DIURNAL PATTERNS OF WHEAT CANOPY PHOTOSYNTHESIS, EVAPOTRANSPIRATION AND WATER-USE EFFICIENCY AT DIFFERENT PHASES OF GROWTH IN THE FIELD

Diurnal patterns of canopy net photosynthetic rates (P(N)), evapotrans piration (E) and water use efficiency (WUE = P(N)/E) of field grown ir rigated wheat (Triticum aestivum L. cv. WH 147) were studied from till ering to maturity at Haryana Agricultural University Farm, Hisar, Indi a. The maximum photosynthetic rates (P(N, max)) increased from tilleri ng to anthesis and declined at a rapid rate thereafter till maturity. Relatively high E(max) but low WUE(max) were recorded at tillering, mi lk ripe and dough stages of growth. Diurnally P(N) followed the course of photosynthetically active radiation (PAR) from tillering to watery ripeness but it declined at high PAR at latter reproductive stages of growth. E was curvilinearly related to air temperature. There was a l ag of about 2 h between P(N, max) (12.00 h) and E(max) (14.00 h) durin g the day. WUE was high from 07.30 to 10.00 h, depressed during midday and a slightly increased from 15.00 to 16.00 h.