DOES TEMPERATURE AFFECT FINAL NUMBERS OF PRIMORDIA IN WHEAT

Photoperiod and vernalisation both affect the number of leaf and spike let primordia and, hence, the duration of the early phases of developm ent in wheat. Temperature also affects duration of developmental phase s but it is not clear whether temperature-related changes in duration are similarly associated with final numbers of primordia. The objectiv e of this study was to examine these temperature-related associations. A phytotron study was conducted in naturally-lit cabinets with four c ultivars (Sunset, Condor, Rosella and Cappelle Desprez) and six temper ature regimes between 10 and 25-degrees-C. Seedlings were vernalised f or 50 days and then grown under a photoperiod of 18 h to avoid confoun ding the responses to vernalisation and photoperiod with those to temp erature. The duration of the period to terminal spikelet initiation wa s reduced by increases in temperature from 10 to 19-degrees-C, but ext ended at temperatures above 19-degrees-C. Cultivars differed in the ti me taken to the terminal spikelet stage. However, final numbers of lea f and spikelet primordia were affected only marginally by temperature. Final leaf number increased slightly with temperature, but this trend was only significant in Cappelle Desprez, and in that case accounted for only 1 leaf over the 15-degrees-C range. There was a curvilinear a ssociation between final spikelet number and temperature, with an opti mum at about 19-degrees-C. However, temperature changed spikelet numbe r by a maximum of only 3 per ear. Although significant correlation was only found for leaf number, there was a general trend for a reduction in number of primordia with longer duration to terminal spikelet init iation. However, these trends were all very small. This suggests that the effects of temperature on the rate of apex development towards ter minal spikelet initiation and on the rate of primordia initiation are similar. Cultivar differences in duration of primordia initiation were translated directly to differences in number of primordia initiated. The relative impact of temperature and other factors on the rates of p rimordia initiation and apex development towards terminal spikelet ini tiation were assessed using a simple model.