Understanding the effect of temperature and pH on xylem sugar concentration in pistachio trees

Background/Question/Methods The variation of sugar concentration in the xylem vessels is the result of the balance of two opposing movements, the efflux of sucrose from vessel associated cells into vessels (from symplast to apoplast) and the influx of sugars through the re-absorption into the symplast. It has been showed that temperature and pH influence the carbohydrate status of xylem apoplast. The change in global climate suggests an increase in temperate areas of diurnal temperature variation with strong implications for trees biology. In fact, it is well-known that temperature has a significant influence on starch synthesis and degradation and transporters’ activity, which in turn regulate the symplast to apoplast interactions. Despite several authors have been investigated the efflux/influx balance, various questions remain still open. Thus, we studied the effect of temperature and pH both on the xylem sap sugar concentration in pistachio trees (Pistacia vera L.). Stems, located inside a chamber to maintain the temperature constant, were perfused by using a peristaltic pump for 24 hours with two solutions (4.5 and 6.5 pH) and applying three temperatures (5, 15 and 30°C). Samples were collected at specific time to measure the sugar concentration and the pH variation all over the cycle.

Results/Conclusions As predicted, the temperature had a significant effect on xylem sugar concentration. At lowest temperature and low pH we found the highest sugar concentration, while using the high pH solution the highest concentrations were observed at low and intermediate temperatures both. As expected, at high temperature the sugar concentration was very low and no differences were found between the two pH. This strong decrease has been related to lower efflux resulting from a lower sucrose concentration gradient between symplast and apoplast, combined with higher influx from vessels into symplast. Regarding the pH variations, when low pH solution was used we found at the end of the cycle an increment of pH at all temperatures, while using the high pH no changes were observed. These findings are in line with a previous work suggesting the key role of the membrane proton pump activity in controlling the apoplastic pH by driving H+ ions into the sap. In the context of climate change, as temperature become more extreme in temperate areas, a more complete understanding of sugar kinetics in xylem will increase the prediction of physiological performance of trees.