Relationship between carbon metabolism indices of peanut leaves and nitrogen fixation ability of nodules

Objectives Comparison of the co-efficiencies of leaf carbon metabolism indices with the nitrogen fixation efficiencies of root nodules of different peanut cultivars would help choosing appropriate indices for the breeding of target peanut cultivars.

Methods A pot experiment was conducted in Laixi Experimental Station of Shandong Peanut Research Institute. A total of nineteen peanut ( Arachis hypogaea) cultivars (lines) were used in the pot experiment, and ¹⁵N isotope tracing method was used to differentiate the nitrogen resources uptaken by peanuts. The chlorophyll content, net photosynthetic rates, soluble sugar content, activity of sucrose synthase and sucrose phosphate synthase, and amount of root nodule fixed N were measured. The relationship between the carbon metabolism indices and nitrogen fixation efficiency was analyzed.

Results 1) There were significant differences among testing peanut varieties in chlorophyll content, net photosynthetic rates, soluble sugar content, activity of sucrose synthase and sucrose phosphate synthase, and amount of fixed N by nodules. The variation coefficient of chlorophyll a, chlorophyll b, carotenoids and total chlorophyll content were all above 12.0%, and that of chlorophyll a was the highest. The net photosynthetic rate was in rage of 14.5–29.7 μmol/(m²·s), with variation coefficient of 21.5%. The highest photosynthetic rate was 36.9% above the average value. The soluble sugar content of Ribenqianyebanman was the highest of the nineteen peanut cultivars, and was 34.4% above the average and 1.9 times of the lowest value of Rihua1. The variation coefficient of activity of sucrose synthase and sucrose phosphate synthase were 18.4% and 24.3%, respectively. The highest sucrose synthase activity was determined in Ribenqianyebanman, and the highest sucrose phosphate synthase was in Yuhua 9326. The variation coefficient of nodule fixed N was over 20.0%, and the highest fixation was 2.6 times of the lowest one. 2) The nodule fixed N amount had significant and positive linear correlations with the indexes of carbon metabolism, with the determination coefficient in rang of 0.793–0.950, and the correlation was in sequence of sucrose synthase activity > soluble sugar content ≈ carotenoids content > sucrose phosphate synthase activity > total chlorophyll content ≈ chlorophyll b content > net photosynthetic rates > chlorophyll a content.

Conclusions Carbon metabolism characteristics and nitrogen fixation of root nodules are genetically stable and closely correlated with each other. Soluble sugar content and sucrose synthase activity are the two most characteristic indices of a cultivar/line, and their correlation coefficiency with the nitrogen fixation efficiency are higher than 0.9, therefore, could be used as indices for high efficient N fixation peanut cultivar.