Photosynthetic Response of an Alpine Plant, Rhododendron delavayi Franch, to Water Stress and Recovery: The Role of Mesophyll Conductance

Rhododendron delavayi Franch is an evergreen shrub or small tree with large scarlet flowers that makes it highly attractive as an ornamental species. The species is native to southwest China and southeast Asia, especially the Himalayan region, showing good adaptability, and tolerance to drought. To understand the water stress coping mechanisms of R. delavayi, we analyzed the plant's photosynthetic performance during water stress and recovery. In particular, we looked at the regulation of stomatal (g(s)) and mesophyll conductance (g(m)), and maximum rate of carboxylation (V(cmax)). After 4 days of water stress treatment, the net CO(2) assimilation rate (A(N)) declined slightly while g(s) and g(m) were not affected and stomatal limitation (S(L)) was therefore negligible. At this stage mesophyll conductance limitation (MC(L)) and biochemical limitation (B(L)) constituted the main limitation factors. After 8 days of water stress treatment, A(N), g(s), and g(m) had decreased notably. At this stage S(L) increased markedly and MC(L) even more so, while B(L) remained relatively constant. After re-watering, the recovery of A(N), g(s), and g(m) was rapid, although remaining below the levels of the control plants, while V(cmax) fully regained control levels after 3 days of re-watering. MC(L) remained the main limitation factor irrespective of the degree of photosynthetic recovery. In conclusion, in our experiment MC(L) was the main photosynthetic limitation factor of R. delavayi under water stress and during the recovery phase, with the regulation of g(m) probably being the result of interactions between the environment and leaf anatomical features.