Salt stress induced sex-related spatial heterogeneity of gas exchange rates over the leaf surface in Populus cathayana Rehd.

The sex-related spatial heterogeneity of gas exchange rates over the leaf surface under salt stress was investigated in the dioecious species, Populus cathayana Rehd. Cuttings were subjected to two salt regimes: 0 and 75 mM NaCl added to the Hoagland solution, the control and the treatment group, respectively. Measurements of gas exchange parameters were taken from over 40 sites on the surfaces of representative 'non-stressed' and 'salttreated' leaves which had the same insertion point for two sexual cuttings. Compared to the control group, the treatment group showed a significant decrease in the mean values of the following: water use efficiency (WUE), Chlorophyll a (Chl a) concentration, chlorophyll b (Chl b) concentration, concentration of carotenoids (Caro), total chlorophyll concentration (TC) in two sexes, and net photosynthesis rate (P-n), stomatal conductance (g(s)), and stomatal length/width ratio (SR) in females. Also, in the treatment group, females exhibited lower WUE, P-n, g(s), E, Chl a, Chl b, TC, and SR than males. Comparison of contour maps showed that the net photosynthesis rate decreased gradually from apical to basal zones over the leaf surface occurred in the two sexes under natural conditions, but under salt stress, the opposite trend was found in females only. The results suggest that the heterogeneity pattern of the gas exchange parameters in response to salt stress between the two sexes is quite different due to different strategies employed by males and females to maintain the photosynthesis rate under salt stress. This heterogeneity phenomenon under salt stress may mainly be attributed to the chlorophyll pigments in males and the stomatal apertures in females.