• Moisture stress delays maturity.
• If stress occurs before flowering, duration of the crop increases.
• If stress occurs after flowering, duration of the crop decreases.
• During ripening, which involves dehydration and certain biochemical processes, moisture regime has very little effect on yield components.
• In cereals, moisture stress at panicle initiation is critical.
• Pod abortion takes place due to drought in several legumes including in soybean.
• Drought decreases leaf sucrose and starch concentrations.
• Drought increases hexose (glucose + fructose) concentrations.
• When the yield is fibre or chemicals where economic product is a small fraction of total dry matter, moderate stress on growth does not have adverse effect on yields.
• Ability of crop to grow satisfactorily under water stress is called drought adaptation.
• Evading the period of drought is the simplest means of adaptation of plants to dry conditions.
• Many desert plants, the so called ephemerals, germinate at the beginning of the rainy season and have an extremely short life period (5 to 6 weeks) confined to the rainy season.
• Ephemerals have no mechanism for overcoming moisture stress, therefore not drought resistant.
• Certain varieties of pearl millet mature within 60 days after sowing.
• Short duration pulses like cowpea, greengram, blackgram included under ephemerals.
• The disadvantage of breeding early varieties is that yield is reduced with reduction in duration.
• Stress avoidance is the ability to maintain water balance, and turgidity even when exposed to drought conditions.
• Favourable water balance under drought conditions achieved by
1. Conserving water by reducing transpiration
2. Accelerating water uptake
• Photosynthetic efficiency is higher in C4 plants than C3 plants.
• In C4 plants, the carboxylating enzyme PEP carboxylase has very high affinity for CO2.
• C4 plants are said to be drought resistant as they are able to grow better even under moisture stress.
• CAM plants are highly drought resistant.
• CAM plants open stomata only during night when they take CO2 into the leaves and store it as organic acid.
• Examples of CAM plants are Pineapple and Agave.
• Soybean, sorghum etc., reduce water loss by depositing lipids on leaves and plant surface.
• In grasses, leaves roll or curl due to moisture stress and reduce the area exposed to solar radiation resulting in low transpiration.
• Leguminous plants show prahelionastic movements i.e., the leaves are oriented parallel to sun rays thus avoiding the load of solar radiation.
• Moisture stressed groundnut plants reduce radiation load during midday by about 60 to 70% due to folding of leaves.
• Pubescence (hairiness) increases leaf reflectance and reduces solar radiation incidence.
• Awned varieties give more yield drought conditions compared to awnless varieties.
• Awns contribute 12% of photosynthates to grain.
• Compared to transpiring area of awns, its contribution of photosynthates to grain is more.
• Water uptake is improved by well developed deep root system.
• Drought increases root growth and root-shoot ratio which is an important mechanism of drought resistance.
• By improving water uptake, high water potential is maintained in leaves and the rate of photosynthesis is not reduced.
• Lowering of resistance to water can be achieved by increasing either diameter of xylem vessels or their number.
• Abscisic acid produced in leaves at lower turgor reduces seed set through its effect on pollen viability.
• By osmotic adjustment, production of abscisic acid is reduced.
• At low osmotic potential, photosynthetic activity and rate of hypocotyl extension are reduced due to inhibition of enzymatic activity.
• Osmotic adjustment allows turgor to be maintained as the leaf water potential and water content decreases.
• Turgor pressure is the pressure exerted on the cell wall by the cell contents inside it when the cell contents are fully turgid.
• In drought tolerance, water potential of plant is reduced and its adverse effects are felt.
• Drought tolerance can be defined as tolerance of the plants to a level of stress at which 50 % of cells die.
• Simplest way of mitigating stress is by resisting dehydration and by maintenance of higher osmotic pressure by accumulating higher amounts of solutes.
• Leaves with thick cuticle resist cell collapse.
• Plastic strain indicates irrevocable loss of plant tissues due to moisture stress.
• Crops with C4 and CAM type of photosynthesis are preferred in dry regions.
• Under normal conditions, contribution of pre anthesis assimilates to grain is less than 20% in most plants except in rice where it ranges from 20 to 40%.
• Under moisture stress conditions pre anthesis assimilates contribution may be up to 50 to 75%.
• Collecting and storing water for subsequent use is known as water harvesting.
• Water harvesting is a method to induce, collect, store and conserve local surface runoff for agriculture in arid and semiarid regions.
• Catchment area is the part of the land that contributes the rain water.
• Command area is where water is used.
• In arid regions, collecting area or catchment area is substantially in higher proportion compared to command area.
In arid lands, runoff is induced in catchment area.
• In semi arid regions, runoff is not induced in catchment area, only excess rainfall is collected and stored.
• Treating soils with chemicals like sodium salts of silicon, latexes, asphalt and wax fill soil pores or make soil repellent to water.
Read More-
