L.) with X-ray micro-CT at the ALS facility at Lawrence Berkeley National Lab CA, USA.
(A) Longitudinal section showing a time series of cavitated vessels refilling in less than 4 hrs; (B) 3D reconstruction of four vessel lumen with water droplets forming on the vessel walls and growing over time to completely fill the embolized conduit.
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