Rachid Serraj

Nitrogen accumulation and nodule activity of field-grown ‘Jackson’ soybean in response to water deficits

Abstract Substantial evidence has accumulated that N 2 fixation in soybean ( Glycine max Merr.) is much more sensitive to drying soil than is leaf gas exchange and plant mass accumulation. In contrast, the soybean cultivar Jackson in controlled environments has been found to have N 2 fixation sensitivity to drying soil that is approximately equivalent to the sensitivity of leaf gas exchange. This study was undertaken to determine if Jackson behaved in a similar way under field conditions. N 2 fixation activity was measured by in situ acetylene reduction rates and N accumulation in shoots. Leaf gas exchange was measured as stomatal conductance and shoot mass accumulation. All measures in this field study indicated that N 2 fixation in Jackson decreased in response to soil drying in about the same proportion as leaf gas exchange. These results with Jackson were in contrast to those with the cultivar Biloxi in which N 2 fixation was decreased to a greater extent by drought than was leaf gas exchange.

Effects of salinity and nitrogen source on growth and nitrogen fixation in alfalfa

Abstract The Interaction between the effects of nitrate (NO3) and sodium chloride (NaCl) concentration on growth) water relations, nitrogen (N) contents and N fixation were investigated in alfalfa (Medicago sativa L. cv. Magali). The plants were grown hydroponically in a growth chamber, in the presence or absence of 3 mM potassium nitrate (KNO3) and exposed to various concentrations of NaCl. Increased salinity resulted in a significant decrease in shoot and root biomass, relative water content and water potential. Shoot growth was more inhibited by NaCl than root biomass. The plants grown in the presence of NO3 were slightly less affected by NaCl than the plants dependent on N fixation for their N nutrition. Nitrogenase activity measured by acetylene reduction activity was substantially inhibited by NaCl, and this inhibition was significantly correlated to the inhibition of shoot growth and total N contents. The comparison of the curves of ARA response to oxygen (O2) partial pressure showed that the salt‐...

Soybean cultivar variability for nodule formation and growth under drought

Both nodulation and nitrogen fixation in soybean [Glycine max (L.) Merr.] are sensitive to soil drying, which can have important negative effects on yield. An exception to this general response has been the identification of the cultivar Jackson as being drought tolerant for N2 fixation. The objectives of this research were to examine nodule formation and growth in Jackson among other soybean cultivars in response to soil drying under field conditions. Two field experiments were conducted to examine the genetic variation in the sensitivity of nodule numbers and dry weights to soil drying. Substantial variation among soybean lines was found, and the drought-tolerance trait was demonstrated again in Jackson. Greenhouse experiments were conducted to further analyze the variation of nodulation response to soil water content. The differences among cultivars observed in the field were confirmed in the greenhouse. Importantly, the relative drought insensitivity of N2 fixation in cultivar Jackson was associated with high individual nodule dry weight under drought conditions, relative to well-watered plants. It was concluded that large variation in nodulation sensitivity to water deficit exists among soybean cultivars and that the response of N2 fixation rates to drought is related in part to nodule formation and growth.

A γ-TIP cross-reacting protein is abundant in the cortex of soybean N2-fixing nodules

Abstract. The distribution and abundance of tonoplast intrinsic protein (γ-TIP), a putative aquaporin which is abundant in the tonoplast of osmoregulated pulvinus motor cells, were determined in nodules of Glycine max (L.) Merr. using chemical fixation and immunolocalization. This protein was highly expressed in the tonoplast of the inner cortical cells of the nodules but poorly expressed in the vascular transfer cells and in infected cells. It is concluded that the differentiation of the inner cortical cells of the nodules like that of pulvinus motor cells, is accompanied by an increased expression of γ-TIP. This result is consistent with our previous hypothesis that a reversible exchange of intercellular water by the inner cortical cells plays a role in the regulation of nodule conductance to O2 diffusion, and hence in subsequent N2-fixing activity.

Recent Advances in the Physiology of Drought Stress Effects on Symbiotic N2 Fixation in Soybean

Oxygen limitation, feedback regulation by ureide accumulation, and carbon shortage, are considered in the present research as three key mechanisms affecting nitrogen fixation in response to drought stress. The interaction of nodule permeability to O2 and drought stress on N2 fixation was examined in soybean nodules, and led to the overall conclusion that 02 limitation seems to be involved only in the initial stages of water deficit stresses in decreasing nodule activity. The involvement of ureides in the drought response of N2 fixation was initially suspected by an increased ureide concentration in shoots and nodules under drought leading to a negative feedback response between ureides and nodule activity. A direct evidence for inhibition of nitrogenase activity by ureides supported this hypothesis. The carbon shortage hypothesis was investigated by 14C labeling and by studying the combined effects of CO2 enrichment and water deficits on nodulation and N2 fixation in soybean. Under drought, in a genotype with drought tolerant N2 fixation, approximately four times the amount of 14C was allocated to nodules compared to a drought sensitive genotype. It was found that an important effect of CO2 enrichment of soybean under drought was an enhancement of photoassimilation, an increased partitioning of carbon to nodules, whose main effect was to sustain nodule growth, which helped sustain N2 rates under soil water deficits. We conclude that all three mechanisms are important in understanding the response of N2 fixation to soil drying.

Ureides Decrease N2-Fixation and Nodule Permeability to Oxygen in Soybean

Sensitivity of legume N2 fixation to drought stress tends to be associated with high ureide production (Sinclair, Serraj, 1995), which suggests a feedback mechanism on N2 fixation during drought by N-compounds (Parsons et al., 1993). To address this hypothesis, we measured the effect of ureides and other N compounds on nitrogenase activity (ARA), on nodule permeability to O2, (Denison, Kinraide, 1995) and on ureide accumulation in plant tissues.