Department of Plant Sciences, University of California, Davis One Shields Avenue, Davis, CA, 95616, USA.
Agricultural Research Organization (ARO), Gilat Center, Petah Tikva, Israel.
Planta. 2017 Sep;246(3):495-508. doi: 10.1007/s00425-017-2707-7. Epub 2017 May 9.
During spring, bud growth relies on long-distance transport of remotely stored carbohydrates. A new hypothesis suggests this transport is achieved by the interplay of xylem and phloem. During the spring, carbohydrate demand of developing buds often exceeds locally available storage, thus requiring the translocation of sugars from distant locations like limbs, stems and roots. Both the phloem and xylem have the capacity for such long-distance transport, but their functional contribution is unclear. To address this ambiguity, the spatial and temporal dynamics of carbohydrate availability in extension shoots of Juglans regia L. were analyzed. A significant loss of extension shoot carbohydrates in remote locations was observed while carbohydrate availability near the buds remained unaffected. This pattern of depletion of carbohydrate reserves supports the notion of long-distance translocation. Girdling and dye perfusion experiments were performed to assess the role of phloem and xylem in the transport of carbohydrate and water towards the buds. Girdling caused a decrease in non-structural carbohydrate concentration above the point of girdling and an unexpected concurrent increase in water content associated with impeded xylem transport. Based on experimental observations and modeling, we propose a novel mechanism for maintenance of spring carbohydrate translocation in trees where xylem transports carbohydrates and this transport is maintained with the recirculation of water by phloem Münch flow. Phloem Münch flow acts as a pump for generating water flux in xylem and allows for transport and mobilization of sugars from distal locations prior to leaves photosynthetic independence and in the absence of transpiration.
在春季，芽的生长依赖于远程储存的碳水化合物的长距离运输。一个新的假设表明，这种运输是通过木质部和韧皮部的相互作用实现的。在春季，发育芽的碳水化合物需求常常超过当地的可用储存量，因此需要从远处的部位（如枝条、茎和根）中转移糖分。韧皮部和木质部都有进行这种长距离运输的能力，但它们的功能贡献尚不清楚。为了解决这个模糊性，分析了 Juglans regia L. 伸长枝中碳水化合物的空间和时间动态。在远程部位观察到伸长枝碳水化合物的显著损失，而靠近芽的碳水化合物可用性不受影响。这种碳水化合物储备耗尽的模式支持长距离转运的概念。环割和染料灌注实验用于评估韧皮部和木质部在向芽运输碳水化合物和水方面的作用。环割导致环割点以上的非结构性碳水化合物浓度降低，同时与木质部运输受阻相关的水分含量意外增加。基于实验观察和建模，我们提出了一个新的机制，用于维持树木春季碳水化合物转运，其中木质部运输碳水化合物，并且这种运输通过韧皮部 Münch 流的再循环来维持。韧皮部 Münch 流作为木质部中水流产生的泵，允许在叶片光合作用独立和蒸腾作用不存在的情况下，从远距离转移和动员糖分。
