Response of leaf stomata and photosynthetic parameters to short-term drought stress in cotton (Gossypium hirsutum L.)

Studying the response of stomata to drought stress can help to understand the behavior of stomatal regulation and improve water utilization efficiency of crops. This study was conducted in an artificial climate chamber using the cotton cultivar 'Guoxin 9'. Four concentrations of PEG 60000 (control), 1.5%, 3%, and 4.5% were added into the nutrient solution. Stomatal morphological parameters, gas exchange parameters, and chlorophyll fluorescence kinetic parameters were measured every two days from the first day to the seventh day of treatment. The results showed that with the elongation of treatment time, the values of leaf water potential, stomatal length, stomatal width, stomatal aperture, net photosynthetic rate (P_n), stomatal conductance (G_s), transpiration rate (T_r), intercellular CO₂ concentration (C_i), maximal photochemical efficiency (F_v/F_m), and actual photochemical production quantum (Yield) all decreased to some extent, whereas stomatal density and non-photochemical quenching coefficient (NPQ) increased. Similar trends were observed in different treatments with an increase in drought stress. After 5 d and 7 d of treatment, leaf water potential, stomatal length, stomatal width, P_n, G_s, T_r, and C_i with 1.5%, 3%, and 4.5% PEG 6000 were significantly different from those with the control (P < 0.05). The magnitudes of the declines in leaf stomatal length, G_s, and T_r were 1.17%-2.61%, 61.62%-69.09%, and 37.62%-67.48%, respectively. Correlation analysis revealed that significant positive correlations existed among leaf stomatal length, width, and aperture (P < 0.01). Stomatal width was not significantly correlated with stomatal density. Stomatal length, width, and aperture were positively correlated with P_n and Yield (P < 0.01), negatively correlated with NPQ (P < 0.01), and not significantly correlated with G_s, T_r, or F_v/F_m. These results suggested that the simulated drought induced by PEG 6000 may result in cotton stomatal closure and an increase in stomatal density, along with decreases in net photosynthetic rate and stomatal conductance. Compared with Yield and NPQ, F_v/F_m was less sensitive to drought stress in cotton.