Comparative physiology of plant adaptation: resource acquisition and constraints of growing season length in balsam poplar from temperate, boreal and arctic environments

在一些被子植物树种的北方种群中，有一种更高的光合速率的趋势，这可能有助于补偿短暂的夏季。我们最近报道了高纬度苦杨光合作用的增强是由于叶片质量和单位面积氮含量的增加，并得到叶肉导度（gm）增强的支持。叶肉电导是衡量二氧化碳在叶片中传播的难易程度的指标，但它的估计是非常不确定的，必须加以验证。一名硕士学生将使用在线同位素识别技术来确认和扩展这些观察结果，以控制苦杨的南北杂交。我们将对苦杨和黑棉进行比较，后者在北方基因型中也表现出更强的光合作用，但得到了更高的气孔导度（gs）的支持。增加gs可以提高氮素利用效率，但降低水分利用效率。相反，增加转基因应该能改善这两方面。这些不同的策略可能反映了水供应的差异，西北海岸的黑棉杨比大陆内陆的苦瓜杨限制较少。沿着其他路线，我们在温哥华和萨斯喀彻温省类似纬度地区建立了广泛收集香脂杨树的共同花园。无论来源如何，在温哥华，所有基因型的花期、生长和发芽都要早5周左右。因此，尽管北方树木可以利用较早的春天，它们可能捕获较少的夏天。在高纬度地区，如果气候变暖足以在冬至前几周导致洪水泛滥，这些固有的反应可能会对树木生长产生强烈的负面影响。我们将从现场和室内实验中获取数据来模拟这些关系。最后，一名博士生将测试从N同位素识别数据中提取氮吸收效率、同化位点和离子通量信息的假设。他将研究来自温暖、高pH、硝酸盐为主的土壤的苦杨与来自寒冷、低pH、氨为主的土壤的树木在来源偏好、歧视和转录本丰度（硝酸盐和铵的同化和运输基因家族）方面的植物地理模式。

There is a trend toward higher photosynthetic rates in northern populations of several angiosperm tree species that may help compensate for short summers. We recently reported that greater photosynthesis in high latitude balsam poplar results from increased leaf mass and nitrogen per area, supported by an enhanced mesophyll conductance (gm). Mesophyll conductance is a measure of the ease with which CO2 travels through leaves, but its estimation is very uncertain and must be verified. An MSc student will use an on-line isotope discrimination technique to confirm and extend these observations in controlled north-south crosses of balsam poplar. We will compare balsam poplar with black cottonwood, which also shows enhanced photosynthesis in northern genotypes, but supported by a higher stomatal conductance (gs). Increased gs can improve nitrogen-use efficiency but reduces water-use efficiency. In contrast, increased gm should improve both. These different strategies may reflect differences in water supply, which is less limiting for black cottonwood from the northwest coast than for balsam poplar from the continental interior. Along other lines, we have established common gardens of a range-wide collection of balsam poplar at Vancouver and at similar latitude in Saskatchewan. Regardless of origin, all genotypes flush, grow and set bud ~5 weeks earlier in Vancouver. Consequently, although boreal trees can take advantage of earlier springs, they may capture less of the summer. At very high latitudes, these inherent responses could lead to strong negative impacts on tree growth if climate warming is sufficient to induce flushing a few extra weeks before the solstice. We will acquire data from field and chamber experiments to model these relationships. Finally, a PhD student will test assumptions that allow information on nitrogen uptake efficiency, sites of assimilation and ion flux to be extracted from N isotope discrimination data. He will examine phytogeographic patterns in source preference, discrimination and transcript abundance (nitrate and ammonium assimilation and transport gene families) in balsam poplar from warmer, high pH, nitrate-dominated soils vs. trees from colder, low pH, ammonium-dominated soils.