Gas exchange and water use efficiency of grain sorghum
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Grain sorghum [Sorghum bicolor (L.) Moench] production is usually confined to environments which are considered too dry and hot for other cereals to be productive. Increasing yield and water use efficiency (WUE) of grain sorghum through identification and utilization of superior germplasm in breeding programs is a major goal in the improvement of this crop. Breeding for increased WUE has been limited by the lack of screening criteria and methods that could be used to select desirable genotypes from large populations under field conditions. This study was conducted to determine whether genotypic differences in WUE and gas exchange traits are present and to determine whether the variation in gas exchange rates and efficiencies reflected the genotypic differences in WUE and thereby, could be used as a screening criteria for identifying genotypes of grain sorghum with higher WUE. Five sorghum genotypes (TX 378, TX 430, SC 35, TX 399, and TX 2741) which are parental lines used in hybrid seed production were tested in the greenhouse and field during 1989 under well watered conditions. In the greenhouse study, individual entries were planted in plastic pots containing 10 kg of sterilized potting mix. The field experiment was conducted on an Amarillo loamy fine sand. In both studies, shoot biomass production, amount of water used, and leaf area development were monitored during die growing season. Water use efficiency was calculated as shoot biomass production per unit water use. Gas exchange measurements were made throughout the vegetative stage on uppermost fully expanded leaves and whole canopy using a portable photosynthesis system. Gas exchange efficiency was expressed as the ratio of photosynthetic rate (A) to transpiration rate (T). The sorghum genotypes exhibited significant variation for A, A/T, shoot biomass production, and WUE. TX 378, TX 430, and SC 35 had higher A, A/T, shoot biomass production, and WUE than TX 399 and TX 2741. No consistent genotypic variation was observed for T or whole plant water use rates. Single leaf measurements of gas exchange traits (A and A/T) reflected single plant and whole canopy WUE differences among the genotypes largely due to the positive correlation between A and shoot biomass production. In addition, there was a positive correlation among the genotypes between leaf area and A. The results indicate that measurements of A and leaf area may be used to select for increased WUE in grain sorghum.