Photosynthetic Differences among Triticum Accessions at Tillering1

Flag leaves of many diploid and tetraploid Tritlcum species are known to have higher photosynthesis rates than flag leaves of cultivated hexaploid wheat (Triticum aestivumL.). Yet these differences are reportedly absent or less pronounced on earlier developed leaves. Our objective was to study the relationship between photosynthesis, stomatal conductance, and water‐use efficiency (WUE) in various Triticumacessions at the tillering stage. Net CO2assimilation per leaf area (A) and associated gas exchange characteristics were measured on newly expanded leaves of 6‐week‐old growth chamber‐grown plants of four wild diploids, six wild tetraploids, and two cultivated hexaploids. Significant variation in A was observed with values ranging from 31.1 μmol CO2m−2s−1in the diploid T. speltoldesto 20.7 in one accession of T. dicoccoides; A for the two hexaploid wheats averaged 22.6. This A range for leaves of plants at tillering is similar to that reported by others measuring A of tlag leaves. Because higher A was associated with higher stomatai conductance (r= 0.86**, significant at the 0.01 probability level), which led to higher transpiration, there was no significant correlation between A and WUE. There was no significant correlation between A and internal CO2concentration (C1), so differences in A among accessions were not explained by differences in CO2supply to the mesophyll. Two T. dicoccoidesaccessions found to differ in A and conductance at tillering also had different flag leaf A and conductance. Comparison of A vs. Ciresponse curves between these two accessions showed that the higher A was related to a higher capacity for mesophyll photosynthesis rather than stomatal conductance per se. The results show that the variation in A among accessions was to a significant extent attributable to nonstomatal factors and that differences in A among Triticumaccessions are detectable at early vegetative growth stages.