Plant Physiology and Fruit Chemistry Division, NARO Institute of Fruit Tree Science, Tsukuba, Ibaraki 305-8605, Japan.
Tree Physiol. 2013 Apr;33(4):398-408. doi: 10.1093/treephys/tpt021. Epub 2013 Apr 5.
In order to elucidate which physiological event(s) are involved in the seasonal changes of carbohydrate dynamics during winter, we examined the effects of different low temperatures on the carbohydrate concentrations of Japanese pear (Pyrus pyrifolia (Burm.) Nakai). For four winter seasons, large increases in the sorbitol concentration of shoot xylem sap occurred during mid- to late December, possibly due to the endodormancy completion and low-temperature responses. When trees were kept at 15 °C from 3 November to 3 December in order to postpone the initiation and completion of chilling accumulation that would break endodormancy, sorbitol accumulation in xylem sap was always higher from trees with sufficient chilling accumulation than from trees that received insufficient chilling. However, an additional increase in xylem sap sorbitol occurred around late December in trees regardless of whether their chilling accumulation naturally progressed or was postponed. To examine different temperature effects more closely, we compared the carbohydrate concentrations of trees subjected to either 6 or 0 °C treatment. The sorbitol concentration in xylem sap tremendously increased at 0 °C treatment compared with 6 °C treatment. However, an additional increase in xylem sap sorbitol occurred at both the temperatures when sufficient chilling accumulated with a peak coinciding with the peak expression in shoots of the sorbitol transporter gene (PpSOT2). Interestingly, the total carbohydrate concentration of shoots tremendously increased with exposure to 0 °C compared with exposure to 6 °C, but was not affected by the amount of accumulated chilling. Instead, as chilling accumulated the ratio of sorbitol to total soluble sugars in shoots increased. We presumed that carbohydrates in the shoot tissues may be converted to sorbitol and loaded into the xylem sap so that the sorbitol accumulation patterns were synchronized with the progression of dormancy, whereas the total carbohydrate transported into shoots from other storage organs may be related to freezing tolerance acquisition independent of dormancy progression. We thus propose that there are different effects of dormancy progression and low-temperature responses on carbohydrate dynamics in Japanese pear.
为了阐明冬季碳水化合物动态季节性变化涉及到哪些生理事件，我们研究了不同低温对日本梨（Pyrus pyrifolia (Burm.) Nakai）木质部汁液中碳水化合物浓度的影响。在四个冬季，12 月中旬至下旬，木质部汁液中的山梨糖醇浓度大幅增加，这可能是由于休眠完成和低温响应。当树木从 11 月 3 日至 12 月 3 日保持在 15°C 以推迟打破休眠的冷积累的开始和完成时，具有足够冷积累的树木的木质部汁液中的山梨糖醇积累始终高于接受不足冷积累的树木。然而，无论冷积累是否自然进行或推迟，12 月下旬，树木中的木质部汁液山梨糖醇都会额外增加。为了更仔细地研究不同的温度效应，我们比较了分别接受 6°C 或 0°C 处理的树木的碳水化合物浓度。与 6°C 处理相比，木质部汁液中山梨糖醇的浓度在 0°C 处理下大大增加。然而，当有足够的冷积累时，木质部汁液中山梨糖醇会额外增加，峰值与叶片中山梨糖醇转运蛋白基因（PpSOT2）的峰值表达一致。有趣的是，与暴露在 6°C 相比，暴露在 0°C 下，芽的总碳水化合物浓度会大大增加，但不受积累冷量的影响。相反，随着冷积累的增加，芽中山梨糖醇与总可溶糖的比例增加。我们推测，芽组织中的碳水化合物可能被转化为山梨糖醇并加载到木质部汁液中，从而使山梨糖醇的积累模式与休眠的进展同步，而从其他储存器官运输到芽中的总碳水化合物可能与独立于休眠进展的抗冻性获得有关。因此，我们提出休眠进展和低温响应对日本梨碳水化合物动态有不同的影响。
