Gossypium barbadense L. (Pima) and
Gossypium hirsutum L. (upland) cottons are the two major
fibre producing species grown in the south-western United States, where lint
yields are adversely affected by high temperatures. In these environments,
heat-adapted upland cultivars show higher yields and heat resistance than
advanced Pima cultivars. Recent studies with an historical series of
commercial Pima cultivars have shown that increases in lint yield and heat
resistance are tightly coupled to increases in stomatal conductance and
photosynthetic rate, and to decreases in leaf area. In the present study, Pima
S-6 and Pima S-7 (advanced Pima cultivars) and Deltapine 90 (advanced upland
cultivar) were compared under field and laboratory conditions to determine
whether the physiological and morphological gradients found in the Pima
historical series extrapolate to upland cotton. In the field, Deltapine 90
showed 25–35% higher stomatal conductance, 35–50%
higher photosynthetic rate and 45% smaller leaf area than Pima S-6. The
higher photosynthetic rate and stomatal conductance of Deltapine 90 leaves
were partially related to their sun-tracking ability. In gas exchange
experiments that prevented sun-tracking, the two cultivars had comparable
photosynthetic rate as a function of incident radiation, while stomatal
conductance was higher in upland cotton. In the 25–35°C range,
photosynthetic rate as a function of temperature remained nearly constant in
both cultivars, and was higher in upland cotton at all temperatures. Stomatal
conductance showed a strong temperature-dependence, and conductance value and
the slope of the stomatal response to temperature were higher in Deltapine 90.
In progeny from a cross between Deltapine 90 and Pima S-7, the segregation of
stomatal conductance in F1 and F2
populations showed a clear genetic component. These results indicate that the
differences in photosynthetic rate, stomatal conductance and leaf area
associated with increases in lint yield and heat resistance in the Pima
historical series are also evident in a comparison between advanced cultivars
of upland and Pima cotton. Upland cotton could be used as a source of genetic
variation for high stomatal conductance in Pima breeding programs.
Enhanced Photosynthesis and Stomatal Conductance of Pima Cotton (Gossypium barbadense L.) Bred for Increased Yield