Relationship between Chloroplast Membrane Fatty Acid Composition and Photosynthetic Response to a Chilling Temperature in Four Alfalfa Cultivars.

The photosynthetic responses of four alfalfa (Medicago sativa L.) cultivars to 10 and 22 C air temperatures were examined and the relationship between the photosynthetic response at 10 C and the fatty acid composition of the chloroplast membranes was determined. Chilling-resistant cultivars exhibited moderate reductions in photosynthesis at 10 C, compared to 22 C, and contained a significantly greater percentage of polyunsaturated fatty acids in the chloroplast membrane and a greater double bond index than the chilling-sensitive cultivars. The chilling-sensitive cultivars exhibited severe reductions in photosynthesis at 10 C, compared to 22 C. The reduction in photosynthesis at 10 C is shown to be negatively correlated (r = -0.94) with the double bond index of the chloroplast membranes of the cultivars observed.The results support the hypothesis that reduced photosynthesis due to chilling temperatures is influenced by the unsaturated fatty acid composition of the chloroplast membrane which affect temperature-induced phase changes in chloroplast membrane lipids.