Expression of Secondary Sexual Dimorphism in the Diurnal Course of Leaf Gas Exchanges Is Modified by the Rhythmic Growth of Ilex paraguariensis Under Monoculture and Agroforestry

Abstract

Dioecious species show a division of labor expressed through the differentiated manifestation of resource acquisition. We hypothesized that the expression of secondary sexual dimorphism (SSD) in the leaf gas exchange of yerba mate would be more intensive in females than in males to permit females the carbon investments necessary to finish the reproductive cycle. This species can present two growth units annually (GU1-fall and GU2-spring) intercalated with two rest periods (R1-summer and R2-winter). The leaf area index (LAI) and the diurnal courses of leaf photosynthesis (Anet), stomatal conductance (gs), leaf transpiration (E), intercellular CO2 concentration (Ci), water use efficiency (WUE), and instantaneous carboxylation efficiency (Anet/Ci) were estimated in female and male plants of yerba mate during four periods of annual rhythmic growth in monoculture (MO) and agroforestry (AFS). Leaf gas exchanges varied over the annual rhythmic growth and were more intensive under MO than under AFS. Anet, Anet/Ci ratios, and WUE were higher in females than in males during the summer (R1) and spring (GU2). Also, gs and E were more intensive in females than males during the summer. Oppositely, higher WUE in males than in females was observed during the fall (GU1) and winter (R2), with males also showing a higher Anet/Ci ratio during the winter and higher E during the spring (GU2). Despite the strong effect of the cultivation system on LAI and leaf gas exchange traits over the diurnal course, SSD expression was rarely modified by the cultivation system, being expressed only in MO for E during the spring (GU2) and WUE during the winter (R2). High WUE in males during the winter would benefit plants during cold and dry periods, improving the balance between carbon acquisition and water loss through transpiration. On the other hand, high Anet during the summer and spring could be considered as a general fitness strategy of female plants to improve photoassimilate supply and support their additional reproduction costs.