Kerton Matt, Newbury H John, Hand David, Pritchard Jeremy
School of Biosciences, University of Birmingham, Birmingham, West Midlands B15 2TT, UK.
J Exp Bot. 2009;60(1):227-35. doi: 10.1093/jxb/ern279. Epub 2008 Nov 13.
The aim of this study is to understand the parameters regulating calcium ion distribution in leaves. Accumulation of ions in leaf tissue is in part dependent on import from the xylem. This import via the transpiration stream is more important for ions such as calcium that are xylem but not phloem mobile and cannot therefore be retranslocated. Accumulation of calcium was measured on bulk coriander leaf tissue (Coriandrum sativum L. cv. Lemon) using ion chromatography and calcium uptake was visualized using phosphor-images of (45)Ca(2+). Leaves of plants grown in hydroponics had elevated calcium in the centre of the leaf compared with the leaf margin, while K(+) was distributed homogeneously over the leaf. This calcium was shown to be localised to the mesophyll vacuoles using EDAX. Stomatal density and evapotranspiration (water loss per unit area of leaf) were equal at inner and outer sections of the leaf. Unequal ion distribution but uniformity of water loss suggested that there was a difference in the extent of uncoupling of calcium and water transport between the inner and outer leaf. Since isolated tissue from the inner and outer leaf were able to accumulate similar amounts of calcium, it is proposed that the spatial variation of leaf calcium concentration is due to differential ion delivery to the two regions rather than tissue/cell-specific differences in ion uptake capacity. There was a positive correlation between whole leaf calcium concentration and the difference in calcium concentration between inner and outer leaf tissue. Exposing the plants to increased humidity reduced transpiration and calcium delivery to the leaf and abolished this spatial variation of calcium concentration. Mechanisms of calcium delivery to leaves are discussed. An understanding of calcium delivery and distribution within coriander will inform strategies to reduce the incidence of calcium-related syndromes such as tip-burn and provides a robust model for the transport of ions and other substances in the leaf xylem.
本研究的目的是了解调节叶片中钙离子分布的参数。离子在叶片组织中的积累部分取决于从木质部的输入。通过蒸腾流的这种输入对于诸如钙等木质部可移动但韧皮部不可移动因而不能再转运的离子更为重要。使用离子色谱法测量了芫荽叶片组织（芫荽属柠檬品种）中的钙积累，并使用(45)Ca(2+)的磷图像观察了钙的吸收情况。水培生长的植物叶片，其中心的钙含量高于叶缘，而钾离子在叶片上分布均匀。使用能谱仪分析表明，这种钙定位于叶肉细胞的液泡中。叶片内部和外部的气孔密度和蒸散量（单位叶面积的水分损失）相等。离子分布不均但水分损失均匀，这表明叶片内部和外部钙与水运输的解偶联程度存在差异。由于从叶片内部和外部分离的组织能够积累相似量的钙，因此推测叶片钙浓度的空间变化是由于离子向这两个区域的输送差异，而非组织/细胞对离子吸收能力的差异。全叶钙浓度与叶片内部和外部组织之间的钙浓度差异呈正相关。将植物置于湿度增加的环境中会降低蒸腾作用和钙向叶片的输送，并消除钙浓度的这种空间变化。本文讨论了钙向叶片输送的机制。了解芫荽中钙的输送和分布情况，将为减少诸如叶尖灼烧等与钙相关症状的发生提供策略，并为叶片木质部中离子和其他物质的运输提供一个有力的模型。
