Severe water stress constrains, or even stops, water transport in the xylem due to embolism formation. Previously, the xylem of poplar trees was shown to respond to embolism formation by accumulating carbohydrates in the apoplast and dropping xylem sap pH. Whether these two processes are functionally linked, however, remains unanswered.
A novel in vivo method to measure xylem apoplast pH during the onset of water stress and a tool allowing for in vitro determination of the temporal kinetics of sugar efflux in response to apoplastic pH variations were developed.
Results/Conclusions
Water stress resulted in pH decrease from ~6.2 in transpiring stems to ~5.6 in severely stressed trees. Moreover, recovery of pre-stress pH is delayed by 24 hours after the recovery to non-stress water potential levels. In a lower pH environment we show in vitro that the xylem apoplast continuously accumulates carbohydrates with glucose and fructose being dominant sugars, while in a higher pH environment sugar accumulation was only temporary with sucrose as the primary component. Apoplastic carbohydrate accumulation reduced significantly after we applied a proton pump blocker (orthovanadate). These observed patterns of sugar accumulation in xylem sap suggest: (1) a significant role of acidic invertases in maintaining a sucrose efflux gradient from stressed cells and (2) the role of proton pumps in maintenance of lower xylem pH during water stress and recovery. The delayed recovery of pH suggests that restoration of xylem transport capacity might be necessary for a tree to return to pre-stress conditions.