SGER: Manipulations of Upper Crown Water Supply to Assess The Role of Low Water Potential in Limiting The Height Growth of Trees

大小是生物体的基本属性，研究限制大小的因素可以提高对生物体功能的遗传和环境约束的理解。化石记录表明，自从大约4亿年前植物开始在陆地上定居以来，它们的体型一直在稳步增长。今天，最大的树木可以达到超过112米的高度，或者接近370英尺（海岸红杉，Sequoia sempervirens），总体积质量超过1485立方米（巨型红杉估计为365,000公斤；Sequoiadendron giganteum）。树木高度的限制还不是很清楚，但可能是由不受重力影响而使水分从土壤上升到最上层叶子的问题决定的。最近的研究表明，植物水分胁迫的一个主要指标，即叶片的“水势”，随着树木的生长逐渐限制光合作用。虽然最高的树木生长在全年土壤保持湿润的地方，但重力的影响似乎随着高度的增加而增加了水分压力，这可能最终限制了树木的生长高度。这一推理预测，如果一棵很高的树的最上面的树枝上有“本地”水源——也就是说，没有在重力作用下抬升的水——那么叶子的水分压力将会减少，光合作用和高度增长将会增加。为了验证这一预测，提出的研究将使用几种新方法来补充110米以上红杉最上层树枝的水分可用性。一套生理和生长测量将揭示实验操作在多大程度上改善水势，释放光合作用和高度生长，从重力的限制。拟议的研究将开发新的方法，可能对研究植物的水分胁迫具有广泛的意义，包括农业上重要的物种。这项研究将增加对高大树木（包括一种重要的木材物种）水资源压力的了解。最后，预计本研究的结果将有助于了解气候变化如何影响森林树木的生长和生产力。资金将用于吸引本科生参与研究过程。

Size is a fundamental attribute of organisms, and studying the factors that limit size can improve understanding of the genetic and environmental constraints on organism function. The fossil record demonstrates that since plants began to colonize land some 400 million years ago they have steadily increased in size. Today the largest trees that can achieve heights in excess of 112 meters, or nearly 370 ft., (coast redwood, Sequoia sempervirens) and total volumetric mass that exceeds 1485 m3 (an estimated 365,000 kg for giant Sequoia; Sequoiadendron giganteum). The limits to tree height are not well understood, but may be determined by the problem of raising water from the soil to the uppermost leaves against the influence of gravity. Recent studies have demonstrated that a major index of plant water stress, the "water potential" of leaves, becomes progressively limiting to photosynthesis as trees grow taller. Although the tallest trees grow where soils remain moist throughout the year, the influence of gravity appears to increase water stress with increasing height, and this may ultimately limit how tall trees can grow. This reasoning predicts that if the uppermost branches of a very tall tree are provided with a "local" source of water - i.e., water that has not been raised under tension against gravity - then leaf water stress will be reduced and photosynthesis and height growth will be increased. To test this prediction, the proposed study will use several novel approaches to supplement water availability to the uppermost branches of redwoods over 110 m in height. A suite of physiological and growth measurements will reveal the extent to which the experimental manipulations improve water potential and release photosynthesis and height growth from the constraints due to gravity. The proposed research will develop new methodologies that may have broad implications for studying water stress in plants, including agriculturally important species. The research will increase understanding of water stress in tall trees, including an important timber species. Finally, it is expected that the results of this study will inform understanding of how climate change may affect the growth and productivity of forest trees. Funds will be used to engage undergraduate students in the research process.