How Plants Work?

Plant energy

Water is important for plant growth

For a plant to successfully grow it needs plenty of energy. Energy which is provided by sunlight and used by the plant to drive the chemical process of photosynthesis (see Plant Evolution), creating plant material by converting carbon dioxide from the air together with chemicals held in solution and obtained by the roots.

Energy is also used by the plant to power the pumping action of lifting water and nutrients in solution from the soil. The energy from the sun evaporates water from the leaves, which sets up thermal and osmotic gradients throughout the plant tissues which powers the pump.

Water also provides energy for plant growth. Only a minute fraction of the water pumped from the roots is actually used within the plant for internal growth. The bulk of the water is used as a medium to extract nutrients from the soil. Nutrients dissolved in the water are sucked into the plant, the nutrients extracted and excess water evaporated.

Growing conditions

Water shortage: Shortage of water will result in a variety of behaviours, because plants have different mechanisms for handling water stress. Many Australian natives are particularly effective in being able to manage a shortage of water, with a whole variety of strategies such as waxy or specially shaped leaves.

One particular mechanism is to slow their reaction rate by cutting back transpiration. These plants do not appear to suffer stress; they simply shut up shop and stop growing, remaining in a state of suspension until water is available. They may even sacrifice entire limbs to survive.

Another survival mechanism employed by some plants is to put down very deep roots which will only operate if the upper soil is dry. The infiltration rate of air through dry soils is relatively high, so under dry conditions deep roots have adequate access to air. Because of their depth, or operating head, these deep roots do not provide for good growth. They simply support the plant. When the soil becomes wet, these deep roots will become starved of air and stop supplying water and nutrients. The more effective shallow roots will take over, giving much higher growth rates.

Grape vines are typical of the sort of crop which have the capability of putting down roots to considerable depths (up to 40 m) in order to survive. However, under these conditions the whole plant will be in survival mode and will avoid making fruit or unnecessary growth until more favourable conditions occur.

Other plants, such as vegetables, do not have these survival mechanisms. When there is no water available, they die.

Excess water, porosity, air and compaction: Roots expel carbon dioxide into the soil and absorb oxygen, which means air and porosity are a crucial part of the plant's environment. A few highly specialised plants, such as mangroves, have the ability to feed air through to the roots. Most plants, however, will stop growing and eventually die if air is excluded from the roots by waterlogging or compaction.

Soil compaction can arise from excessive application of water, as occurs in flood irrigation, or mechanical action, as from large sprinkler drops. Plant growth is reduced by the lack of air in the root zone.

Nutrients: Nutrients are obtained by adding fertiliser, by the decomposition of organic matter, and by biological action.

Many soils, particularly clays, can be rich in nutrients, but these are locked into the soil and are not available to the plant. Micro-biological action can release these nutrients.

Water shortage and nutrients: If there is an excess of nutrients - for example, the plant is over fertilised when water is short - the solution in the soil will be stronger than in the plant. The plant will be killed by osmotic pressure sucking water out of the plant.

Excess water and nutrients: Excess water in the soil with insufficient nutrients creates weak solutions and a high osmotic gradient. The plant will be able to draw in and evaporate large amounts of water but will not be able to extract sufficient food, resulting in reduced growth. Obtaining the right balance of water, air and nutrients is vital to healthy growth.

Moisture transfer

Cotyledons is shown above soil - the root below will extract nutrients

Water and nutrients are continuously moving through the soil. In the heat of a summer day, roots will be extracting both moisture and nutrients from within the root zone faster then they can be replaced, setting up moisture gradients throughout the soil. At night, when the plant is not feeding, moisture and nutrients will move from beyond the root zone to restore the balance.

Gravity, surface tension, evaporation and condensation, hydraulic pressure and friction control the movement of water through the soil.

The role of gravity is obvious. Surface tension is the "wicking" action and is dependent on pore size. Surface tension forces in coarse sand are very low so there is little lateral movement; finer soil will have a higher surface tension, which means water will move through quite readily. If the pore size is extremely small, such as in very fine clays, the surface tension forces will be quite high but the soil may be very tightly packed and water will move very slowly.

Often, gravity and surface tension forces are in equilibrium. Gravity will be pulling water downwards against surface tension forces which are pulling the water upwards. This equilibrium condition is the field capacity, and is less than saturation, where all the voids are filled with water.

Osmotic pressures are created when different levels of solution are in contact, water being attracted towards the stronger solution.

Good soils have significant pores. If the soil is moist, water will be continuously evaporating and condensing, so the air inside the pores is saturated with water vapour. If the soil is uniformly moist, there appears to be no movement of the water because the rates of evaporation and condensation are in balance. The whole system is in equilibrium, with no net movement of water.

However, if one area such as the root zone is drier than the rest of the soil, then water condensing on either the roots themselves or on soil within the root zone will result in water movement from areas outside the plant to the root zone. Movement of water by evaporation and condensation can be significant with changes in temperature from day to night.