RATE OF LEAF APPEARANCE AND FINAL NUMBER OF LEAVES IN WHEAT - EFFECTSOF DURATION AND RATE OF CHANGE OF PHOTOPERIOD

This study was conducted to test the hypothesis that photoperiod or it s rate of change significantly affects the rate of leaf appearance (RL A) and final number of leaves (FNL) in wheat, as suggested from severa l time-of-sowing experiments. Two wheat cultivars (Condor and Thatcher ) were sown in the field on 2 Sep. 1992 at Melbourne (38 degrees S). P hotoperiod was extended artificially to give five treatments up to ter minal spikelet initiation (TS) viz.: natural photoperiod (rate of chan ge of photoperiod = 2 min d(-1)), two faster rates of change (8.5 and 13.3 min d(-1)) and two constant photoperiods of 14.0 and 15.5 h. Afte r TS, the two constant photoperiods were extended to 15.0 and 16.5 h, respectively, and treatments were re-randomised, i.e. some plots recei ved different photoperiod regimes before and after TS. The rate of lea f appearance maintained strong linear relationships with thermal time. It was greater for Condor [0.012-0.013 (degrees C d)(-1)] than for Th atcher [0.011-0.012 (degrees C d)-1] and did not alter during plant de velopment or in response to the change in photoperiod al TS. Rate of l eaf appearance on the main culm was not influenced by the rate of chan ge of photoperiod nor by the average photoperiod. Cultivar and photope riod significantly affected FNL on the main culm. Condor produced more leaves than Thatcher under long but not under short photoperiods. The rate of change of photoperiod did not affect FNL independently of the effect of average photoperiod. Most of the variation in FNL due to ph otoperiod resulted from differences in duration of leaf initiation. Th e lack of effects of the photoperiod treatments on RLA contrast with p revious reports of its effects on the rate of phasic development from seedling emergence to double ridge. Therefore, the number of visible l eaves on the main culm (NL) at double ridge and at TS were not constan t. However, NL on the main culm at double ridge was closely correlated with FNL.