Amazing things plants

By John Fitzsimmons

We all know plants are wonderful, sustaining life and wellbeing in so many ways, but we can sometimes benefit from a reminder of just what amazing ‘machines’ they are too –  amazing hydraulic and chemical machines!

It’s amazing how ‘vascular’ plants can turn sunlight into chemical energy, and sometimes startling how (tree) roots and stems can physically disrupt hard paving and even affect otherwise solid buildings. We know most plants need some water, but how often do we think of the almost incredible hydraulic forces they employ to access that water and utilise it? The pressure differentials involved are truly surprising. It’s a good step in thinking beyond just ‘giving them a drink’ when needed.

Most plants, excluding fungi, algae, lichens and most mosses, that concern horticulturists are vascular (from vasculum, Latin for duct). These are the flowering plants, shrubs, trees, grasses and ferns that have xylem (lignified) cells which facilitate the movement of water and nutrients (including minerals) through the plant, and phloem (non-lignified) cells to facilitate movement of water, minerals and the products of photosynthesis. Nutrients resulting from photosynthesis are dissolved in water and transported from areas of high concentration (e.g. roots) to areas of lower concentration such as the flowers, stems and leaves, for growth and reproduction. Most vascular plants have roots, stems and leaves, with evolutionary variations of these structures reflecting responses to various environments.

Water is mostly absorbed by the roots, and moves up through the stems and ultimately is transpired through stomata on leaf surfaces. The volume and pace of water movement through the plant are factors of plant species and physiology, the properties of soil or growing media surrounding the roots, the size and growth stage of the plant, and the terrestrial conditions including humidity, wind, and light levels. The plant’s opening and/or closing of stomata is a key control measure, with transpiration ‘driving’ the process. 

Depending on species, stage of growth, and growing conditions, plants are commonly 60-95% water.

The water acts as a carrier of nutrients in solution, and contributes to cell volume and structural support (turgor). Chemically, it also provides elements (especially hydrogen) for photosynthesis and other biochemical reactions to occur, and contributes to stabilising the plant’s temperature as both a heat energy buffer and via the cooling action of evaporation from leaf surfaces.

Under extremes of temperature or water availability, the stomata ‘shut down’ as a stress response and photosynthesis (and therefore growth) will stop. Remember that stomata open up to allow CO₂ ‘in’ for photosynthesis, which also allows water ‘out’ for transpiration; it’s a fine balance with an inherent trade-off.

The water ‘loss’ through this process is evapotranspiration (ET).

A plant often only uses up to 5% of its water intake. Depending on species and conditions (solar radiation, temperature, relative humidity, wind), it has been estimated that a mature tree might transpire 300 litres or more per day. In contrast, one hectare of corn might transpire 35,000+ litres per day! In Australia, individual wheat plants might transpire 1-5 litres per day (depending on growing conditions and plant genetics). As water is lost through transpiration, the plant will work to import more via the roots. Plants wilt when insufficient water is available because the pressure inside the plant’s stems and leaves drops, making it insufficient to keep them firm and upright. Obviously, waterlogging will deny the rootzone of oxygen leading to a breakdown of the system and ultimately death of the plant, unless plants are adapted to waterlogging as in mangroves with ‘snorkel’ roots. 

The gradient in pressure required to move water ‘from root to tip’ in plants (Water Potential Gradient) can be substantial. Transpiration is also affected by the plant’s structure, age, and health. But to illustrate, the plant could access readily available water in the root zone (soil) at a modest pressure of perhaps -0.3MPa (atmospheric pressure is usually about 1013MPa). Through the xylem in the trunk of a tree, the pressure to move water upwards might increase to, say, -1.0MPa. To finally escape a tree via its topmost leaves, that pressure could have increased to between -7 to -100MPa. And how does water overcome that resistance to flow to the highest parts of the canopy? It ‘grows’ there, cell by cell, as the plant develops.

Horticulturists are familiar with the adaptation of plants to various environments. For example, desert plants transpire at slower rates than those adapted to humid environments. They may keep their stomata closed during the heat of the day to reduce transpiration. Other plants may tolerate some wilting to reduce transpiration. Small, silvery, reflective, or hairy leaves also reduce transpirational water loss.

The standard recommendation to watering deeply in mornings or evenings allows the plant’s cells to absorb water before they experience higher daytime temperatures, so when the heat arrives, they will be more resilient.

Plant breeders consider many of the above factors when pursuing new varieties that perform better in hot and dry (or cool and humid) conditions. They might focus on genes that improve photosynthesis, that affect stomatal openings or enhance the internal ‘flow’ of CO₂ to deliver the performance and resilience that buyers seek. For example, Australian research has done much to produce wheat varieties that meet or exceed yield benchmarks while requiring less water (as naturally occurring rainfall).

Other research in ornamentals has also identified irrigation patterns that produce plants of marketable size using less water than industry standards. Knowing the real needs of the plants is the first step, with some noting a relative of shortage of good objective science on that subject. Knowing, accurately, the delivery of water to your plants via the irrigation system is the second.

The latter has been made much easier through recent rapid advances in technology, especially of improved sensors (of soil/media moisture levels, humidity and cell turgor, as examples) and hugely flexible and responsive control systems (computing power). The cost and reliability of these technologies has, in real terms, also fallen dramatically.

One approach, resulting from research, suggests using soil (or media) moisture sensors as cut-off switches, so programmed irrigation cycles are only ‘permitted’ to operate when substrate moisture levels are below (or up to) a nominal point (say, 40%). The plants then effectively ‘control’ their own moisture needs even as growing conditions change.

So, next time you irrigate your plants by whatever method, spare a thought for the full range of processes you are participating in.