Submitted to: Annals Of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/3/2006
Publication Date: 8/19/2006
Citation: Schulze, J., Temple, G., Temple, S.J., Beschow, H., Vance, C.P. 2006. Nitrogen fixation by white lupin under phosphorus deficiency. Annals of Botany. 98:731-740. Interpretive Summary: Phosphorus (P) is one of the most important nutrients required for plant growth. The use of P in agriculture is problematic. In the developed world farms fertilize with excess P resulting in contamination of surface waters. By comparison, in the developing world adequate P is unavailable for crop production. Discovery of mechanisms that allow plants to obtain P more efficiently would be extremely beneficial for agriculture. The legume plant white lupin has several unique adaptations for growth in P-deficient soils. One of these adaptations is the formation of dense lateral roots called cluster roots. White lupin also has another adaptation to obtain the nutrient nitrogen (N) through symbiotic nitrogen fixation (SNF). The process of SNF in legumes occurs when a successful symbiosis is established between plant roots and the soil bacteria known collectively as rhizobia. This process occurs in small wart-like structures on roots termed nodules. In this study we tested the hypothesis that SNF in white lupin was efficient under P-deficient conditions. The results show that cluster root zones in P-deficient roots appear to be more susceptible to root nodule formation. Moreover, SNF in P-deficient plants continued at rates comparable to that found in P-sufficient plants for 21-28 days. After this period SNF in P-deficient plants was significantly reduced. The high efficiency of SNF during the first 21-28 days of growth appeared to be related to increased energy utilization in nodules. These studies are important because new mechanisms were discovered that help plants adapt to low P environments. Increased energy use efficiency in white lupin nodules grown under P-stress may be applicable to other legume plants.
Technical Abstract: White lupin is highly adapted to growth in a low P environment. The objective of the present study was to evaluate whether white lupin grown under P-stress has adaptations in nodulation and N2 fixation that facilitate continued functioning. Nodulated plants were grown in silica sand supplied with N-free nutrient solution containing 0 to 0.5 mM P. At 21 and 37 days after inoculation (DAI) growth, nodulation, P and N concentration, N2 fixation (15N2 uptake and H2 evolution), root/nodule net CO2 evolution, and CO2 fixation (14CO2 uptake) were measured. Furthermore, at 21 DAI in vitro activities and transcript abundance of key enzymes of the C and N metabolism in nodules were determined. Moreover, nodulation in cluster root zones was evaluated. The -P treatment led to lower P concentration in shoots, roots, and nodules. In both treatments, the P concentration in nodules was greater than that in the other organs. At 21 DAI nitrogen fixation rates did not differ between treatments and the plants displayed no symptoms of P or N deficiency on their shoots. Although nodule number at 21 DAI increased in response to P-deficiency, total nodule mass remained constant. Increased nodule number in P-deficient plants was associated with cluster root formation. A higher root/nodule CO2 fixation in the -P treatment led to a lower net CO2 release per unit fixed N although the total CO2 released per unit fixed N was higher in the -P treatment. The higher CO2 fixation was correlated with increased transcript abundance and enzyme activities of PEPC and MDH in nodules. Between 21 and 37 DAI -P plant shoots developed symptoms of N- and P-deficiency. By 37 DAI the P concentration decreased in all organs of the -P treatment. At 37 DAI nitrogen fixation in the -P treatment had almost ceased. Enhanced nodulation in cluster root zones and increased potential for organic acid production in root nodules appear to contribute to white lupin's resilience to P-deficiency.