| Summary: | The relative growth-rate of plants grown on a vermiculite culture medium in an arctic climate during the growing season was about a quarter of that of comparable plants on the same medium in a temperate climate. In both climates the relative growth-rate was lower on natural soils than on vermiculite. Net assimilation rates and, to a lesser extent, leaf-area ratios were depressed by arctic climates and soils. Net assimilation rates of seven species in various habitats in two arctic regions were about 0.1–0.3g dm−2wk−1. Previous suggestions that net assimilation rates in arctic regions equal or exceed those in temperate regions are attributed to misinterpretation of data or to inadequate methods. There is evidence that the depression of net assimilation rates in arctic regions is due to the low temperatures, which, especially when associated with soil nitrogen deficiency, reduce the rate at which assimilates are used in respiration and new growth; this causes sugars to accumulate to levels at which they depress assimilation.
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