Sex-specific responses of Populus deltoides to defoliation

There has been an increasing interest in understanding the differential effects of sexual dimorphism on plant stress responses. However, there is no clear pattern in the responses of the sexes to defoliation. In this study, the effects of different severity of artificial defoliation on biomass production, total nonstructural carbohydrate (NSC) concentration, and photosynthetic rate (<em>P</em>_N) of male and female <em>Populus deltoides</em> were examined. We used half and full defoliation to observe the differences between the sexes in three harvest dates (1 week, 4 weeks, and 8 weeks after treatments). We hypothesized that female and male <em>P. deltoides</em> compared with an undefoliated control would have compensatory growth in response to defoliation treatments. Results showed that half and full defoliation reduced the growth of both sexes. Following half defoliation, root growth was reduced, especially in males, at T2 (4 weeks after defoliation) and T3 (8 weeks after defoliation), while males showed an increase in height increment under the half defoliation compared with the nondefoliation treatments. By contrast, females were more negatively affected by defoliation than males in terms of biomass after 8 weeks. One week after defoliation, <em>P</em>_N increased significantly in females and males under half defoliation (+30%, +32%, respectively) and full defoliation (+58%, +56%, respectively). However, 8 weeks after defoliation, there was little difference in <em>P</em>_N between defoliated and undefoliated female cuttings. Increases in stomatal conductance (g_s) and leaf nitrogen were observed under fully defoliated female and male cuttings. Moreover, males had less NSC concentrations following half defoliation compared with females. Our results indicate that leaf compensatory growth in male cuttings of <em>P. deltoides</em> was maintained by obtaining greater photosynthetic capacity, higher leaf nitrogen, and lower NSC concentration following half and full defoliation. Our results highlight that females suffered from greater negative effects than did males following half defoliation, but under full defoliation, the differences between both sexes were subtle.