Genetic variability in nodulation and root growth affects nitrogen fixation and accumulation in pea

Abstract

Background and aims: Legume nitrogen is derived from two different sources, symbiotically fixed atmospheric N(2) and soil N. The effect of genetic variability of root and nodule establishment on N acquisition and seed protein yield was investigated under field conditions in pea (Pisum sativum). In addition, these parameters were related to the variability in preference for rhizobial genotypes.

Methods: Five different spring pea lines (two hypernodulating mutants and three cultivars), previously identified in artificial conditions as contrasted for both root and nodule development, were characterized under field conditions. Root and nodule establishment was examined from the four-leaf stage up to the beginning of seed filling and was related to the patterns of shoot dry matter and nitrogen accumulation. The genetic structure of rhizobial populations associated with the pea lines was obtained by analysis of nodule samples. The fraction of nitrogen derived from symbiotic fixation was estimated at the beginning of seed filling and at physiological maturity, when seed protein content and yield were determined.

Key results: The hypernodulating mutants established nodules earlier and maintained them longer than was the case for the three cultivars, whereas their root development and nitrogen accumulation were lower. The seed protein yield was higher in 'Athos' and 'Austin', the two cultivars with increased root development, consistent with their higher N absorption during seed filling.

Conclusion: The hypernodulating mutants did not accumulate more nitrogen, probably due to the C cost for nodulation being higher than for root development. Enhancing exogenous nitrogen supply at the end of the growth cycle, by increasing the potential for root N uptake from soil, seems a good option for improving pea seed filling.

Fig. 1.

(A) Shoot dry matter and (B) shoot nitrogen of five pea genotypes, from the four-leaf stage until physiological maturity. 4L, 10L, BF, BSF and PM indicate the developmental stages, and correspond to four-leaf stage, ten-leaf stage, beginning of flowering, beginning of seed filling and physiological maturity, respectively. Each point is the mean value of three replicates. Vertical bars represent LSD (P < 0·05).

Fig. 2.

(A) Nodulated root dry matter, (B) root dry matter, (C) tap root length and (D) total root length of five pea genotypes, from the four-leaf stage until physiological maturity or the beginning of seed filling. 4L, 10L, BF, BSF and PM indicate the developmental stage and correspond to four-leaf stage, ten-leaf stage, beginning of flowering, beginning of seed filling and physiological maturity, respectively. Each point is the mean value of three replicates. Vertical bars represent LSD (P < 0·05).

Fig. 3.

(A) Nodule number and (B) nodule dry matter of five pea genotypes, from the four-leaf stage until physiological maturity or the beginning of seed filling. 4L, 10L, BF, BSF and PM indicate the developmental stage, and correspond to four-leaf stage, ten-leaf stage, beginning of flowering, beginning of seed filling and physiological maturity, respectively. Each point is the mean value of three replicates. Vertical bars represent LSD (P < 0·05).