APEX SIZE, FLOWERING AND GRAIN-YIELD OF WHEAT AS AFFECTED BY SOWING DATE

Wheat (Triticum aestivum L.) cultivars Hartog (a spring type) and Rosella (winter type) were sown at seven dates in 1987-88 using conventional and minimum tillage in the central-west of N.S.W. Apex or spike sizes were measured on six to eight occasions until terminal-spikelet formation. Some plants at the boot stage were transferred from the field to a glasshouse so that their grain matured in a common thermal environment, to allow estimation of the effect of time of sowing and environment prior to anthesis on yield and yield components. Double-ridge formation occurred when apices reached certain sizes, irrespective of sowing dates and tillage practices. The results support the hypothesis that apex development depends on the shoot apex reaching a certain size irrespective of environment. Apex volume increased exponentially with thermal time (degrees-Cd) and the relative rate of increase in apex size appeared to be less before double-ridge formation than between double-ridge and terminal-spikelet formation. The thermal time required for apex development was reduced in later sowings, presumably because of the influences of photoperiod and vernalization in addition to that of day degrees on speed of development. The timing of developmental events could be predicted for the spring cultivar but not the winter cultivar from simple Australian models using thermal time and photoperiod and for both cultivars using the CERES Wheat model which incorporated a vernalization function in addition to thermal time and photoperiod. The importance of environment prior to anthesis was indicated by grain yield and its components generally decreasing with lateness of sowing, even in plants which were transferred at the boot stage from the field to a glasshouse so that grain grew in a common thermal environment. Moreover, the relationships among yield components (paths to yield, ascertained by path coefficient analysis) varied among sowings and cultivars in plants in which grain growth occurred in a glasshouse. The importance of environment after anthesis was indicated by a single, general relationship (correlation) between grain yield in the field and grain yield from plants which matured in the glasshouse, in which field yield was 0.76 of glasshouse yield irrespective of sowing date.