The Effect of Changing Sowing Date on Leaf Structure and Gas Exchange Characteristics of Wheat Flag Leaves Grown under Mediterranean Climate Conditions

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Abstract
Comparisons of leaf structure and gas exchange characteristics have been made between flag leaves of four old genotypes of cultivated tetraploid wheats and three current varieties of hexaploid Triticum aestivum grown under Mediterranean climate conditions. For some genotypes the effect of varying the sowing date was investigated. In the hexaploid wheat Kolibri the effect of sowing date on leaf structure and gas exchange of the penultimate leaf was also studied. Flag leaves differed significantly in photosynthetic capacity and leaf structure characteristics between genotypes, within each ploidy level. When the mean values for each ploidy level were considered, there were no significant differences in values for photosynthesis per unit leaf area, stomatal conductance, intercellular CO2 concentration, residual CO2 conductance and water-use efficiency between the tetraploid and hexaploid wheats sown on the same date. When comparisons were made of leaf structure the only significant differences observed were in adaxial and abaxial stomatal frequencies and leaf area: mean values of these parameters were higher in tetraploid than in hexaploid wheats. The changes in leaf structure in response to varying sowing date were significant and followed the same pattern in all the genotypes studied: a xeromorphic adaptation was observed in later sowings in response to warmer climate. Such structural changes affected some gas exchange characteristics. For example, the area of flag leaves decreased by 60% in the hexaploid wheat Kolibri from first to last sowing, which led to a transpirative loss of 49% per single leaf, in spite of the fact that transpiration rate per unit leaf area increased by 26%. Penultimate leaves of cv. Kolibri followed a fairly similar adaptive pattern in later sowings as compared with flag leaves. The adaptive significance of changes in leaf structure and gas exchange characteristics with varying sowing date under Mediterranean climate conditions is discussed.