Physiological acclimations to chilling temperature in symbiotically grown alfalfa

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Abstract
An apparent shoot rest period was induced in the 2nd month of growth of alfalfa (Medicago sativa L.) seedlings by a drop in growth temperature from 25:20 °C to 10:7 °C. After prolonged chilling the shoots were replaced by new shoots. Temperature profiles of nodulated root respiration and nitrogenase activity (acetylene reduction and H2 evolution) were measured simultaneously in experiments with a flow-through gassing system during 3 months of cold treatment. Net photosynthesis of whole plants was measured in a closed system. More than half the total initial nitrogenase activity and relative efficiency (RE) were lost during the rest period and recovered during regrowth. Acetylene reduction by chilled plants was insensitive to temperature in the 5 – 15 °C range, unlike the temperature dependence of respiration and H2 evolution in air. In all temperature profiles of RE the RE was highest at 5 – 10 °C. The RE was minimum 10 – 15 °C during the rest period. The optimum temperature for whole plant net photosynthesis also declined to 10 – 15 °C during chilling and it later flattened out in the cold-acclimated regrowth. Possible mechanisms are discussed.