Second in the Water Series

Try to imagine life without water….No matter how dry it may seem to be here, the soil cracked open in supplication, the lawns toasted and tan, Rhododendrons with their leaves curled and burnt along their margins, Vine and Japanese Maples, their leaves crisped blowing down curbs in late summer’s heat, there is water…everywhere, locked deeply in tissues, bound tightly to soil particles. Like most things, there are no absolutes with water. It is not simply here then gone, but on a continuum of availability. Biological scientists and agronomists will often speak of ‘dry weight’ when looking at growth trying to minimize the variability of water weight in living organisms. They bake the subject in autoclaves reducing water weight to zero without igniting and burning the carbon and more ‘solid’ structure to ash. There is water throughout the structure of plants, hydrating their cells, making possible the many processes at work within them. There is water in the atmosphere even on a blistering hot and clear day in the form of vapor effecting everything from the Evapotranspiration Rate, (ET), to how hot or cold we may feel beyond what the thermometer reads; and there is water in the soil though our plants be wilting or dead of desiccation, and it effectively sucks the moisture from our skin when we work bare handed in it. Water is everywhere even in the dry periods within the desert and, nature is okay with that and has in fact adjusted to it. Our gardens, however, are anomalies we’ve created. We are invested in them and as gardeners we do what we can to assure their survival, and more, their success!

Where the Roots Hit the Soil: Xylem Tissue, Water & Xylem Sap

Most simply put plants ‘pump’ water from the soil and transport it throughout their structure to enable growth and life. Their roots penetrate and ‘colonize’ the soil both available and amenable to their growth. Different plants have different root structures, some possessing heavy woody structural roots that branch smaller and smaller down to tiny ‘hair’ like roots, while others possess fleshy unbranched roots. All of the less woody, structural roots, are continuously being shed and replaced. In Epiphytes, roots serve primarily as anchorage to their ‘perches’ in trees or on rocks, while many water plants simply float in the water itself the roots providing no anchorage at all. What I’m going to be discussing here are the terrestrial plants that most of us garden with…the others are more exceptional and their roots specialized.

As tool wielding humans, pumping conjures up an image of a mechanical pump creating a pressure differential in a pipe or tube, allowing us to move water against gravity and friction. Plants, however, have no pump per se, but they are extremely effective at moving water from the soil and throughout their structures. Some plants, like the Coast Redwood, can move water over 300’ vertically (though these also take in some with their foliage, high in their canopies, to add into the mix), a challenge that building mechanical engineers can appreciate. Suck if you can on a 10’ length of tube from a pool until you have drawn water it’s length vertically to your mouth! In plants, the ‘pump’ is the canopy, the many stomata in their leaves combining to, draw the water up rather than pushing it from the roots. Negative vapor pressure, suction, is created as water is used by the plant and/or exits through the stomata in the undersides of leaves. There is not one ‘pipe’ ascending each plant or tree, but many, many tiny tubes. The small diameter of these xylem ‘tubes’ provide a huge amount of surface area that while it produces friction, slowing flow, aids flow by providing the water a surface to adhere to while the water clings to itself. It produces a capillary effect, a positive force pulling the column of water up though it is nowhere strong enough to lift it into a tree’s canopy. For any such pump to work requires that the system be sealed, that it have no vacuum leaks. If it does air will fill the ‘piping’, in this case the cambium’s xylum tissue and the water column will collapse. Girdling a vascular plant (Not all plants have a vascular system to move water.  Some simply absorb it through their cell walls directly to where it’s needed.) breaks this vacuum, and leads to tissue and plant death. A wound, limited to part of the plant’s circumference, will cause the plant above and below, xylem generally does not spiral or branch, to desiccate and die in that portion interrupted by the wound. Think of Verticillium Wilt, a fungal disease that blocks the vascular tissue as it spreads resulting in the death of tissue above it, often leaving a side of the plant dead while the rest of the plant remains unscathed.

Staying with the same mechanical example, pumps draw water from a contained volume of water, not from soil. The pump could technically do it but because it depends on a single-point-large inlet it lacks the needed contact with the water that is contained within the soil. Roots, in a sense, give many tens of thousands, millions even, of ‘inlets’ for a pump to draw on, each one tiny to maintain contact with moist soil. There is also air in soil, air that is necessary for various functions within plants, but that cannot be allowed to interrupt the many columns of water climbing up through and moving throughout the plant.   Plants, as organisms comprised of specialized cells and tissues, have evolved the ability to draw water and dissolved minerals and even other bits of life, if tiny enough, through the highly specialized membranes of their cellular walls. The roots, then, do not have openings per se acting as one-way valves, they draw it through their walls through capillary action. Water moves through it to ‘equalize’ the difference. (Not everything different is allowed through. Some plant toxins will find the wall ‘closed’ to them. Some ‘salts’, like nitrogenous fertilizers, can pass through in excess and cause ‘burning’ of tissues unless flushed through with even more fresh water.) Even this is too simple.

