Contrasting leaf structural, photosynthetic and allocation responses to elevated [CO2] in different-aged leaves of tropical fruit trees Persea americana and Annona muricata

Responses of leaf photosynthetic traits to elevated growth [CO₂] vary among species, but there is limited understanding of underlying trait trade-offs, especially for tropical species with continuous leaf formation. Persea americana and Annona muricata with significant investments in defense structures (idioblasts) were used to study the impacts of growth [CO₂] (400 vs 800 μmol mol^-1) on leaf structural, chemical, and photosynthetic characteristics at different leaf developmental stages. Growth at elevated [CO₂] increased whole plant leaf area (S_T) and whole plant average leaf dry mass per unit area (M_Av) in P. americana, whereas both S_T and M_Av were reduced in A. muricata. Elevated [CO₂] moderately reduced foliage N and P contents per dry mass in P. americana but increased in A. muricata. In P. americana, elevated [CO₂] increased anthocyanin content in young leaves and decreased in mature leaves, and increased the share of idioblast tissue fraction (f_I) with moderate downregulation of photosynthesis (A). In A. muricata, elevated [CO₂] reduced anthocyanin content in young leaves and f_I was unaffected, whereas a major downregulation in A was observed. In this species, photosynthetic downregulation was not associated with nutrient starvation, but occurred due to direct inhibition of stomatal conductance by elevated [CO₂], ultimately limiting leaf development and growth and curbing V_cmax and J_max in mature leaves. These results demonstrate a limited impact of primary/secondary metabolism trade-off on photosynthetic response to growth [CO₂], underscore major species differences in response to elevated [CO₂] and emphasize the impact of leaf age in determining whole plant growth response.