The productivity of a crop depends on genotype x environment interaction. Temperature and light influence various physiological processes including photosynthesis and respiration; and consequently the rate and duration of growth and thus the productivity of wheat. Investigation was made on the photothermal unit requirement of the newly released wheat variety `Satabdhr under different irrigation regimes. Full irrigated wheat required more days for onset, duration and thermal units for expression of various phenophases or for maturity than that of non-irrigated one; but almost similar of the units within the treatments irrigated at 2 or 3 stages. A relatively fixed amount of heat units was needed to proceed from one phenophase to the other or sowing to maturity, irrespective of the sowing date studied. The 'growing-degree days' (GDD) and 'crop heat unit' (CHU) indices were found superior to other units in explaining wheat maturity. Heat use efficiency for production of seed yield and total dry matter production was evaluated. The highest heat use efficiency (2.26 and 6.66 kg/ha/deg.-day for seed yield and dry-matter, respectively) were obtained in well irrigated treatment, and the lowest (1.14 kg/ha/deg.- • day) in full deficit treatment.