Rate of Leaf Appearance and Final Number of Leaves in Wheat: Effects of Duration and Rate of Change of Photoperiod

This study was conducted to test the hypothesis that photoperiodor its rate of change significantly affects the rate of leafappearance (RLA) and final number of leaves (FNL) in wheat,as suggested from several time-of-sowing experiments. Two wheatcultivars (Condor and Thatcher) were sown in the field on 2Sep. 1992 at Melbourne (38°S). Photoperiod was extendedartificially to give five treatments up to terminal spikeletinitiation (TS) viz.: natural photoperiod (rate of change ofphotoperiod = 2 min d^-1), two faster rates of change (8·5and 13·3 min d^-1) and two constant photoperiods of 14·0and 15·5 h. After TS, the two constant photoperiods wereextended to 15·0 and 16·5 h, respectively, andtreatments were re-randomised, i.e. some plots received differentphotoperiod regimes before and after TS. The rate of leaf appearance maintained strong linear relationshipswith thermal time. It was greater for Condor [0·012-0·013(°C d)^-1] than for Thatcher [0·011-0·012 (°Cd)^-1] and did not alter during plant development or in responseto the change in photoperiod at TS. Rate of leaf appearanceon the main culm was not influenced by the rate of change ofphotoperiod nor by the average photoperiod. Cultivar and photoperiod significantly affected FNL on the mainculm. Condor produced more leaves than Thatcher under long butnot under short photoperiods. The rate of change of photoperioddid not affect FNL independently of the effect of average photoperiod.Most of the variation in FNL due to photoperiod resulted fromdifferences in duration of leaf initiation. The lack of effects of the photoperiod treatments on RLA contrastwith previous reports of its effects on the rate of phasic developmentfrom seedling emergence to double ridge. Therefore, the numberof visible leaves on the main culm (NL) at double ridge andat TS were not constant. However, NL on the main culm at doubleridge was closely correlated with FNL.Copyright 1994, 1999 AcademicPress Triticum aestivum L., wheat, leaf appearance, phyllochron, photoperiod