Jongebloed Udo, Szederkényi Judit, Hartig Katja, Schobert Christian, Komor Ewald
Pflanzenphysiologie, Universität Bayreuth, D-95440 Bayreuth, Germany.
Physiol Plant. 2004 Feb;120(2):338-346. doi: 10.1111/j.0031-9317.2004.0245.x.
The development of castor bean (Ricinus communis L. var. sanguineus) leaves from bud break to abscission was studied to determine whether senescence of phloem precedes or follows chlorophyll degradation in the course of natural ageing of leaves. The castor bean leaf blade took 20 days for full expansion and its average life span was 60 days. From the day of full expansion on it suffered a substantial loss in N, a small loss in C, K and P and a gain in Ca, Mg and S. The content of soluble sugars increased with time, paralleled by a decrease of photosynthetic activity. Starch accumulated shortly before chlorophyll breakdown. The amino acid level in the leaves decreased steadily together with nitrate reductase and glutamine synthetase activity. Reactive oxygen species increased and oxidation-protecting compounds decreased during the life span of the leaves. Shortly after full leaf expansion an increasing number of sieve plates showed strong callose depositions when visualized by aniline blue method. At day 40 only half of the sieve tubes appeared functional. Chlorophyll breakdown followed these processes with a time lag of approximately 10 days. The sieve tube sap of ageing leaves had the same sucrose concentrations as young leaves, whereas amino acid concentrations decreased. High levels of reduced ascorbic acid and glutathione together with increasing levels of glutaredoxin indicated oxidative strain during senescence. We speculate that the gradual increase of reactive oxygen species during ageing together with the import of calcium ions lead to the stimulation of callose synthesis in plasmodesmata and sieve plates with the consequence of inhibition of phloem transport leading to carbohydrate back-up in the leaf blade. The latter may finally induce chlorophyll breakdown and, at the end, leaf abscission at the petiole base. Thus phloem blockage would precede and may be causal for chlorophyll degradation in leaf senescence.
研究了蓖麻（Ricinus communis L. var. sanguineus）叶片从萌芽到脱落的发育过程，以确定在叶片自然衰老过程中韧皮部衰老是否先于或后于叶绿素降解。蓖麻叶片完全展开需要20天，平均寿命为60天。从完全展开之日起，其氮含量大幅下降，碳、钾和磷含量略有下降，而钙、镁和硫含量增加。可溶性糖含量随时间增加，同时光合活性下降。淀粉在叶绿素分解前不久积累。叶片中的氨基酸水平与硝酸还原酶和谷氨酰胺合成酶活性一起稳步下降。在叶片寿命期间，活性氧增加，抗氧化保护化合物减少。叶片完全展开后不久，用苯胺蓝法观察到越来越多的筛板显示出强烈的胼胝质沉积。在第40天，只有一半的筛管看起来有功能。叶绿素分解在这些过程之后出现，时间滞后约10天。衰老叶片的筛管汁液中蔗糖浓度与幼叶相同，而氨基酸浓度下降。高水平的还原型抗坏血酸和谷胱甘肽以及谷氧还蛋白水平的增加表明衰老过程中存在氧化应激。我们推测，衰老过程中活性氧的逐渐增加以及钙离子的导入导致胞间连丝和筛板中胼胝质合成的刺激，结果是韧皮部运输受到抑制，导致叶片中碳水化合物积累。后者最终可能诱导叶绿素分解，最后导致叶柄基部叶片脱落。因此，在叶片衰老过程中，韧皮部堵塞先于叶绿素降解，并且可能是其原因。
