Clifford S C, Arndt S K, Popp M, Jones H G
Horticulture Research International, Wellesbourne, Warwick CV35 9EF, UK.
J Exp Bot. 2002 Jan;53(366):131-8.
The drought-tolerant tree species Ziziphus mauritiana Lamk. and Z. rotundifolia Lamk. were shown to have similar high mucilage concentrations (7-10% dry weight) in their leaves, with large numbers of mucilage-containing cells in the upper epidermis and extracellular mucilage-containing cavities in the leaf veins and stem cortex. The main sugar constituents of the water-soluble mucilage extract were rhamnose, glucose and galactose. During drought-stress in two independent studies, foliar mucilage content was unaffected in both species, but glucose and starch contents declined significantly in crude mucilage extracts from droughted leaves. Enzymatic hydrolysis of the mucilage extract using alpha-amylase and amyloglucosidase released glucose, indicating that a mucilage-associated water-soluble glucan, with alpha-1,4- and alpha-1,6-linkages, may exist which was extracted together with the mucilage. From the current data, it is not possible to localize the glucan to determine whether or not it is associated with mucilage-containing cells. Data from pressure-volume analyses of drought-stressed and control leaves showed that, in line with their similar mucilage contents, the relative leaf capacitance isotherm (change in relative water content per unit change in water potential) was similar in both species. During drought-stress, reduced relative capacitance resulted from osmotic adjustment and decreased wall elasticity. Data suggest that in Ziziphus leaves, intracellular mucilages play no part in buffering leaf water status during progressive drought. In Ziziphus species, growing in environments with erratic rainfall, the primary role of foliar mucilage and glucans, rather than as hydraulic capacitors, may be as sources for the remobilization of solutes for osmotic adjustment, thus enabling more effective water uptake and assimilate redistribution into roots and stems prior to defoliation as the drought-stress intensified.
耐旱树种毛叶枣（Ziziphus mauritiana Lamk.）和圆叶枣（Z. rotundifolia Lamk.）的叶片显示出相似的高黏液浓度（干重的7-10%），上表皮有大量含黏液细胞，叶脉和茎皮层有细胞外含黏液腔。水溶性黏液提取物的主要糖类成分是鼠李糖、葡萄糖和半乳糖。在两项独立研究的干旱胁迫期间，两个物种的叶片黏液含量均未受影响，但干旱叶片粗黏液提取物中的葡萄糖和淀粉含量显著下降。使用α-淀粉酶和糖化酶对黏液提取物进行酶水解释放出葡萄糖，表明可能存在一种与黏液一起提取的、具有α-1,4-和α-1,6-键的与黏液相关的水溶性葡聚糖。根据目前的数据，无法确定葡聚糖的位置以确定它是否与含黏液细胞相关。干旱胁迫叶片和对照叶片的压力-体积分析数据表明，与它们相似的黏液含量一致，两个物种的相对叶电容等温线（水势每单位变化时相对含水量的变化）相似。在干旱胁迫期间，相对电容降低是由渗透调节和壁弹性降低导致的。数据表明，在毛叶枣叶片中，细胞内黏液在渐进干旱期间对缓冲叶片水分状况不起作用。在降雨不稳定环境中生长的毛叶枣属物种中，叶片黏液和葡聚糖的主要作用，而非作为水力电容器，可能是作为溶质再转运的来源用于渗透调节，从而在干旱胁迫加剧导致落叶之前，使水分吸收和同化物再分配到根和茎中更有效。
