Males exhibit competitive advantages over females of Populus deltoides under salinity stress.

Sexual competition among dioecious plants affects sex ratios and the spatial distribution of the sexes in different environments. At present, little is known about sexual dimorphisms induced by different competition patterns under salinity stress. We employed Populus deltoides as a model to investigate sex-related growth as well as physiological and biochemical responses to salinity stress under conditions of intrasexual and intersexual competition. Potted seedlings (two seedlings per pot; two females, two males, or one female and one male) were exposed to two salt levels (0 and 50 mM NaCl) and salinity- and competition-driven differences in growth, assimilation rate, water use, contents of leaf pigments and osmotica, hydrogen peroxide (H2O2), and antioxidant enzyme and nitrate reductase activity were examined. In the absence of salinity, no significant differences in competitive ability between males and females subjected to intrasexual competition were observed, although the growth of females was moderately greater under intersexual competition. The salinity treatment significantly increased the sex differences in competitive ability, especially under intersexual competition. Under salinity stress, males showed decreased height, but displayed greater capacity for osmotic adjustment, enhancement of long-term water-use efficiency and increase in antioxidant enzyme activities. The absolute values of these traits were greater in salt-stressed males than in females under intersexual competition. In addition, salt-stressed males accumulated less Cl- and had lower H2O2 contents than females. These data collectively demonstrate that the competitive advantage of females in non-stressed conditions is lost under salinity. Greater salinity resistance of males growing intermixed with females under salt stress can importantly affect the sex ratio of P. deltoides populations.