Investigating critical growth stage of cotton subject to water deficit stress . Fiaz Ahmad*, Shabab Ud Din, Asia Perveen and Mohammad Naveed Afzal

Critical growth stage of cotton crop was investigated by artificially imposing water stress under field conditions. The crop was given water deficit stress for a period of 30 days at squaring (SS) and first boll split (BS) phases by maintaining the leaf water potential (yw) at –2.2+0.2 MPa whereas in control plots yw was maintained at -1.6+ 0.2 MPa by irrigation scheduling. The average irrigation water applied during two years was 2432 m3, 2174 m3, and 2194 m3 in NS, SS, and BS treatments, respectively. The results revealed that the imposed water stress, at either stage of crop, had adverse effects on cotton crop performance. The main stem height decreased by 12% and 7.4%, inter-nodal length by 9.2% and 4.3% in SS and BS, respectively over NS treatment. The decrease in dry biomass production was 4% and 7% in leaf; 21% and 11% in stalk and 20% and 13% in fruit in SS and BS treatments, respectively. Gas exchange characteristics were also adversely affected by imposed water stress showing a decrease of 18% and 28% in stomatal conductance, 16% and 22% in transpiration rate and 24% and 30% in net photosynthetic rate in SS and BS treatments, respectively. The decrease in seed cotton yield was 9.5% and 2.8% in SS and BS treatments, respectively. Chlorophyll (SPAD values), electrolyte leakage and cell injury values increased over non stressed crop. Lint percentage and fibre strength increased whereas fibre length decreased significantly in BS treatment. It was concluded that the squaring phase is more critical to water deficit stress in cotton.