Drought stress causes changes in vein and stomatal density. The objectives of this study were to determine (1) if the changes in vein and stomatal density are coordinated in cotton (Gossypium hirsutum L.) and (2) how these changes affect water-use efficiency (WUE). The results showed significant positive correlations between vein density and stomatal density when cotton was grown under different degrees of drought stress. WUE was significantly positively correlated with the densities of both veins and stomata. Stomatal pore area and stomatal density on the abaxial leaf side, but not the adaxial side, were significantly correlated with WUE, stomatal conductance, leaf net photosynthetic rate, and transpiration rate. In conclusion, coordinated changes in vein and stomatal density improve the WUE of cotton under drought stress. The abaxial leaf side plays a more important role than the adaxial side in WUE and gas exchange.

Mesophyll conductance (g_m) is a crucial leaf trait contributing to the photosynthetic rate (A_N). Plant domestication typically leads to an enhancement of A_N that is often associated with profound anatomical modifications, but it is unclear which of these structural alterations influence g_m. We analyzed the implication of domestication on leaf anatomy and its effect on g_m in 26 wild and 31 domesticated cotton genotypes (Gossypium sp.) grown under field conditions. We found that domesticated genotypes had higher A_N but similar g_m to wild genotypes. Consistent with this, domestication did not translate into significant differences in the fraction of mesophyll occupied by intercellular air spaces (f_ias) or mesophyll and chloroplast surface area exposed to intercellular air space (S_m/S and S_c/S, respectively). However, leaves of domesticated genotypes were significantly thicker, with larger but fewer mesophyll cells with thinner cell walls. Moreover, domesticated genotypes had higher cell wall conductance (g_cw) but smaller cytoplasmic conductance (g_cyt) than wild genotypes. It appears that domestication in cotton has not generally led to significant improvement in g_m, in part because their thinner mesophyll cell walls (increasing g_cw) compensate for their lower g_cyt, itself due to larger distance between plasmalemma and chloroplast envelopes.

The induction and relaxation of photochemistry and non-photochemical quenching (NPQ) are not instantaneous and require time to respond to fluctuating environments. There is a lack of integrated understanding on how photochemistry and NPQ influence photosynthesis in fluctuating environments. We measured the induction and relaxation of chlorophyll a fluorescence and gas exchange in poplar and cotton at varying temperatures under saturating and fluctuating lights. When the light shifted from dark to high, the fraction of open reaction centers in photosystem II (q_L) gradually increased while NPQ increased suddenly and then remained stable. Temperature significantly changed the response of q_L but not that of NPQ during the dark to high light transition. Increased q_L led to higher photosynthesis but their precise relationship was affected by NPQ and temperature. q_L was significantly related to biochemical capacity. Thus, q_L appears to be a strong indicator of the activation of carboxylase, leading to the similar dynamics between q_L and photosynthesis. When the light shifted from high to low intensity, NPQ is still engaged at a high level, causing a stronger decline in photosynthesis. Our finding suggests that the dynamic effects of photochemistry and NPQ on photosynthesis depend on the phases of environmental fluctuations and interactive effects of light and temperature. Across the full spectra of light fluctuation, the slow induction of q_L is a more important limiting factor than the slow relaxation of NPQ for photosynthesis in typical ranges of temperature for photosynthesis. The findings provided a new perspective to improve photosynthetic productivity with molecular biology under natural fluctuating environments.