Defoliation-induced compensatory transpiration is compromised in SUT4-RNAi Populus

Leaf sucrose contents are high in species of the genus Populus and other temperate tree taxa. Sucrose is subcellularly compartmentalized, but assumptions about the physiological relevance of such partitioning remain largely unexplored. In this study, the effects of partial defoliation treatments on water uptake, leaf gas exchange properties, non-structural carbohydrate abundance in source and sink organs, and growth were compared in poplars with normal or impaired intracellular sucrose trafficking. The tonoplast sucrose transporter PtaSUT4 is well expressed in leaves of P. tremula x P. alba (INRA 717-IB4), and its inhibition by RNA-interference (RNAi) is known to affect leaf sucrose abundance. After partial defoliation, maximum photosynthesis rates increased while intercellular CO2 decreased with trajectories that were similar in wild type and SUT4-RNAi lines. Leaf transpiration increased more robustly in wild-type than RNAi plants, while leaf water content increased more in RNAi lines. Stomatal conductance did not differ between genotypes, nor did it increase with defoliation. Post-defoliation reductions in steady-state levels of sucrose, the major hexoses (glucose and fructose) and starch were similar in wild-type and SUT4-RNAi shoot sinks. Water uptake and stem growth after partial defoliation were not as well sustained in RNAi as in wild-type plants. The data suggest that vacuolar efflux of sucrose by PtaSUT4 more clearly facilitated adjustments in water uptake than sucrose export following leaf removal.