Many plant species exhibit compensatory growth following defoliation. This response is often attributable in part to a defoliation-induced increase in photosynthesis. It has been suggested that the stimulation of photosynthesis following defoliation may relate to: 1) enhanced CO2 availability, resulting from increased stomatal conductance (gs), or 2) improved biochemical capacity, associated with increased leaf nitrogen (N) levels and/or release from end-product inhibition. We contend that both of these mechanisms likely play important, temporally specific roles, with a defoliation-induced rise in biochemical capacity following that in gs. To test this hypothesis, we studied the potential mechanisms underlying compensatory photosynthesis in defoliated aspen (Populus tremuloides Michx) at different times following defoliation. We removed 50% of total leaf area of three-year-old potted aspen saplings, and monitored light-saturated net photosynthesis (Asat), gs, biochemical capacity and concentrations of N and soluble sugars in foliage of both defoliated and nondefoliated saplings for 30 days.
Results/Conclusions
Leaf Asat and gs were higher in defoliated than nondefoliated saplings (P < 0.05) during the 6th to 30th day after defoliation. Across treatments, a strong positive relationship between Asat and gs was also observed during that period (r ≥ 0.84; P < 0.001). The maximum rate of RuBP carboxylation (Vcmax) was greater for defoliated saplings on the 30th day after defoliation (P < 0.05). Overall, a positive relationship between Asat and Vcmax (r = 0.89; P < 0.001), as well as a negative relationship between Vcmax and soluble sugars (r = -0.60; P = 0.05), was observed at the same time. These results help to clarify the temporally specific roles of multiple mechanisms underlying leaf photosynthetic responses to defoliation: early after defoliation, Asat is enhanced by gs-mediated increases in CO2 availability; subsequently, Asat is stimulated by increases in both gs and Vcmax. The eventual rise in Vcmax may be attributable to an increase in the activity rather than amount of photosynthetic proteins.