Soil temperature affects both the rate and thoroughness with which a plant root system permeates soil. Root system expansion is a function of two temperature-dependent processes, growth and development. Growth processes, like cell elongation, increase root length and diameter. Development controls duration of growth and initiation of new roots and reproductive organs. Interpreting root temperature responses requires an understanding of how development and growth interact. Soil temperature affects growth of root system components, initiation and branching, orientation and direction of growth, and root turnover. Genotypic differences in root response to soil temperature exist between and within plant species. In natural soil profiles, root system expansion is affected by seasonal patterns of soil temperature. As soil warming advances downward, progressively deeper soil layers become suitable for root growth. In temperate regions, soil temperature often limits the rate of rooting-depth increase and the maximum depth attainable. A simple temperature-based model to predict rooting depth with time indicates that rooting depth may follow the downward progression of a particular isotherm, which has sometimes been observed in the field.
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