**2. Effects of drought on plant growth and development**

Water stress has significant effect on plant cellular processes, growth, development and economic yield. At cellular level, it effects structure of membranes and organelles, hydration and structure of proteins and nucleic acids, pressure differential across the membrane cell wall complex. Water stress is usually measured as leaf water potential since leaves are directly involved with production of assimilates for growth and yield. Osmotic adjustment or Osmoregulation is active accumulation of solutes in the cells during stress period. Different solutes like sugars, fructans and most importantly inorganic ion K+ . Osmotic adjustment is finite and sufficient time is required for solute accumulation.

The development of water stress in the field and its effects on the crop is as follows. Initially, a moderate level of stress develops at which leaf expansion ceases but photosynthesis continues a part of photosynthate is used for osmotic adjustment. Thus plant growth is inhibited by moderate level of stress that reduce cell enlargement, increased root/shoot ratio, leaf area prevention. Osmotic adjustment would occur which will protect cell from extreme desiccation and allow a continued gas exchange. As water stress increases, older leaves senesce to various degrees, reduced leaf area, decline in water use, increased hydraulic resistant, stomata remain open and photosynthesis may continue as long as bulk leaf turgor is maintained in leaves due to osmotic adjustment. The level of water stress in reproductive meristems is lower than that in the transpiring leaves of a plant at any given time. During increased water stress, turgor is lost, stomata close fully, growth ceases, leave roll up, gas exchange drops to zero, carbon is lost by respiration, tissue water continues to decrease slowly, plant enters the pre lethal, nonreproductive stage of survival, leaves temperature increases to lethal levels, and the meristem dies then plant is considered as died. Time scale of plant processes which may influence their drought tolerance includes turnover of some proteins, stomatal movement within minutes of stress. In hours, production of heat shock proteins (hsp) or dehydrins, leaf movement, wilting, osmotic adjustment response to ABA. Cellular hardening, induction of housekeeping genes, floral initiation in 1–2 days of stress.