Within the soil the ‘line’ between plant and soil blurs. Roots are specialized plant structures, that, our ‘reductionist’ scientific thinking, has clearly defined as part of the plant. Everything else is other and much of the ‘other’ has been said to exist separate from the mineral soil. There is an incredibly complex organic community within the soil, which for years, was discounted as superfluous, incidental or at most, inconsequential. More inclusive thinking has refuted these divisions. Systemic thinking looks at these as a whole. The soil is not ‘inert’. It is rather a piece of a very complex whole. Roots are conduits supplying the larger plant, but roots themselves are colonized in unique and complex ways by many other organisms including fungi, bacteria and a great many others, many of which effectively ‘expand’ the reach and ‘precision’ with which plants get what they need from the soil. This soil/root zone forms complex communities that we are finding are essential to the optimal performance of plants and landscapes that themselves function in sustainable relationship, with minimal to no resource inputs from elsewhere. Hyphae merge with roots with rhizo-bacteria and mycorrhizae. Shedding roots, hyphae and their exudates work to create a soil and crumb structure that better suits the plants. They all work together to retain and cycle nutrients in a manner that ‘conventional’ agriculture and horticulture has largely ignored. We fertilize the soil while all along healthy soil is ‘working’ to build and retain a storehouse of nutrients.  The root ‘systems’ reaching down, overlapping or going deeper than those of their neighbors, ‘capturing’ water soluble nitrogen and others before they might leach away lost. Newer thinking views the plant itself as a community its constituent parts working together toward a common goal of health and vitality, (See anyone of several of Lynn Margulis’ books on microbiology that are aimed at the layman.  She, along with James Lovelock, proposed the Gaia Theory.)…of the larger biotic community. And all of this goes on within the matrix of moist hydrated soil. Just as we do not ‘feed’ our plants, when we water our gardens, we are not giving plants a drink. We are encouraging a community, a system, which has evolved over millions of years.

Getting Back to the Water Column: Internal Plant Processes, Phloem Sap and the Movement of Water

While simple physics may explain the movement of water it does not fully explain the structure of a plant and the processes necessary to its living. Capillary action, adhesive and cohesive forces, respiration and evaporation, photosynthesis, cellular metabolism, transpiration, cell division, protein and carbohydrate synthesis, various internal process that create compounds to defend it from pests, like latex, or to protect them from freeze damage, the ‘triggers’ for the growth of reproductive structures, reproduction itself, pollination, seed formation the growth of structures to aid dispersal, all of these things go on with in the living plant, determined by DNA, effected by environment operating within the limits of a physics of biological life. Water moves through a plant carrying with it the ‘building blocks of life’, it’s catalytic tools that will help shape and determine its individual and collective future. It is never ‘just’ water. All of these things are shaped by the physical nature and limitations of water. ‘Tug’ on a column of water and it will rise held together and helped along by the molecular forces within it. The shape and size of the conducting vascular tissue is determined by the particular solution they carry. This ‘water’, xylem sap, a mixture of nutrients, mineral elements and hormones, is drawn up the plants stem and out its branches into its leaves where a portion is ‘lost’ through leaf stomata as water vapor, the site of most of photosynthesis producing the sugars that will then be carried to the meristematic tissue in the cambium and buds to power the growth of the plant, of new leaves, phloem and xylem, all of the transient root hairs, floral parts and seeds. This ‘production’, and the ‘loss’ through the stomata, produces a ‘pull’ on the water that would otherwise only be drawn in as high as simple capillary action and osmosis could pull it negating the possibility of trees and any plants with much height at all. The taller the tree, the higher the column of water, the greater the negative ‘vapor’ pressure in the xylem, caused by the ‘tugging’ of leaves, that can, in the very tallest trees, like Coast Redwood, lead to cavitation, the formation of air bubbles in the xylem and the death of the tissue. There are real physical limits to the height of trees.

