Growth Reductions of Rice at Low Root Temperature: Decreases in Nutrient Uptake and Development of Chlorosis

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Abstract
The physiological mechanisms for growth reductions of rice at low root temperatures were investigated in detail via time courses in nutrient status of several cultivars. During short-term exposure to low temperature, i.e. between 0–2.5 d with roots at 10°C, leaf extension rates were reduced approximately 80%-95% in all cultivars. In contrast, relative growth rates of shoots on a dry weight basis were often even greater for plants with roots at 10°C relative to 30°C. During long-term growth at low root temperatures, i.e. between 2.5–10 d, relative growth rates of shoots were reduced, chlorosis developed and cultivar differences were observed which were consistent with field observations of cold-tolerant and cold-intolerant cultivars. The results indicate that decreases in nutrient concentrations in plants could not account for growth reductions during short-term exposure to low root temperatures. However, it is possible that they are responsible for most of the growth reductions and chlorosis later than 2.5 d. The latter suggestion is not proven unequivocally but is supported by: (i) similar results when plants were transferred to CaSO4 solutions at 30°C in terms of growth, nutrient decreases with time and chlorosiθ (ii) N and sometimes P concentrations falling below critical levels for rice and (iii) lower nutrient uptakes and concentrations, particularly of N, in a cold-intolerant than a cold-tolerant cultivar.

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