EFFECTS OF HIGHER TEMPERATURES, PHOTOPERIOD AND SEED VERNALIZATION ON DEVELOPMENT IN 2 SPRING WHEATS

The intention of this work was to examine to what degree development in wheat is modified by increased temperature when other environmental factors are also varied. The genotypes used were an early season wheat, Hartog, and a closely related mid-season line, Late Hartog. Plants were grown throughout in temperature regimes of 25/15-degrees-C 17/7-degrees-C and photoperiods of 9, 11, 13 or 15 h with 9 h of natural summer radiation. Seeds were vernalised for 0, 2 or 4 weeks. Increased temperature extended the thermal time to ear emergence, particularly in Late Hartog, and particularly under short days after plants had been vernalised for 4 weeks. Phyllochron intervals on the main shoot were increased by higher temperature, as were spikelet numbers in Late Hartog under long photoperiod. However, in Hartog and in Late Hartog exposed to short days, spikelet numbers were reduced by increased temperature. Long seed vernalisation significantly reduced leaf and spikelet number in Late Hartog but only at the higher temperature. Fewer spikelets were produced per leaf at short photoperiod. The apparently complex interactions between temperature, photoperiod, genotype, and seed vernalisation on development are explained in terms of their effects on the timing of floral initiation and the related numbers of primordia accumulated on the apex at that stage. We hypothesise that, in the absence of source limitation, delays in floral initiation such as occur through shortening of photoperiod can increase numbers of leaves and spikelets, though at a decreasing rate with increasing delay. When source is limiting, as under higher temperature and unchanged or reduced radiation, long delays result in a greater reduction in the rate of production of organs and their final number. We further propose that much of the change may be associated with the interplay between the plastochron and phyllochron intervals. Published data are examined to show that high temperatures delay ear emergence in other genotypes both in controlled environments and in the field.