Phloem Sap moves from the leaves, the site of photosynthesis and the creation of carbohydrates, along with other mineral elements and hormones via the ‘Pressure Flow Hypothesis’. It is thought that it moves wherever it is needed by ‘diffusion’. The high concentration of carbohydrates in the sap creates a hydrostatic, positive pressure, which pushes it towards areas with a lower concentration of carbohydrates. For those of you dying to know, this is known as an osmotic gradient, substances in solution have a tendency to even out and move toward cells actively metabolizing or acting as ‘sinks’. Much of the sugars that aren’t needed immediately, are pushed to roots where it is stored during the summer when carbohydrate production is high, and later dispersed throughout the plant in spring when the demand of new spring growth out strips the plant’s ability to produce needed carbohydrates. (Presumably, in tropical climes where plants retain their foliage and are actively growing year round, and in evergreen plants in general which are always ‘ready’ to photosynthesize and grow, the sugars are needed continuously for active cell metabolism, as long as it’s not stymied by drought and the need for a reserve is much less, as long as drought periods are significant.) Phloem Sap is bi-directional, it will flow in the direction of lowest concentration, up or down the phloem. The Phloem Sap moves towards the ‘sinks’, roots, stems and seeds, where the sugars are removed from solution, concentrated and stored as starches, the water drawn off and returned to the Xylem. It is also thought, but not understood, that there is an internal ‘communication’ system that ‘calls’ for more or less to specific locations. During the active tissue growth of spring stored starches in the roots are re-dissolved into xylem sap and sent back into phloem for use. This Phloem Sap is the sugar rich product that gives Maple Sugar its sweetness and provides various ant species a food source, if their guts contain other helpful organisms that allow them to digest it. Water, originally brought in through the roots provides the fluid that transports everything within the plant. It serves another purpose as well.

Water and Photosynthesis

In elementary school most of us once learned that plants had the capacity to capture the energy from sunlight, in the presence of that magical substance, chlorophyll, converting it into chemical energy that powers the growth of all plants. Of course it’s a lot more complicated than that.

6CO₂ + 6H₂O ——> C₆H₁₂O₆ + 6O₂
^{Sunlight energy}

 Where: CO₂ = carbon dioxide

H₂O = water

Light energy is required

C₆H₁₂O₆ = glucose

O₂ = oxygen

Six molecules of that soil water, from that xylem sap, and six molecules of CO₂ are broken down and recombined in creating one energy rich molecule of glucose, a simple sugar utilized in cell metabolism, in the process freeing six oxygen molecules to the atmosphere. The sunlight is ‘captured’ in the glucose molecule and stored there until needed. When that glucose molecule is brought to a living cell through the process above or by animals/humans consuming plant material, that same amount of energy is released within a cell by that cell’s mitochondria in the process known as respiration. Respiration is the reverse chemical process, combining six molecules of oxygen with the one glucose, releasing, rather than absorbing energy in its ‘conversion’ to a simpler more stable form. What’s the point? The net energy gained from the sun and spent in the action of life. The sun ultimately powers all life on Earth and it does this through photosynthesis, a very elegant process…not possible without water.

There is more going on here inside the plant. Plants don’t do this in a sterile well equipped lab. No…remember everything is more complicated. There are intermediate steps along the way, there is a process through which the equation above is manifested within the plant. The equation itself only lays out the elements…the idea of it. The sun does not ‘wave’ its magic wand over chlorophyll and….

The C3 Photosynthetic Pathway

Water is brought into the leaves through its system of veins that are comprised of extensions of the xylem and phloem. CO₂ is brought in through the stomata in the undersides of leaves. Water, in the xylem sap, is drawn into the cells of the leaves themselves, specifically, into the pallisaide cells of the mesophyll and they are brought into them by the same process that drew it up the stem. The CO₂ moves by diffusion, from plenitude to deficit. Within the mesophyll are choroplasts. This is where the chlorophyll resides, where the energy is collected from sunlight. The carbon, hydrogen and oxygen doesn’t simply become free atoms within a plant, it is a bio-chemical process.   They are first transformed into a more complex 3-carbon molecule from the materials brought into the mesophyll. The chlorophyll is able to ‘capture’ ‘excited’ electrons within ATP and other molecules created in the process to power this series of chemical changes. It creates and utilizes various enzymes to move the process along down a pathway utilizing multiple smaller steps. This is a biological, living, process, it is precise and effective manufacturing everything it needs along the way from what is at hand and it is a process developed and refined over millions of years. There is nothing simple about it. Energy is transformed. Molecullar building blocks are created. Complexity is added. ‘More’ results. More life. More living mass. It is consumptive and creative, ultimately adding to life. It is ‘miraculous’. C3 is the basic process of photosynthesis that occurs in all photosynthetic plants. Other processes/modifications have been added to it over time under very different growing conditions across the Earth.

