Physiological applications for determining water use efficiency among cotton genotypes



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Drought stress can substantially alter plant metabolism by decreasing plant growth and photosynthesis. The lack of rapid and reliable screening criteria and measurement techniques for determining water use efficiency (WUE) of crop plants has greatly restricted progress in this critical area of crop improvement. In grain sorghum (Sorghum bicolor L.), WUE was associated with the transpiration ratio [CO2 assimilation (A) / transpiration rate (E), A:E] from leaf gas exchange measurements. Research is needed to identify drought effects on plant productivity and to exploit the use of this knowledge in breeding and agronomic efforts. Therefore, the objectives of this study were to determine if differences in A:E and other physiological parameters existed between two selected cotton (Gossypium hirsutum L.) genotypes and to evaluate the response of cotton genotypes experiencing water stress at two different growth stages on biomass production and yield. Two experiments were conducted using two cotton genotypes differing in drought tolerance. Each experiment was repeated three times in a randomized complete block design with six replications. In Experiment I, the water stress treatment was induced by withholding water when the plants reached the 4-node growth stage. The water stress treatment in Experiment II was imposed at early bloom. Gas exchange and chlorophyll fluorescence measurements were collected during dry-down and recovery periods to determine water stress effects on plant physiology. Biomass was partitioned following the recovery period, to examine phenotypic responses of plants exposed to water stress. The results of these experiments indicate that A:E is significantly increased as leaf water potential (?L) decreases with no differences in A:E between the two genotypes. Gas exchange measurements showed significant decreases with declining ?L and significant increases upon re-watering; yet, no differences were observed between the two genotypes. Chlorophyll fluorescence was not different between genotypes in either light- or dark-adapted leaves. In Experiment I TAM 89E-51 had a significantly greater seedcotton yield; however, in Experiment II TAMCOT 22 had the greater yield. These experiments suggest that the effects of water stress on cotton are a function of the intensity of the stress and the growth stage in which the stress is experienced.