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Yield of Pima cotton (Gossypium barbadense L.) has tripled over the last 40 years with the development of new cultivars. Six genetic lines representing successive stages in the breeding process (one primitive noncultivated accession, four cultivars with release dates from 1949 to 1983, and one unreleased breeding line) were grown in a greenhouse, and their gas exchange properties were compared. Among the cultivated types, genetic advances were closely associated with increasing single-leaf photosynthetic rate (A) and stomatal conductance (g.), especially in the morning. The A and g, of the primitive line approached those of the cultivated types early in the morning, but were much lower for the rest of the day. In both morning and afternoon, A was correlated with g, across genotypes but was not correlated with leaf thickness, concentrations of chlorophyll or starch, or intercellular CO2 concentration (c). In the oldest cultivar, the relationship of A to ci did not change between morning and afternoon. In the two most recent lines, the slopes of the A:ci curves at limiting cl exceeded that of the oldest cultivar by 25 to 50% in the morning, but the differences were much smaller in the afternoon. The maximum A of the newer lines at high ci exceeded that of the oldest cultivar only in the moming. Breeding for increasing yield has enhanced the photosynthetic capacity and stomatal conductance of Pima cotton and altered the diurnal regulation of photosynthesis.Leaf gas exchange is a complex, highly regulated process dependent upon interactions between mesophyll cells and stomata (9). Cowan and Farquhar (6) Wong et al. (24) found that stomata tend to maintain c, at the optimum level for WUE, often 60 to 70% of ambient CO2 concentration.The hypothesis of optimized gas exchange is based upon the initial assumption that water availability limits reproductive success, thus assuring strong selection pressure for maintenance of maximum WUE. However, this assumption may not generally apply to agricultural production. Many crop plants are the result of years of breeding in excellent soils, in some cases with supplemental irrigation to relieve water deficits. Under these conditions water is unlikely to limit productivity, and selection pressure to maximize WUE may be slight. Indeed, in upland cotton (Gossypium hirsutum L.) in a hot environment, stomatal behavior does not maximize WUE unless the crop becomes water-stressed (5,14,16,19). Rather, midday gs is so high that it provides a relatively small limitation to gaseous diffusion, and ci exceeds 80% ofambient CO2 concentration. Under these conditions, the stomatal limitation to A is minimized, and leaves are cooled considerably by evaporation of large amounts of water. This behavior has been interpreted in terms ofheat resistance (15). Radin (16)

Advanced lines of Pima cotton (Gossypium barbadense L.) bred for higher yield potential and heat resistance have higher stomata conductance and smaller leaf areas than those of obsolete lines. In controlled experiments, five commercial lines of Pima cotton having increasing lint yield and heat resistance showed a gradient of increasing stomatal conductance and decreasing leaf size. In field experiments, heat‐sensitive, low yield Pima lines showed a lower stomatal conductance than high yielding, advanced lines. This indicates that selection for high yield potential and heat resistance has imposed a selection pressure for higher stomatal conductance and smaller leaf areas. The higher stomatal conductance and smaller leaf area in the advanced lines resulted in a lower leaf temperature in both controlled environments and in the field. The largest leaf temperature differences between obsolete and advanced lines were observed in the afternoon. These differences coincided with the largest differences in stomatal conductance and the highest air temperatures. Measurements of stomatal conductance and leaf temperature in field‐grown progeny from a cross between the advanced line, Pima S‐6. and the obsolete line, Pima 32, showed that genetically determined differences in stomatal conductance resulted in corresponding differences in leaf temperature. None of the altered physiological traits were selected for in the breeding program, indicating that selection for the desired agronomic traits imposed selection pressures on the altered physiological traits. The increases in stomatal conductance and decreases in leaf area could represent an integrated response to selection pressures on enhanced evaporative cooling, ensuing from selection for heat resistance.

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