Scientist continue to study the processes within plants, within the cells and their organelles to better understand it. Not surprisingly, some scientists, and the companies that employ them or fund their research, have become very interested in the processes of chorophyll and photosynthesis themselves and how they capture energy, specifically, splitting hydrogen from water molecules, which takes energy. They are trying to figure out how to utilize free solar energy to produce hydrogen which we can store to later use in hydrogen fuel cells to power everything, the only by product of which, when ‘burned’, would be pure water. They are trying to ‘grow’ a pure fuel source without consuming dirty or imported fuel sources…like plants do.

The above-described photosynthetic process is known as C3 photosynthesis, for its utilization of 3-Carbon molecules. It is believed to be the oldest form of photosynthesis, developed at a time on Earth when oxygen was much less common and  and CO₂  was more. C3 photosynthesis is most efficient in a moist environment with moderate temperatures. As temperatures rise, and/or water becomes less available, the C3 pathway has problems. The stomata begin to close cutting off both the supply of CO₂ and the outlet for the transpiration of water. But even at moderate temperatures C3 photosynthesis is a very inefficient user of water as it ‘wastes’ over 90% of the water it draws up into its tissues through its stomata through photorespiration. This is a process that uses RuBisCO, an essential enzyme plants manufacture, in the ‘fixing’, or creation, of the intermediate carbon molecules. This same enzyme, if it is near O₂, reverses the process and the energy is instead used to power respiration producing and giving off CO₂, stopping the production of carbohydrate and actually consuming it. As temperatures increase over 80F, the plants depending solely on C3, begin to close down their stomata slowing down and eventually stopping photosynthesis as they rise through the 90’s. When temperatures drop at night, C3 plants, in the absence of light, wait in limbo.

The C4 and CAM Photosynthetic Pathways

Some plants have evolved modifications to this basic pathway to accomplish their photosynthetic needs. Some of these, developing independently of one another, share a general process. As a loose group these live in drier tropical and sub-tropical regions. They produce a different group of Carbon molecules with 4 Carbon atoms in each molecule, hence this pathway is called C4. (The C2 pathway is the process of respiration in which CO₂ is the primary byproduct.) C4 utilizes chlorophyll but follows a different chemical path, sharing some of the enzymes of C3 plants as well as others and have modified structures within the leaves to accomplish this. These physical adaptations isolate the enzyme RuBisCO away from oxygen, where it is saturated by CO₂ eliminating the option of the wasteful inefficiencies of photorespiration. C4 is more efficient than C3 especially at higher temperatures and with less available water. Because there are extra steps in this process more energy in the form of ATP is required, about double. It is then an adjustment that requires more energy, but less water in a hotter environment.  Water is conserved.  Sunlight, the energy source, is adequate to meet the demand.  Once this portion of the process is completed these plants finish producing carbohydrate by utilizing the final portion of the C3 pathway.

The third major pathway is CAM, Crassulacean Acid Metabolism, so named because the process was discovered while studying the Jade Plant a member of genus Crassula. I’ve mentioned all three of these in previous postings and provided links to sites containing ‘simple’ descriptions. CAM is utilized by a small group of plants, primarily of desert origin, where water and high temperatures are major limiting factors. In CAM plants the leaf stomata are closed during the day to reduce water loss via photorespiration.  There are also many less stomata. The stomata open at night to take on CO₂. This is stored as a 4 carbon acid, Malate, in vacuoles, little ‘bubbles’, within mesophyll cells. During daylight, when light can provide the energy necessary to make the conversion, it is changed back to CO₂ within the chloroplasts where it is concentrated around the enzyme RuBisCO increasing the efficiency of photosynthesis and the rest of the process completed. The final phase in CAM plants is also the C3 process to create the carbohydrates.  The C4 and CAM modifications make it possible for plants to live in environments otherwise completely inhospitable to them.

Sharing Pathways Within Plants and/or Over Time

Now, to complicate this further, are the plants that use CAM part of the time. CAM is an adaptation to drought and some plants switch from C3, when water is available, to CAM when they enter drought conditions…and back again. Then there are others like certain species of Cactaceae that also have leaves, the leaves utilize C3, but may be shed during drought while the stems always function as CAM. Some plants don’t have enough capacity to store all of the malate in their vacuoles and so utilize C3 for the rest of their needs. These are known as strong or weak CAM depending on their capacities.

