Mitamura Masako, Yamamura Yasuo, Nakano Takashi
Laboratory of Ecology, Faculty of Science, Ibaraki University, 2-1-1, Bunkyo, Mito 310-8512, Japan.
Tree Physiol. 2009 Jan;29(1):137-45. doi: 10.1093/treephys/tpn014. Epub 2008 Dec 5.
Although the environmental change by canopy gap formation in a forest improves the light availability for the saplings on the forest floor, it may result in stresses on the saplings due to high radiation and drought. In large-scale gaps, the photosynthesis of shade-tolerant species may be inhibited by high radiation and drought stress if they lack effective tolerance or avoidance mechanisms for the stresses. We investigated the photosynthetic traits and water relations of Abies veitchii Lindl. saplings in an open habitat created by an avalanche and in a nearby forest floor habitat undisturbed by the avalanche. We analyzed the influence of exposed conditions on sapling photosynthesis. The maximum photosynthetic rate of the saplings in the open habitat was lower than that in the forest habitat. The ratio of variable to maximum chlorophyll fluorescence (F(v)/F(m)) was lower in the open habitat than that in the forest habitat during the late growing season, indicating that the open habitat saplings suffer photoinhibition of photosystem II for a long period. A lower Rubisco concentration in needles in the open habitat indicated the breakdown of this photosynthetic protein because of excess solar energy resulting from serious photoinhibition. The shoot water potential of the saplings in the open habitat at daytime was higher than that of the saplings in the forest habitat because of less transpiration caused by the remarkable stomatal closure in the open habitat. Although these acclimations to high radiation improve the tolerance of A. veitchii saplings to high radiation and drought stress, they would result in low gain of daily carbon and a reduction in growth in the open habitat.
尽管森林中林冠间隙形成所带来的环境变化提高了林冠下层幼树可利用的光照，但这也可能因高辐射和干旱而给幼树带来压力。在大规模的林冠间隙中，如果耐荫物种缺乏有效的胁迫耐受或规避机制，其光合作用可能会受到高辐射和干旱胁迫的抑制。我们调查了岷江冷杉幼树在雪崩形成的开阔生境以及附近未受雪崩干扰的林冠下层生境中的光合特性和水分关系。我们分析了暴露条件对幼树光合作用的影响。开阔生境中幼树的最大光合速率低于森林生境中的。在生长季后期，开阔生境中可变荧光与最大荧光之比（F(v)/F(m)）低于森林生境中的，这表明开阔生境中的幼树长期遭受光系统II的光抑制。开阔生境中针叶里较低的核酮糖-1,5-二磷酸羧化酶（Rubisco）浓度表明，由于严重光抑制导致的太阳能过剩，这种光合蛋白遭到了破坏。由于开阔生境中气孔显著关闭导致蒸腾作用减弱，开阔生境中幼树白天的嫩梢水势高于森林生境中的。尽管这些对高辐射的适应性提高了岷江冷杉幼树对高辐射和干旱胁迫的耐受性，但它们会导致开阔生境中每日碳积累量较低以及生长减缓。
