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更好地理解遗传和环境对生物体功能的制约。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，它们的大小一直在稳步增加。今天，最大的树木可以达到超过112米(近370英尺)的高度(海岸红杉、半边红杉)和总体积超过1485立方米(巨型红杉估计为365,000公斤；巨型红杉)。树高的极限还没有被很好地理解，但可能是由土壤中的水分在重力的影响下上升到最上面的叶子的问题决定的。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”，随着树木的生长而逐渐限制光合作用。虽然最高的树生长在土壤全年保持潮湿的地方，但重力的影响似乎会随着高度的增加而增加水分压力，这最终可能会限制树的生长高度。这一推理预测，如果为一棵非常高大的树的最上面的树枝提供一种“本地”水源--也就是没有在重力的压力下升高的水--那么树叶的水分压力将会减少，光合作用和高度生长将会增加。为了验证这一预测，拟议中的研究将使用几种新的方法来补充高度超过110米的红杉最上面的树枝的水分供应。一套生理和生长测量将揭示实验操作在多大程度上改善了水势，释放了光合作用和身高增长，摆脱了重力的限制。这项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影响，包括农业上重要的物种。这项研究将增加对高大树木水分胁迫的了解，包括一种重要的木材物种。最后，预计这项研究的结果将有助于理解气候变化可能如何影响林木的生长和生产力。资金将用于让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.