| Summary: | Sexually dimorphic response to stress has been observed in assorted natural dioecious plants. Up to now, few studies have focused on the difference of stress responses between artificially bred siblings. To determine the sexual dimorphism between artificially bred sibling poplar trees, we conducted a study comparing the response to water deficit between male and female <i>Populus</i> × <i>euramericana</i> siblings. This pair of hybrids was analyzed in terms of growth, photosynthesis, membrane injury and repair systems, as well as gene regulation patterns. The female and male siblings presented distinct responses to water deficit, with greater inhibition in females’ growth and photosynthesis. The results also displayed that in females, relative electrolyte leakage and malonaldehyde content were higher than those in males under water deficit conditions. On the other hand, water deficit caused a greater increase in both SOD activity and POD activity in males than those in females. Consistent with these physiological differences, the expression of several stress-related genes, including <i>SOD</i>, <i>GST</i>, <i>bHLH35</i>, and <i>PsbX1</i>, was regulated differently between female and male hybrids by water deficit stress. Higher expression of <i>SOD</i> in moderate-water-deficit-treated females and higher <i>GST</i>, <i>bHLH35</i> expression in both moderate- and severe-water-deficit-treated females suggest that the female sib is more sensitive, whilst higher expression of <i>SOD</i> in severe-water-deficit-treated males and higher <i>PsbX1</i> expression in water-deficit-treated males testify that males protect cells better. To achieve an integrated view, all these variables were analyzed through the use of a principal component analysis and a total discrepancy between the sexes in their response to water deficit was demonstrated. The results indicate that, compared with male poplar sibs, females are more sensitive, but deploy a weaker protective apparatus to deal with water deficit.
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