Sink-Source Transition in Leaves: A Model

随着叶片的发育，它从碳水化合物的净使用者转变为净出口国。这一事件影响了增长模式和生产力。糖从叶片向韧皮部的转移称为韧皮部负荷。这一过程的公认模型是蔗糖通过选择性载体在细胞膜上转移。另一种机制被提出，其中蔗糖通过胞间连丝传递到韧皮部，胞间连丝是提供细胞质连续性的微小原生质连接。在韧皮部，较小的蔗糖分子转化为不能通过胞间连丝的较大的糖分子。糖就这样被困在韧皮部细胞中。我们将研究胞间连丝的形态和尺寸，以及将蔗糖转化为大糖的酶。//

As a leaf develops it is converted from a net user of carbohydrate to a net exporter. This event influences patterns of growth and productivity. The transfer of sugars form leaf to phloem is called phloem loading. The accepted model for this process is that surcose is transferred across cell membranes by selective carriers. An alternative mechanism is proposed in which sucrose passes from cell to cell to the phloem through plasmodesmata, tiny protoplasmic connections that provide cytoplasmic continuity between cells. In the phloem, the smaller sucrose molecules are converted to larger sugar molecule that cannot pass back through the plasmodesmata. The sugar is thus trapped in the phloem cells. The morphology and dimensions of plasmodesmata will be studied along with the enzymes involved in converting sucrose into larger sugars.//