The present study compares the demands for P of the initial nodule formation, and of the later growth and functioning of the nodulated root system in two inoculated lines of common bean (Coco blanc and BAT477). After germination and inoculation, seedlings were divided into two lots. One lot was grown under constant P supply, either 15 (low) or 250 (high) µM P, corresponding to provision of 120 and 2000 µmol P, respectively. In the second, seedlings were cultivated on the same medium supplied with 15 µM P for 24 days after germination, and then with 60 or 250 µM (total provision of P: respectively, 390 and 1530 µmol). Nodule number and biomass were significantly diminished by the low P (120 µmol) treatment, as compared with the other treatments. However, the intrinsic characteristics of the nodules (individual biomass and size, P concentration and efficiency of N fixation) did not depend on P availability. Although the bean line BAT477 was distinguished from the Coco blanc line through higher nodule number, size, biomass, and nitrogen fixation, both lines displayed analogous responses to P availability.

Iron is an important nutrient in N(2)-fixing legume nodules. The demand for this micronutrient increases during the symbiosis establishment, where the metal is utilized for the synthesis of various iron-containing proteins in both the plant and the bacteroid. Unfortunately, in spite of its importance, iron is poorly available to plant uptake since its solubility is very low when in its oxidized form Fe(III). In the present study, the effect of iron deficiency on the activity of some proteins involved in Strategy I response, such as Fe-chelate reductase (FC-R), H(+)-ATPase, and phosphoenolpyruvate carboxylase (PEPC) and the protein level of iron regulated transporter (IRT1) and H(+)-ATPase proteins has been investigated in both roots and nodules of a tolerant (Flamingo) and a susceptible (Coco blanc) cultivar of common bean plants. The main results of this study show that the symbiotic tolerance of Flamingo can be ascribed to a greater increase in the FC-R and H(+)-ATPase activities in both roots and nodules, leading to a more efficient Fe supply to nodulating tissues. The strong increase in PEPC activity and organic acid content, in the Flamingo root nodules, suggests that under iron deficiency nodules can modify their metabolism in order to sustain those activities necessary to acquire Fe directly from the soil solution.

The impact of phosphorus (P) availability on root proliferation, proton efflux, and acid phosphatase activities in roots and leaves was investigated in two lines of common bean (Phaseolus vulgaris): BAT 477 and CocoT. Phosphorus was supplied as KH 2 PO 4 at 0 and 60µmol per plant (0P and 60P, respectively). Under P shortage, the plant growth was more restricted in CocoT than in BAT 477, shoots being more affected than roots. The root area increased significantly at 0P in both lines. Up to 1 week following P shortage, the proton efflux increased in both lines despite a higher extent in BAT 477 as compared to CocoT. Root acid phosphatase activity was significantly higher under P limitation in the both lines, this trend being more pronounced in BAT 477 than in CocoT. This was also true for the leaf acid phosphatase. Regardless of the bean line, higher values were recorded for the old leaves as compared to the young ones for this parameter. Interestingly, a significant correlation between Pi content in old leaves and their acid phosphatase activity was found in P-lacking (0P) plants of the both bean lines, suggesting that acid phosphatase may contribute to increase the phosphorus use efficiency in bean through the P remobilization from the old leaves. As a whole, our results highlight the significance of the root H + extrusion and the acid phosphatase activity rather than the root proliferation in the relative tolerance of BAT 477 to severe P deficiency.

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