Sink-Source Transition in Leaves: A Model

随着叶子的发育，它从碳水化合物的净使用者转变为净出口者。这一事件影响着增长和生产力的模式。糖从叶片到韧皮部的转移称为韧皮部运输。对于这一过程，公认的模型是，蔗糖通过选择性载体跨细胞膜转移。提出了另一种机制，即蔗糖通过胞间连丝从一个细胞传递到另一个细胞到韧皮部，胞间连丝是提供细胞间细胞质连续性的微小原生质连线。在韧皮部中，较小的蔗糖分子被转化为较大的糖分子，不能通过胞间连丝返回。因此，糖被困在韧皮部细胞中。将研究胞间连丝的形态和尺寸，以及将蔗糖转化为更大糖的酶。

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.//