The effect of water stress on plant growth and development, and the utilisation of water
by plants, is discussed below and also in the section on Irrigation, 1N, pages 93–100.
transpiration by plants.
Plants take up moisture through their roots from the store of available water in the
capacity and that held at the permanent wilting point. Field capacity is the percentage
of moisture held in the soil 2–3 days after being saturated and after free drainage has
ceased. The permanent wilting point is the moisture content of the soil at which plants
fail to recover (to regain full turgidity) when water is added again to the soil.
The amount of available water in the soil depends mainly on the soil texture and
profile:
soils. In other words sandy soils hold less water in reserve for plant growth than clay
soils, and so sandy soils need more frequent rainfall or irrigation to support a crop.
distance below the surface through which neither water nor roots can easily pass. Thus
water is held near to the soil surface, and in addition roots cannot penetrate the soil
deeply. Plants growing in these soils rapidly use up the available water, then suffer from
water stress and fail to reach their full potential.
Plants have a fixed capacity to utilise water from the soil. This water is transpired
through very small holes, or pores, called stomata, which are found mainly on the
underside of leaves. Stomata allow the plant to take in carbon dioxide (CO 2 ) for the
However, as the temperature rises above about 32°C, and when there are strong
winds, the root system cannot replace the lost water fast enough for the uptake of CO 2 to
continue, and the stomata close. As a result, water movement within the plant ceases and
the plant wilts and stops making sugars—see paragraph on wilting 1Ef, page 39.
If this process is repeated for a few hours every day, the plant begins to draw on the
moisture within its own plant cells after a few days, and the flowers and young fruits
begin to fall to the ground.
Plants are damaged by heat stress at high temperatures even if there is adequate soil
Some crops are more successful at surviving drought than others. For example, fast
growing crops such as the millets, grass pea and sorghum often avoid drought because
their life cycle is very short—drought resistance via drought avoidance.
production of carbohydrate, and give out oxygen as a waste product—discussed in
9
- Soil Water. Soils lose water by drainage, by evaporation from the soil surface and by
- Soil Texture. Sandy soils have a lower wilting point and field capacity than clay
soil—this water is the difference between the volume of water held at field (moisture)
- Soil Profile. Many soils have an impervious layer (a soil pan or hardpan) a short
GROWING FOOD – THE FOOD PRODUCTION HANDBOOK
1Ed. “Photosynthesis/Respiration”, page 39.
moisture. The artificial cooling of crops is discussed in 1N. “Irrigation”, page 93.