Submitted to: American Society of Plant Physiologists Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 7/17/2000
Publication Date: N/A
Technical Abstract: White lupin has elegant adaptations for survival in a low phosphorus (P) environment including proteoid (cluster) root formation, enhanced organic acid exudation from roots, and the release of acid phosphatase into the rhizosphere. However, little is known of whether root adaptations to low P affect root nodulation and symbiotic nitrogen (N2) fixation. Our objectives swere to determine the impact of low P on root nodule symbiosis between white lupin and Bradyrhizobium lupini. Plants were grown on N-free quartz sand at two different P levels in the nutrient solution (0 and 0.5 mM P). At day 21 after emergence, the roots of the -P plants had developed more proteoid (cluster) root zones and the nodule number as compared to the +P plants. However, since the nodules were smaller, total nodule mass did not differ between the treatments. The nodules in the -P plants were located largely in the vicinity of proteoid root zones at lateral roots, while in the +P plants most nodules developed on the main root. Although the P concentration (mg P/mg DM) in the -P plants was only about 1/3 in shoots and roots and 1/2 in nodules compared to the +P plants, the shoots showed no visible signs of P deficiency and neither the % N nor the total N in any of these organs was reduced. Nitrogenase activity, measured through 15N2 uptake on intact plants, was not different between the treatments and the amount of N2 fixed per mg P in the nodule was increased in the -P treatment by 92%. To explain these adaptations in the P efficiency of nitrogen fixation in white lupin, data on nodule enzyme expression, photosynthetic carbon allocation to the nodules, in vivo nodule dark CO2 fixation and nitrogen fixation (15N2 uptake and H2 evolution) at low and high external oxygen pressures will be presented.