It can be hard to tell which pathway a plant may use simply by looking at it. Many plants have made external modifications to reduce their water loss, but just because they have doesn’t mean that they are automatically C4 or CAM plants. Certain genera maybe wholly one or another, but likely not. Some plants, like family Cactaceae, as seedlings use C3 switching over as they grow. No grasses are CAM but they are divided between C3 and C4. Corn and Sorgham, two of the world’s larger agricultural crops, are annuals and C4 plants.  Rice is C3. Then there are some of the dicots, like some of the Pelargonium, that are, surprising to me, CAM, with much lower water requirements. Some Senecio are CAM. Euphorbiaceae are split between all three. What about those Pachypodiums? with their squat, fat, water retaining stems and odd leaves they drop in severe drought?  No trees are CAM. CAM plants tend to be smaller in stature. Is this because their efficiencies with water reduce the negative vapor pressure within their vascular system so they don’t possess the ability to pull water to heights of trees? They tend to be slower growing, frugal with water and unable to survive in the wetter conditions that C3 plants require.

What about Eucalypts? Some of their species are among the tallest plants in the world. They often occupy very arid parts of Australia and have phyllodes, modified extensions of twigs, in place of leaves. Their phyllodes would appear to be adaptations to drought. I would guess that they are C4, but then I wonder, could a C4 tree be capable of ‘pumping’ water so high into their canopies? What about other plants with phyllodes, like Phyllocladus alpinus, the Celery Pine from New Zealand, a plant I’ve apparently just killed in a large pot by allowing it to get too dry?

Okay, so you read all of this, and now you want a list, to see which plants are which, so you can sort this out in your head, ground some of this ‘hocus-pocus’. Well, from my internet search, though not exhaustive, there is no such complete list. The information is scattered in, perhaps, thousands of scientific papers written by those who have done the specific lab testing to determine a plant, or group of plant’s, specific pathway. Remember there are many thousands of species, and, even more names, and I’m talking ‘legitimate’ botanical names, requiring even more cross checking…. Remember, however, that the vast bulk of plant species are thought to be C3, and that, C4 and CAM plants, tend to be indigenous to arid and desert regions.  While it may not be accurate to assume that other genera from the same areas share the same pathway, it helps you begin to see patterns, and helps me to put it into perspective.

Photosynthesis requires water as a physical component.  It breaks it down converting it into other necessary molecules and this creates a ‘need’ at the leaf level for water, which all combined adds to the vacuum pressure that draws water up through a plant.  It is likely, however, that without the inefficiencies of photo-transpiration and its resulting massive loss of water out the stomata of the leaves, that tall trees, at least, would not exist.  It is the total vacuum pressure that draws the water up to the heights.

Plants have evolved to survive in almost every climatic condition on Earth. We plant our little gardens and often have a narrow, limited idea of what it means to garden. We invite plants into them and are often not aware of what they actually require. Knowing our plants and the conditions in our gardens are both essential if we want to garden well. If we choose plants from the same or related regions, they are more likely going to be able to grow in the conditions we provide for them, they will also, because of the physical adaptations they have made to place, look like they belong together.

Conclusion: Plant’s Need for Water

All plants need water, but vary greatly in their need for it to complete their various internal processes. Most plant species are C3 and, being the least efficient photosynthesizers and most wasteful of water, have the highest demand for it. (They are ‘inefficient’ because after going to all of the effort of ‘fixing’ carbon into organic compounds, they then ‘consume’ carbohydrate through photo-respiration converting them back to CO₂), but even within this broad group of plants, their need can vary widely. You cannot put a C3 plant on a water diet, forcing it to be more efficient in its use of water. There will be a point where photosynthesis stops and wilt begins, where the plant lacks the water to fully hydrate its cells. If this state continues too long or scarcity drops the internal water levels even more, ‘permanent wilt’ will set in, and no amount of irrigation will bring it back. Death of the wilted tissue or even the whole plant can occur. Under the very same water conditions your C4 and CAM plants will likely be fine.  Improving ‘drought’ resistance in C3 plants means encouraging the broadest healthiest root system possible and making sure that you are going into drought conditions with hydrated soil, soils containing available water, not saturated soils.  Saturated soils, from overwatering, under many temperature scenarios, are very likely to cause roots to rot.

