Costs and benefits in branching and the mechanical stability of plants

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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.