Stomatal and nonstomatal limitations of photosynthesis in field grown cotton and sorghum



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Texas Tech University


The nature of the relationship between photosynthesis and stomatal conductance of 00« was investigated in field-grown cotton (Gossypium hirsutum L.) and sorghum (Sorghum bicolor L. Moench) across growth stages and a range of soil water conditions. The purpose of the study was to determine the degree to which the stomata represent a direct limitation of the photosynthetic process under field environmental conditions. Potential genetic variation in photosynthetic rates, leaf conductances, and photosynthesis:leaf conductance ratios was investigated in 14 strains of cotton and in 20 lines and 9 hybrids of sorghum under both irrigated and dryland conditions. The results of the research provide insight into the potential for genetically increasing C0« fixation without concurrently increasing water loss. Diurnal and seasonal observations made across numerous strains of cotton indicated that gross photosynthetic rates were related to stomatal conductances of upper canopy leaves in a curvilinear rather than a strictly linear manner. Increases in leaf conductance of CO^ in excess of 0.8 to 1.2 cm s" did not result in consistent increases in gross photosynthetic rates. Increases in external C0« concentration to levels above ambient did not produce consistent increases in photosynthetic rates even though intercellular COp availability increased. These results indicate that factors in addition to stomatal control of CO^ diffusion are involved in determining photosynthetic rates of cotton. A more nearly linear photosynthesis: conductance relationship was observed in sorghum, but observed photosynthesis and conductance levels were poorly correlated on a long term basis. Data collected across the entire season and including all lines and hybrids resulted in low coefficients of determination (r) for the photosynthesis: conductance relationship, generally less than 0.3. A portion of the variability in the longterm relationship between photosynthesis and conductance was associated with the influence of growth stage and moderate to severe water stress on the photosynthesis:conductance relationship. Pooling across all genotypes, photosynthesis:conductance ratios were generally lower during grain filling than prior to flowering, and also declined with moderate to severe water stress. In both crop species, measurements were made throughout both vegetative and reproductive growth stages to evaluate the consistency of genetic ranking in each parameter. On all sampling dates, significant variation existed among strains within each species for gross photosynthesis, leaf conductance, and photosynthesis per unit leaf conductance. Variation in these parameters was evident among cotton strains, but conductance levels were more variable than photosynthetic rates, In contrast, considerable genetic variability was evident in both photosynthesis and conductance among sorghum lines and hybrids. Particularly in some strains of sorghum, the relative genetic ranking of photosynthesis and leaf conductance was variable both with irrigation treatment and growth stage. However, individual entries were identified which exhibited consistently high photosynthetic rates and photosynthesis: conductance ratios over a considerable portion of the season. The large genetic variability in photosynthesis, conductance, and. photosynthesis:conductance ratios identified a viable approach toward genetic improvement of photosynthetic productivity and water use efficiency in cotton and sorghum.