Schröder Roswitha, Clark Christopher J, Sharrock Keith, Hallett Ian C, MacRae Elspeth A
The Horticulture and Food Research Institute of New Zealand, Mt. Albert Research Centre, Private Bag 92169, Auckland, New Zealand.
J Plant Physiol. 2004 Apr;161(4):371-9. doi: 10.1078/0176-1617-01275.
The white part of citrus peel, the albedo, has a special role in water relations of both fruit and leaves from early on in fruit development. In times of drought, this tissue acts as a water reservoir for juice sacs, seeds and leaves. When water was injected into the albedo, free water was undetectable using magnetic resonance imaging. Microscopy showed tightly packed cells with little intercellular space, and thick cell walls. Cell wall material comprised 21% of the fresh albedo weight, and contained 26.1% galacturonic acid, the main constituent of pectin. From this, we postulated that pectin of the cell wall was responsible for the high water-binding capacity of the immature lemon albedo. Cell wall material was extracted using mild procedures that keep polymers intact, and four pectic fractions were recovered. Of these fractions, the SDS and chelator-soluble fractions showed viscosities ten and twenty times higher than laboratory-grade citrus pectin or the other albedo-derived pectins. The yield of these two pectins represented 28% of the cell walls and 62% of the galacturonic acid content of immature lemon albedo. We concluded that, from viscosity and abundance, these types of pectin account for the high water-binding capacity of this tissue. Compositional analyses showed that the two highly viscous pectic fractions differ in galacturonic acid content, degree of branching and length of side chains from the less viscous albedo-derived pectins. The most striking feature of these highly viscous pectins, however, was their high molecular weight distribution compared to the other pectic fractions.
柑橘果皮的白色部分，即内果皮，在果实发育早期对果实和叶片的水分关系起着特殊作用。在干旱时期，该组织充当汁囊、种子和叶片的蓄水池。向内果皮注入水后，使用磁共振成像无法检测到自由水。显微镜观察显示细胞紧密排列，细胞间隙小，细胞壁厚。细胞壁物质占新鲜内果皮重量的21%，含有26.1%的半乳糖醛酸，这是果胶的主要成分。由此，我们推测细胞壁中的果胶是未成熟柠檬内果皮高持水能力的原因。采用温和的方法提取细胞壁物质以保持聚合物完整，并回收了四种果胶组分。在这些组分中，十二烷基硫酸钠（SDS）和螯合剂可溶组分的粘度分别比实验室级柑橘果胶或其他源自内果皮的果胶高10倍和20倍。这两种果胶的产量占未成熟柠檬内果皮细胞壁的28%，半乳糖醛酸含量的62%。我们得出结论，从粘度和含量来看，这些类型的果胶是该组织高持水能力的原因。成分分析表明，与粘性较低的源自内果皮的果胶相比，这两种高粘性果胶组分在半乳糖醛酸含量、分支度和侧链长度方面存在差异。然而，这些高粘性果胶最显著的特征是与其他果胶组分相比，它们具有高分子量分布。
