Hoffmann-Thoma G, van Bel A J, Ehlers K
Institut für Allgemeine Botanik und Pflanzenphysiologie, Justus-Liebig-Universität Giessen, Germany.
Planta. 2001 Jan;212(2):231-42. doi: 10.1007/s004250000382.
Minor-vein ultrastructure and sugar export were studied in mature summer and winter leaves of the three broadleaf-evergreen species Ajuga reptans var. artropurpurescens L., Aucuba japonica Thunb. and Hedera helix L. to assess temperature effects on phloem loading. Leaves of the perennial herb Ajuga exported substantial amounts of assimilates in form of raffinose-family oligosaccharides (RFOs). Its minor-vein companion cells represent typical intermediary cells (ICs), with numerous small vacuoles and abundant plasmodesmal connectivity to the bundle sheath. The woody plants Hedera and Aucuba translocated sucrose as the dominant sugar species, and only traces of RFOs. Their minor-vein phloem possessed a layer of highly vacuolated cells (VCs) intervening between mesophyll and sieve elements. Depending on their location and ontogeny, VCs were classified either as companion or parenchyma cells. Both cell types showed symplasmic continuity to the adjacent mesophyll tissue although at a lower plasmodesmal frequency compared to the Ajuga ICs. p-Chloromercuribenzenesulfonic acid did not reduce leaf sugar export in any of the plants, indicating a symplasmic mode of phloem loading. Winter leaves did not show symptoms of frost injury, and the vacuolar pattern in ICs and VCs was equally prominent in both seasons. Starch accumulation as a result of reduced phloem loading was not observed to be triggered by low temperature. In contrast, high amounts of starch were found in mesophyll and bundle-sheath cells of summer leaves. Physiological data on season-dependent leaf exudation showed the maintenance of sugar export in cold-acclimated winter leaves.
对三种阔叶常绿植物紫背金盘（Ajuga reptans var. artropurpurescens L.）、桃叶珊瑚（Aucuba japonica Thunb.）和常春藤（Hedera helix L.）成熟的夏季和冬季叶片的小叶脉超微结构及糖分输出进行了研究，以评估温度对韧皮部装载的影响。多年生草本植物紫背金盘的叶片以棉子糖家族寡糖（RFOs）的形式输出大量同化物。其小叶脉伴胞代表典型的中间细胞（ICs），有许多小液泡，与维管束鞘有丰富的胞间连丝连接。木本植物常春藤和桃叶珊瑚以蔗糖作为主要糖类进行转运，仅含有微量的RFOs。它们的小叶脉韧皮部在叶肉和筛管分子之间有一层高度液泡化的细胞（VCs）。根据其位置和个体发育，VCs可分为伴胞或薄壁细胞。这两种细胞类型与相邻叶肉组织均表现出共质体连续性，不过与紫背金盘的ICs相比，胞间连丝频率较低。对氯汞苯磺酸并未降低任何一种植物的叶片糖分输出，表明韧皮部装载为共质体模式。冬季叶片未表现出冻害症状，ICs和VCs中的液泡模式在两个季节同样显著。未观察到低温引发因韧皮部装载减少导致的淀粉积累。相反，在夏季叶片的叶肉和维管束鞘细胞中发现了大量淀粉。关于季节性叶片渗出的生理数据表明，冷驯化的冬季叶片能够维持糖分输出。
