Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, 02138, USA.
Center for Nanoscale Systems, Harvard University, Cambridge, MA, 02138, USA.
New Phytol. 2021 Oct;232(1):418-424. doi: 10.1111/nph.17465. Epub 2021 Jul 22.
A robust understanding of phloem functioning in tall trees evades us because current methods for collecting phloem sap do not lend themselves to measuring actively photosynthesizing canopy leaves. We show that Raman spectroscopy can be used as a quantitative tool to assess sucrose concentration in leaf samples. Specifically, we found that Raman spectroscopy can predict physiologically relevant sucrose concentrations (adjusted R of 0.9) in frozen leaf extract spiked with sucrose. We then apply this method to estimate sieve element sucrose concentration in rapidly frozen petioles of canopy red oak (Quercus rubra) trees and found that sucrose concentrations are > 1100 mM at midday and midnight. This concentration is predicted to generate a sieve element turgor pressure high enough to generate bulk flow through the phloem, but is potentially too high to allow for sucrose diffusion from photosynthetic cells. Our findings support the Münch hypothesis for phloem transport once the carbon is in the phloem and challenge the passive-loading hypothesis for carbon movement into the phloem for red oak. This study provides the first ˜in-situ (frozen in the functioning state) source sieve element sucrose concentration characterization in any plant, opening a new avenue for investigation of phloem functioning.
我们对高大树木韧皮部功能的理解还很不完善，因为目前收集韧皮部汁液的方法并不适用于测量活跃进行光合作用的树冠叶片。我们表明，拉曼光谱可以用作一种定量工具来评估叶片样本中的蔗糖浓度。具体来说，我们发现拉曼光谱可以预测冷冻叶片提取物中添加蔗糖的生理相关蔗糖浓度（调整后的 R² 为 0.9）。然后，我们将此方法应用于估算树冠红橡树（Quercus rubra）叶柄中筛管蔗糖浓度，发现中午和午夜时蔗糖浓度>1100mM。该浓度预计会产生足以通过韧皮部产生整体流动的筛管膨压，但可能过高而无法允许蔗糖从光合细胞扩散。我们的发现支持了韧皮部运输的 Münch 假说，一旦碳进入韧皮部，就挑战了红橡树碳进入韧皮部的被动加载假说。本研究首次对任何植物的功能状态下的源筛管蔗糖浓度进行了˜原位（在功能状态下冷冻）特征描述，为研究韧皮部功能开辟了新途径。
