Costs and benefits in branching and the mechanical stability of plants

植物结构是与植物的光截获和机械稳定性有关的大体形态。我们的目标是验证这样一种假设，即植物在给定的环境中，其叶片的发育能够有效地拦截光线。植物被单独种植或在密林中生长以适应不同的光照气候。在最初的两年里，我们用一年生藜属植物进行了一次实验。我们描述了独生林和密林植物的结构，并分析了它们的生长。根据叶面积分布，我们计算了植物对光的截获。为了描述植物的结构，我们使用了植物体，它是植物生长的单位，每个单位由节间、叶柄和叶片组成。分析了单株和密林植物在分枝格局和资源利用上的差异。密林植物比单株更细、更高。单株植株产生更多更长的枝条。密植植物以牺牲机械稳定性为代价长得更高。密林植物的光能利用效率高于单株植物，氮素利用效率低于单株植物。从第二年开始，我们使用另一种一年生植物加拿大苍耳来确定影响植物伸长生长和机械稳定性的因素。我们测量了节间的弹性模量和断裂应力，发现单株的弹性模量和断裂应力都低于密林植物，但单株的抗屈曲安全系数较大，这是因为单株的地位较低。单株与密植树节间干物质密度与弹性模量、断裂应力呈正相关，回归差异显著，分析了相对伸长率与节间含水率的关系。水分含量与节间干物质密度呈极显著正相关。这一关系式是通过一个简单的模型推导出来的，该模型包括干质量密度和空气空间分数作为参数。

Plant architecture is gross morphology pertaining to light interception and mechanical stability of plants. We aimed to test the hypothesis that plants develop their leaves to efficiently intercept light in a given environment. Plants were grown solitarily or in dense stands to have different light climates.In the first two years, we conducted an experiment using an annual Chenopodium album. We described the architecture of solitary and dense-stand plants and analyzed their growth. From leaf area distribution, we calculated light interception of the plants. To describe plant architecture, we used phytomers, a unit of plant growth each composed of an internode, petiole and leaf blade. Differences in branching pattern and resource use between solitary and dense-stand plants were analyzed. Dense-stand plants were thinner and taller than solitary plants. Solitary plants produced more and longer branches. Dense-plants grew taller at the expense of mechanical stability. Light use efficiency was higher and nitrogen use efficiency was lower in dense-stand plants than solitary plants.From the second year, we used another annual species Xanthium canadense to identify factors involved in extension growth and in mechanical stability of plants. We measured the elastic modulus and breaking stress of internodes and found that both were lower in solitary than dense-stand plants but that the safety factor against buckling was larger in solitary plants due to their lower statue. There were positive correlations between elastic modulus, breaking stress and dry mass density of internodes, but the regressions were significantly different between solitary and dense-stand plants.Relationships between the relative elongation rate and water content of internodes were analyzed. There was a strong correlation between water content and dry mass density of internodes. This relationship was derived with a simple model that includes density of dry mass and fraction of air space as parameters.