Loss of stem hydraulic conductance due to the intrinsic vulnerability to embolism in xylem is a serious burden that occurs not only as result of a severe water deficit but during low or moderate water stress conditions. Cavitation can be reverted by refilling and recent studies suggest that it may happen despite presence of moderate tension in the xylem. This leaves an open question as to what mechanism is responsible for this process. For successful refilling water must be drawn into embolized vessels possibly from living parenchyma cells could supply the water necessary to refill xylem. Water membrane channel proteins (PIP) are likely to be involved in this process, and to test this hypothesis we monitored the expression of 16 PIP genes.
Here, we demonstrate that P. tricocharpa is highly vulnerabile to drought-induced cavitation and that embolism-refilling cycle are diurnally present even at low-level drought stress, showing a small degree of embolism formation during the morning but recovered during afternoon. Further, we show that cavitation is magnified by increasing degree of water stress and refilling requires conditions of reduced stress, like irrigation. In moderate stressed-plants a full recovery of stem water potential and stem conductivity was observed within few hours, despite presence of tension and transpiration, whereas in severe stressed-plants a full recovery did not occur until the next morning, despite stem water potential recovery occurring within a few hours.
Diurnal pattern of the expression PoptrPIP2 and PoptrPIP1 subfamilies, in wood of well-watered plants, did not change. On the contrary, gene expression profiling revealed that, during drought conditions, PIP2, known to be active water channel proteins, had variable and low expression levels. Surprisingly PIP1 genes, a subfamily usually characterized by low or no water channel activity, were very responsive to water stress, suggesting a major role for this subfamily as moderators of refilling. Upon re-watering, most of PIP2 and PIP1 genes returned to a similar expression level to control plants within few hours after re-watering. This response was uniform across all tested genes and suggested that water stress and not the presence of embolism, was the key driver behind changes on aquaporin expression levels.