High Photon Irradiance Research Articles

Factors Controlling Endosperm Cell Number and Grain Dry Weight in Wheat: Effects of Shading on Intact Plants and of Variation in Nutritional Supply to Detached, Cultured Ears

The association between endosperm cell number and grain dry weight, and the dependence of endosperm cell division on the availability of organic nutrients, have been investigated in wheat. Two different procedures were used to vary the supply of nutrients to the grains during the phase of cell division. Detached ears were cultured in solutions of sucrose (0-60 g 1-1) and glutamine (0.125-0.75 g N 1-1), or intact plants were exposed to high (560 �mol m-2 s-1) or low (55 �mol m-2 s-1) photon irradiance. Cell number per endosperm, and grain dry weight, were both responsive to the concentration of nutrients in the external medium, and to the level of photon exposure. Average dry weight per cell was relatively independent of the level of nutrition or of photon exposure until cell division had ceased but, in the later stages of grain-filling, dry weight per cell in the cultured ears displayed a dependence upon the concentration of nutrients in the external medium. Amounts of sucrose, other soluble carbohydrates and soluble amino nitrogen were extracted from the grains and, on a per grain basis, the amounts of all fractions varied in response to variation in the level of nutrients supplied to the ears, and to photon exposure. However, concentrations of these nutrients in the developing grains, calculated on a dry weight or water basis, were not associated with the rate of cell division in the grains. While the evidence gathered supports the notion that growth (in cell number, and dry weight) of the developing endosperm is controlled inter alia by the provision of organic nutrients, the nature of the controlling mechanism is obscure. It seems that cellular division is not affected directly by nutritional supply through a mechanism involving the concentration of substrates for energy and protein synthesis within the developing grain.

An equivalent circuit model for microwave-biased extrinsic photoconductors∗

An equivalent circuit model for microwave-biased extrinsic photoconductors is introduced. Theoretical performance results calculated from the model are compared to experimental values of a microwave-biased mercury-doped germanium photoconductor operating at high background photon irradiance levels. The comparison indicates that the equivalent circuit model is adequate to describe the performance of microwave-biased extrinsic photoconductors. Experimental and theoretical results indicate that microwave-biased extrinsic photoconductors are high gain (G ∼10 4) devices with a simultaneous submicrosecond speed of response.