Gardening demands that we make choices constantly. When, how much and even how to water are important questions that are directly linked to our climate, soil, aspect, design and plant choices. Gardening is a dance of all of these elements and water is an essential in it. We cannot ignore the role of water in our plants.

For those who want to see a ‘simple’ list with a few diagrams showing the differences between C3, C4 and CAM photosynthesis see this link.

This whole discussion of pathways, and the ‘function’ of water within a plant, brings up more questions for me. Why don’t all plants of a particular area share the same photosynthetic pathway, shouldn’t they since they share the same conditions? If we could locate all plants on a water use continuum how much overlap would there be between the three different pathways? Out there in the real world are there clear lines between them? In genera where different species have taken different pathways, do any Genus ‘bedmates’ exist side by side with their cousins that share different pathways? Why? Or is life truly more a matter of a roll of the dice?

How much difference in water use do surface leaf adaptations alone, like thicker or ‘hairy’ cuticles, make to water loss/ transpiration? Why don’t leaf surface adaptation always go along with the internal modifications that go with the three photosynthetic pathways? Going to their foliage, some plants add and drop their leaves in response to drought and abundance, like many desert and California Chaparral plants, e.g., Ocotillo or Fouquieria splendens, and the California Buckeye or Aesuclus californica, while many plants are simply weakened if they lose their leaves prematurely from summer drought?

What about their roots? How do plants that dominate in wet/boggy soils get the water they need and the air for gas exchange? What about plants like the Bald Cypress, Taxodium distichum, from the Cypress Swamps of the American SE, which do that, and, are mostly drought tolerant for us in heavier soils in the Maritime Pacific Northwest? And, more about roots, are there physical characteristics of roots that make them more or less efficient at collecting water? Very fine roots vs. relatively fat ‘fleshy’ roots like on Bananas? Nutrients, including the micro-nutrients, in soil water, move from higher concentration to lower, from the soil into the plant, which can lead to toxic levels in a plant…how much ‘control’ does the plant have, or is the interface ‘transparent’?

Most plants can’t survive for long in saturated soils, where even the pore space is filled with water, where ‘free’ water surrounds the root hairs.  At what level does soil water cease being available to plants? Where is the sweet spot?  Does this vary between plant species? Is this determined by different root structures?

As I read back through this I ask myself, “How much of this does a gardener need to know?” and, I answer, “not that much.” But it is one of the things that has always drawn me to plants and horticulture, not need, but curiosity, coupled with a wonder of the world that surrounds us and an amazement at how much of this so many people, mostly non-gardeners, are completely ignorant of and, so often, indifferent to. So, I write these little pieces, these little explorations, and try to convey a piece of my sense of wonder to those not yet fully awakened to it, in hope that I might stir something in them, and find that, often, it does. The natural world and its wonders resonate inside of us! If only we weren’t all so distracted! Working, raising families, mindfully attending our many relationships and the physical needs of our own bodies, pursuing our activities, finding moments of peace and beauty to restore ourselves within a crazy, whirling life that often spins madly about us. For many people, their religious beliefs have effectively resulted in a diminution in ‘value’ of the natural world. Modern society has been split away from the natural world, a condition we now have a name for, Nature Deficit Disorder, leaving us with science alone to reestablish our connection. Gardening pulls me into what matters. It teaches me, offering lessons into what is important in life and its staggering complexity and beauty, the inter-connectedness. They are never just plants or landscapes. Looking at them that way demeans all life…us included.

If you remember nothing else remember this:

• Plants vary in their requirement for water.
• It is the living processes themselves within the plant, working together, that make it possible…even those inefficiencies within the plant, the ‘waste’, that makes it all possible.  Improvements in efficiency, were they possible, would change the function and form of a plant.
• Plants’ themselves and their root structures have evolved over millennia adapting to the particular conditions where they evolved, including the water conditions.  Change this at your peril.
• Plants have developed modifications in their structure and appearance that help reduce their water losses through their leaves from reducing their leaf area and surface temperatures by shielding them from the sun’s radiation, all of the way to dropping them when drought becomes too extreme.
• Plants follow different biochemical pathways while they manufacture the components they require for life and these pathways help determine their requirements for water.
• The time to prepare your garden for drought is before the drought begins!
• Plants possess a limited capacity to adapt to changes in available water.  It is thus very important to choose appropriate plants that will meet the conditions on your site, including your decision of whether or not you will irrigate, how often you will do it and how you will apply it.