Department of Crop Science, Agricultural University of Athens, Athens, Greece.
Department of Crop Science, Agricultural University of Athens, Athens, Greece.
J Plant Physiol. 2018 Dec;231:337-345. doi: 10.1016/j.jplph.2018.10.012. Epub 2018 Oct 17.
Boron (B) toxicity frequently affects plant performances and productivity, especially in arid and semi-arid environments. In this experiment, loquat seedlings were subjected to 25 μM (control) or 400 μM B (B excess) to test the hypothesis that (i) B alters sugar/polyol metabolism in polyol-producing tree species as loquat and (ii) changes of leaf and stem anatomy assist young tissues against toxic effect of B. Gas exchange was monitored from the beginning of the experiment (FBE) till one week after the first visible symptoms of B toxicity appeared in the upper part of the stems (147 d FBE). At 147 FBE, plant biometric parameters and pattern of B accumulation, leaf and stem anatomy, chlorophyll a fluorescence kinetics as well as biochemical measurements were assessed in top (asymptomatic) leaves and upper stem bark. Boron accumulated principally (in the row) in top leaves > top bark > top wood in B-stressed plants, but no changes in allocation pattern were found between controls and B-stressed plants. Excess B promoted the increase in the spongy layer of top leaves and caused the development of cork and numerous collenchyma cells with increased cell wall thickness. This mechanism, which has never been described before, can be considered an attempt to store excessive B in tissues where B ions are less harmful. The accumulation of sorbitol (B-complexing polyol) in top leaves and stem bark can be considered as a further attempt to detoxify B excess. However, B toxicity drastically affects the photosynthetic rate of top leaves, mainly due to non-stomatal limitations, i.e., reduction of ambient CO use efficiency and of photosystem II (PSII) efficiency, modification of the partitioning excess energy dissipation in PSII, thus leading to an increased level of lipid peroxidation. Our results suggest that changes in sugar metabolism associated with leaf and stem bark thickening partially assist (but not totally preserve) young tissues of loquat plants under B stress.
硼（B）毒性经常影响植物的表现和生产力，特别是在干旱和半干旱环境中。在这项实验中，枇杷幼苗分别接受 25μM（对照）或 400μM B（B 过剩）处理，以检验以下假设：（i）B 改变了枇杷等产多元醇树种的糖/多元醇代谢；（ii）叶片和茎解剖结构的变化有助于幼嫩组织抵抗 B 的毒性作用。从实验开始（FBE）一直监测气体交换，直到 B 毒性在茎上部首次出现可见症状后的一周（147 d FBE）。在 147 d FBE 时，评估了植物生物计量参数和 B 积累模式、叶片和茎解剖结构、叶绿素 a 荧光动力学以及生化测量值，评估对象为无症状（顶端）叶片和上部茎皮。在 B 胁迫植物中，B 主要（顺序）积累在顶叶＞顶皮＞顶木中，但在对照和 B 胁迫植物之间未发现分配模式的变化。过量的 B 促进了顶叶海绵层的增加，并导致软木和大量厚壁细胞形成的韧皮细胞的发育。这种机制以前从未被描述过，可被视为将过量的 B 储存在 B 离子危害较小的组织中的尝试。在顶叶和茎皮中积累山梨糖醇（B 复合多元醇）可被视为进一步解毒 B 过剩的尝试。然而，B 毒性严重影响顶叶的光合速率，主要是由于非气孔限制，即环境 CO2利用效率和 PSII 效率降低，PSII 过量能量耗散的分配发生变化，从而导致脂质过氧化水平升高。我们的结果表明，与叶片和茎皮变厚相关的糖代谢变化部分有助于（但不完全保护）枇杷幼嫩组织在 B 胁迫下生存。
