Increased sink strength offsets the inhibitory effect of sucrose on sugarcane photosynthesis

Spraying sucrose inhibits photosynthesis by impairing Rubisco activity and stomatal conductance (g_s), whereas increasing sink demand by partially darkening the plant stimulates sugarcane photosynthesis. We hypothesized that the stimulatory effect of darkness can offset the inhibitory effect of exogenous sucrose on photosynthesis. Source-sink relationship was perturbed in two sugarcane cultivars by imposing partial darkness, spraying a sucrose solution (50mM) and their combination. Five days after the onset of the treatments, the maximum Rubisco carboxylation rate (V_cmax) and the initial slope of A-C_i curve (k) were estimated by measuring leaf gas exchange and chlorophyll fluorescence. Photosynthesis was inhibited by sucrose spraying in both genotypes, through decreases in V_cmax, k, g_s and ATP production driven by electron transport (J_atp). Photosynthesis of plants subjected to the combination of partial darkness and sucrose spraying was similar to photosynthesis of reference plants for both genotypes. Significant increases in V_cmax, g_s and J_atp and marginal increases in k were noticed when combining partial darkness and sucrose spraying compared with sucrose spraying alone. Our data also revealed that increases in sink strength due to partial darkness offset the inhibition of sugarcane photosynthesis caused by sucrose spraying, enhancing the knowledge on endogenous regulation of sugarcane photosynthesis through the source-sink relationship.