Glaucousness in Wheat: Its Development and Effect on Water-use Efficiency, Gas Exchange and Photosynthetic Tissue Temperatures*

Abstract

Glaucousness, which is the visual manifestation of epicuticular wax, has previously been found to increase the yield of grain and dry matter of droughted wheat. This study was designed, first, to monitor the development of epicuticular wax in a pair of durum (Triticum turgidum L. var. durum) wheats isogenic for glaucousness and, second, to determine the likely reasons for increased yields of glaucous durum and common (T. aestivum L.) wheats over their non-glaucous isogenic counterparts. Glaucousness first appears on the leaf sheath at the time of stem elongation. It rapidly reaches maximum expression, particularly on the flag leaf sheath and the abaxial surface of the flag leaf lamina, as well as on the emerging head. In glasshouse experiments using isogenic pairs of both common and durum wheats, the water-use efficiency measured between sowing and maturity in droughted treatments was on average 9% higher in the glaucous lines. Glaucous leaves were retained longer than non-glaucous leaves in the droughted treatment but not in the irrigated control. In droughted field-grown plants, temperatures of photosynthetic tissues were up to 0.7"C cooler in glaucous than non-glaucous lines, depending on the time of day. Similarly, in well-watered glasshouse plants, glaucous plant parts were on average 0.3"C cooler than non-glaucous plants. Gas-exchange studies of the durum lines in both irrigated and droughted treatments in a glasshouse indicated that ears of non-glaucous plants had higher rates of photosynthesis and day- and night-time transpiration, a higher stomata1 conductance and a greater rate of increase in photosynthesis with increasing light levels. However, the ratio of photosynthesis to transpiration was higher in the ears of glaucous plants. No differences were found for flag leaves when the adaxial surface was exposed to light. This study further documents that glaucousness can be a yield positive character under water-limited conditions, primarily through its effect on wateruse efficiency, on an extended period of transpiration and on the timing of the deposition of wax.