The importance of water as a factor affecting crop production cannot be overemphasized. Large expanse of dry but fertile land could be brought into productive cultivation if water is supplied through irrigation.
On the other hand, lands usually covered by water need some elaborate drainage system to make them productive.
Importance and Uses of Water to Plants
On the average, crop plants use 220 to 320 of water to produce a single kilogram of dry plant matter. Water deficiency commonly limits plant growth; in several agricultural areas, water is one of the most important component in farming. Water is vital to growers because of the several functions it serves in plant growth:
- Plant cells are largely made up of water. Plant tissue is 50% to 90% water, depending upon the type of tissue.
- When plant cells are full of water, the plant is stiff (turgid) or semi-rigid because of water pressure in plant tissue. This keeps stems upright and leaves expanded to receive sunlight.
- Photosynthesis uses water as a building block in the manufacture of carbohydrates.
- Transpiration, or evaporation of water from the leaf, helps cool the plant.
- Plant nutrients are dissolved in soil water and move towards roots through the water. Water is thus important in making nutrients available to plants.
- Water carries materials such as nutrients and carbohydrates throughout the plant.
Effects of Water Stress
Water stress is caused by a shortage of water in plant tissue. As will be explained later, stress can occur even at moisture levels that do not cause wilting. Part of the reason for such stress is that, as the soil dries, it becomes increasingly difficult for the plant becomes deficient in water guard cells to begin to close the stomata thereby slowing down the exchange of oxygen and carbon dioxide.
As a result of the reduced exchange of the two gases, photosynthesis must also slow down. With less photosynthesis, plant growth is inhibited.
As the soil dries further, or if the weather is hot and dry, the plant becomes even more deficient in water. The plant begins to lose water faster than it can be absorbed and the plant temporarily wilts. At this temporary wilting point, the plant will recover when conditions improve.
Wetter soil, cooler temperatures, a more humid atmosphere, shade, or less wind can help the plant recover. Although the plant recovers, episodes of water stress can reduce plant growth and crop yields. With further drying, the permanent wilting point is reached. At this stage, the plant will not recover even if conditions improve.
Plants suffering from chronic water stress are small and sparse with small, poorly coloured leaves. Old leaves often turn yellow and drop off. Some plants show specific symptoms of water stress. For example, the leaves of corn plants curl when they need water.
Seed germination is very sensitive to water shortage. While seeds efficiently absorb moisture through the seed coat, the emerging seedling is easily injured by dry soil.