Water requirement for physiological functions

Transpiration

Evapotranspiration from a rice crop canopy is a function of the size of the crop (leaf area), water availability and the environmental conditions. Evapotranspiration is low at young stages and achieve maximum towards heading. Hence the frequency of irrigation should increase and adjusted accordingly (Please see section on total water requirement for lowland rice).

Rice plant uses less than 5% of the total water absorbed through roots while the rest is lost through transpiration. This helps to maintain leaf energy balance.

Removal of water through transpiration reduces leaf water potential, which results stomatal closure. This in turn

• • decreases transpiration
  • improves water use efficiency under water stress
  • decreases carbon assimilation (reducing CO₂entry via stomata).
  • increase leaf temperature, affecting rates of biochemical processes
  • decreases translocation of assimilates due to reduction is source and sink water potentials.

Root structure

• • There is a marked genotypic variation in rooting pattern in rice in response to water stress, where drought resistant varieties possess deep and thick roots.
  • Thick roots positively correlate with xylem vessel area which actively support water transport from roots to upper parts and meet evaporative demand. Water stress also reduces the nutrient uptake as most of the elements are absorbed through passive diffusion.
  • Direct seeded rice is more tolerant to water stress than transplanted rice which could be due to its superior root system.
  • Increases nitrogen fertilizer increases the susceptibility to water stress.

Drought resistance

• • The ability of a plant to grow satisfactorily when exposed to periods of water stress is called drought resistance.
  • Mechanism of drought resistance in rice could be
    1. tolerance – (Ability of plants to withstand water stress, degree of tolerance vary among varieties and growth stages e.g. Osmorogulation in certain varieties)
    2. avoidance- (possess mechanisms to maintain favourable water status, either by conserving water or by their ability to supply water to above ground organs even during drought stress. E.g. increased root depth in upland rice, adjusting life cycle to evade drought)
    3. escape (e.g. leaf rolling, reduced leaf area, stomatal closure and delayed flowering under water stress)
    4. recovery (Ability to recover after water stress varies with the variety, the severity of stress and growth stage)