Mechanisms underlying compensatory stimulation of leaf photosynthesis following defoliation in Populus tremuloides

Background/Question/Methods

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 (g_s), 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 g_s. 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 (A_sat), g_s, biochemical capacity and concentrations of N and soluble sugars in foliage of both defoliated and nondefoliated saplings for 30 days.

Results/Conclusions

Leaf A_sat and g_s were higher in defoliated than nondefoliated saplings (P < 0.05) during the 6^th to 30^th day after defoliation. Across treatments, a strong positive relationship between A_sat and g_s was also observed during that period (r ≥ 0.84; P < 0.001). The maximum rate of RuBP carboxylation (V_cmax) was greater for defoliated saplings on the 30^th day after defoliation (P < 0.05). Overall, a positive relationship between A_sat and V_cmax (r = 0.89; P < 0.001), as well as a negative relationship between V_cmax 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, A_sat is enhanced by g_s-mediated increases in CO₂ availability; subsequently, A_sat is stimulated by increases in both g_s and V_cmax. The eventual rise in V_cmax may be attributable to an increase in the activity rather than amount of photosynthetic proteins.