THE EFFECT OF HIGH-TEMPERATURE ON KERNEL DIMENSIONS AND THE TYPE AND OCCURRENCE OF KERNEL DAMAGE IN RICE

Rice (Oryza sativa L. cv. Calrose) growing at 27/22-degrees-C was either transferred to day/night temperatures ranging from 24/19 to 39/34-degrees-C 7 days after heading and held at these temperatures until maturity, or transferred to a temperature of 36/31-degrees-C for 8 day periods at regular intervals commencing from heading. Kernel dimensions were measured directly and the types of kernel damage at maturity were characterized by direct viewing under the light microscope of intact and sectioned kernels, or by scanning electron microscopy of the exposed surface of kernels cut transversely with a razor blade. Kernel thickness was reduced most by high temperature treatments commencing 12 days after heading, but other kernel dimensions (length and width) were most sensitive to high temperature earlier in development. Sterility and pathenocarpy were most evident when temperature treatments commenced at heading (approximately 2 days before anthesis) and were greatest at the highest temperature (39/34-degrees-C). Abortive and opaque kernels were most evident when the high temperature commenced 4 days after heading and were also most numerous at the highest temperature tested. From observations on the morphology of the kernels it appears that temperatures above 27/22-degrees-C can interfere with the early stages of cell division and development in the endosperm. Chalky endosperm tissue occurred in several forms depending on both the temperature level and the timing of the temperature treatment. White-core kernels were only evident at a temperature of 27/22-degrees-C. White-back kernels were most numerous at 36/31-degrees-C and when the high temperature treatment commenced 16 days after heading. Milky-white kernels were found in all but the lowest temperature treatment (24/19-degrees-C), with a peak at 36/31-degrees-C and this type of damage was most evident when the high temperature treatment commenced 12 days after heading. Differences in endosperm cellular morphology were observed between the different types of damage, and in accord with other published data the chalky appearance was associated with the development of numerous air spaces between loosely packed starch granules and a change in light refraction.