Temperature of rice spikelets: thermal damage and the concept of a thermalburden

The temperature of an inanimate object can be expressed as the sum of air t emperature and a temperature term called the 'thermal burden'. The thermal burden depends on the size and shape of an object and its environment. The temperature of rice spikelets can be thought of in the same way except that they can offset some of the thermal burden by transpirational cooling. Usi ng simple theory and measurements we derive Values of the thermal burden, t he boundary layer resistance and resistance to water loss of spikelets by c omparing adjacent panicles, one of which had been severed by a stem borer a nd was unable to transpire. Close to midday a spikelet at the top of the ca nopy had a thermal burden of approximately 5 degrees C, the maximum achieva ble thermal burden was calculated to be approximately 20 degrees C. Spikele t resistance to water vapour loss varied from 173 to 851 s m(-1), boundary layer resistance was approximately 17 s m(-1). The fertility of rice spikek ets declines when the daily maximum air temperatures is above 33 degrees C, to zero at 40 degrees C. Because of the interaction between the thermal bu rden and spikelet transpiration, tissue temperature should be used to descr ibe thermal damage rather than air temperature. The objective of the work d escribed was to develop a simple model of thermal exchanges that could be u sed to investigate thermal damage to rice spikelets. (C) Inra/Elsevier, Par is.