Institute for Phytopathology and Applied Zoology, Heinrich-Buff-Ring 26-32, Gießen, Germany.
Plant Cell Environ. 2013 Jan;36(1):237-47. doi: 10.1111/j.1365-3040.2012.02571.x. Epub 2012 Aug 14.
Collection of cucurbit exudates from cut petioles has been a powerful tool for gaining knowledge on phloem sap composition without full notion of the complex exudation mechanism. Only few publications explicitly mentioned that exudates were collected from the basal side of the cut, which exudes more copiously than the apical side. This is surprising since only exudation from the apical side is supposedly driven by phloem pressure gradients. Composition of carbohydrates and pH values at both wounding sides are equal, whereas protein concentration is higher at the basal side. Apparently, exudation is far more complex than just the delivery of phloem sap. Xylem involvement is indicated by lower protein concentrations after elimination of root pressure. Moreover, dye was sucked into xylem vessels owing to relaxation of negative pressure after cutting. The lateral water efflux from the vessels increases turgor of surrounding cells including sieve elements. Simultaneously, detached parietal proteins (PP1/PP2) induce occlusion of sieve plates and cover wound surface. If root pressure is strong enough, pure xylem sap can be collected after removal of the occlusion plug at the wound surface. The present findings provide a mechanism of sap exudation in Cucurbita maxima, in which the contribution of xylem water is integrated.
从切断的叶柄中收集葫芦科渗出物一直是一种强大的工具，可以在不完全了解复杂渗出机制的情况下了解韧皮部汁液的组成。只有少数出版物明确提到渗出物是从切口的基部收集的，基部的渗出量比顶部多。这令人惊讶，因为据称只有顶部的渗出物是由韧皮部压力梯度驱动的。碳水化合物的组成和两侧的 pH 值相等，而蛋白质浓度在基部较高。显然，渗出物远比输送韧皮部汁液复杂。木质部的参与表明，在消除根压后，蛋白质浓度降低。此外，由于切割后负压的松弛，染料被吸入木质部导管。从导管中渗出的侧向水流会增加包括筛管在内的周围细胞的膨压。同时，分离的周质蛋白（PP1/PP2）会引起筛板的闭塞，并覆盖伤口表面。如果根压足够强，在去除伤口表面的闭塞塞后，可以收集到纯木质部汁液。本研究结果提供了一种南瓜中汁液渗出的机制，其中木质部水分的贡献是整合的。
