Phosphorus and nitrogen interactions in field-grown soybean as related to genetic attributes of root morphological and nodular traits

Two field experiments with different soybean (Glycine max L.) materials were conducted to investigate the interactions between phosphorus (P) and nitrogen (N) as related to the genetic attributes of root morphological and nodular traits. In experiment one, 13 cultivated soybean varieties were grown in a field with relatively low soil P and N availability. P application with 160 kg P/hm(2) as triple superphosphate produced a significant simultaneous increase in the content of both P and N in shoot, demonstrating positive P and N interactions. The addition of P also increased root dry weight, root nodule number, nodule mass, nodule size, and nodulation index, but decreased root length and root surface area, indicating that P may affect N nutrition in soybean through a number of root morphological and nodular traits. Interestingly, like P content, N content appeared to be more correlated with root morphological traits (root weight, root length, and root surface area) than with root nodular traits (nodule number, nodule size, nodule mass, and nodulation index) at both P levels, implying that N taken up by the roots may contribute more to the plant N status than biological N-2 fixation under the present experimental conditions. In experiment two, 57 soybean lines of a recombinant inbred line (RIL) population derived from a cross between a cultivated variety and a wild genotype were grown on another field site with moderately sufficient P and N levels to further characterize the genetic attributes of root morphological and nodular traits and their relationships with P and N interactions. The results indicated that all morphological and nodular traits measured continually segregated in the RIL population with a normal distribution of the phenotypic values, indicating that these traits are possibly controlled by quantitative trait loci (QTLs). Genetic analysis revealed that all these root traits had relatively low heritabilities (h(b)(2)=74.12, 70.65, 73.76, 56.34, 52.59, and 52.24 for root weight, root length, root surface area, nodule number, nodule mass, and nodule size, respectively), suggesting that root morphology and nodule formation are influenced greatly by environmental factors. Correlation analysis of the RILs showed that shoot N content was significantly correlated with P content, confirming positive PXN interactions. Similar to experiment one, shoot N content was only significantly correlated with root morphological traits, but not with root nodular traits, again denoting the fact that the N status in soybean could be attributed more to N uptake from the soil than to biological N2 fixation under the present experimental conditions.