Canopy Closure in Field-Grown Gossypium Hirsutum Decreases Differences in Photosynthetic Thermotolerance During Leaf Expansion

Previous research on tree species has shown that at mid-day, younger leaves are often warmer, because of limited transpirational cooling, and more photosynthetically thermotolerant than mature leaves. This difference in heat tolerance between leaf stages, however, diminishes as the growing season progresses. We test the hypotheses that in open canopies with higher vapor pressure deficit (VPD), if stomatal conductance is depressed and leaf temperatures rise, then young leaves will be more thermotolerant and proportionately greater contributors to overall photosynthesis. Alternatively, as the canopy closes and VPD decreases, differences between the leaf stages will decrease and the proportional impact of young leaves on total photosynthesis will decline. To that end, we studied upper canopy cotton (Gossypium hirsutum) leaf stages, in greenhouse environments and in the field. We found that under high VPD conditions (early in the growing season or in the greenhouse) young leaves at midday had significantly higher abaxial surface temperatures and lower stomatal conductance than mature leaves (e.g., young leaves in the open canopy were often 15°C warmer with 46% lower stomatal conductance than mature leaves). These young leaves also had higher photosynthetic thermal optima than mature leaves (e.g., young leaves had an optima of 30-40°C compared with 25-35°C for more mature leaves in the open canopy). After canopy closure occurred and VPD decreased, leaf temperature, stomatal conductance, and photosynthetic thermotolerance were similar amongst the leaf age classes. When the relative photosynthetic activity of the leaf stages was measured (using chlorophyll a fluorescence), younger leaves had a greater impact during open canopy measurements, than when measured after canopy closure had occurred (~25 compared with ~18%, respectively). We conclude that these differences may be a result of an increased sensitivity to ambient vapor pressure deficit in young leaves, possibly because of an under-developed internal cuticle.