THE RESPONSE OF WHEAT TO HIGH-TEMPERATURE FOLLOWING ANTHESIS .2. SUCROSE ACCUMULATION AND METABOLISM BY ISOLATED KERNELS

The effect of temperature on [C-14]sucrose accumulation and the incorporation of C-14-label into starch by isolated kernels was followed in a range of wheat (Triticum aestivum L.) cultivars. To facilitate uptake the outer pericarp and embryo were removed from each kernel which was then split along the crease region to expose the starchy endosperm directly to a 4% [C-14]sucrose solution. There was an initial high rate of C-14-accumulation by the kernel followed by a slower linear increase in activity from 6 to 24 h after the start of the uptake period. The rate of accumulation of C-14 by the endosperm of individual kernels from plants growing at 15/10 degrees C day/night was greatest 30-42 days after anthesis, with a considerable fall in both the accumulation of [C-14]sucrose and the incorporation of label into starch 50 days after anthesis. Differences between cultivars in the accumulation of [C-14]sucrose and starch synthesis by the starchy endosperm of isolated kernels followed differences in the growth rate of kernels on the plant, although the absolute rates of sucrose accumulation and incorporation into starch were only about 65% of those observed in the intact plant system. Differences between cultivars in their response to high temperature were, however, not evident in the [C-14]sucrose uptake studies. The optimum temperature for kernel respiration, in the short term, was greater than 30 degrees C. However, above 25 degrees C the Q(10) for kernel respiration was below 2, which is an indication that physical factors may influence cell metabolism at this temperature. We suggest that greater consideration should be given to the effect of temperature on sucrose transfer from the vascular system in the crease region of the kernel, through the endosperm cavity to the starchy endosperm.