We hypothesized that adventitious roots may improve crop adaptation to low-phosphorus soils by enhancing topsoil foraging. In a tropical field study, phosphorus stress stimulated adventitious rooting in two phosphorus-efficient genotypes of common bean (Phaseolus vulgaris L.) but not in two phosphorus-inefficient genotypes. Although phosphorus availability had no consistent effects on the length or biomass of whole root systems, it had differential effects on adventitious, basal, and taproots within root systems in a genotype-dependent manner, resulting in increased allocation to adventitious roots in efficient genotypes. Adventitious roots had greater length per unit biomass than other root types, especially under phosphorus stress. Adventitious roots had less construction cost than basal roots, despite having similar tissue nitrogen content. Phosphorus stress reduced lateral root density, and adventitious roots had less lateral root density than basal roots. Lateral roots formed further from the root tip in adventitious roots compared with basal roots, especially under phosphorus stress. Field results were confirmed in controlled environments in solid and liquid media. Stimulation of adventitious rooting by phosphorus stress tended to be greater in wild genotypes than in cultivated genotypes. We propose that adventitious rooting is a useful adaptation to low phosphorus availability, because adventitious roots explore topsoil horizons more efficiently than other root types.

The diversity of 16 strains of chickpea-infective rhizobia from various geographical origins was analysed using genotypic and phenotypic approaches. Multilocus enzyme electrophoresis was performed, and restriction fragment length polymorphisms of the amplified 16S + IGS (intergenic spacer) rRNA gene, assimilation of 147 carbon sources, antibiotic resistance, and tolerance to NaCl and extreme pH values and temperatures were tested. These approaches had different discriminating powers. Esterase polymorphisms gave a unique pattern for each strain, allowing this method to be used for strain fingerprinting. Genetic distances between strains were estimated. The three approaches used in this study yielded consistent results. They evidenced high heterogeneity among the strains, and made it possible to classify the strains into two clusters. Isozyme patterns for superoxide dismutase were particularly interesting, since they delineated the same two groups. The phenotypic tests clearly confirmed the existence of two genetic groups on the basis of 11 phenotypic characters. Owing to the large phylogenetic distance between the two groups of strains, the taxonomic status of chickpea-infective strains is discussed.

Understanding the magnitude of N2 fixation and export of plant N, particularly in the harvested grain and straw, is necessary to assess the potential for grain legumes to contribute to long‐term agricultural production stability. The proportions of total plant N derived from N2 fixation in four grain legume crops, faba bean (Vicia faba L.), chickpea (Cicer arietinum L.), lentil (Lens culinaris L), and pea (Pisum sativum L.), were estimated in three field experiments conducted for two seasons in Syria (Vertic Chromoxeralf soil) and one season in France (Calcaric Cambisol). Since cultural practices and cultivars affect grain yield, the impact on N2 fixation of sowing date in chickpea and Sitona insect control in lentil, and of cultivar selection of pea and faba bean, was evaluated. By calculating the proportion of total plant N derived from N2 fixation (%Ndfa) using 15N isotope dilution with barley (Hordeum vulgare L.) and non‐nodulating chickpea as reference crops, the effects of removal of N in grain and straw, relative to N fixed and plant uptake of soil mineral N, were estimated. Pea and lentil had similar %Ndfa values across locations, seasons, and cultural practices, with an average 70%Ndfa. In chickpea, winter sowing increased %Ndfa to 72 from 26% in the spring‐sown crop. Dinitrogen fixation in the spring‐sown crop was higher in France (44%Ndfa), while fixation in the winter crop was higher in Syria (80%). Faba bean obtained 9O%Ndfa in France, but only 69% in Syria. The large‐seeded type fixed higher N (74%Ndfa) than the small‐seeded type (64%) in Syria but not in France. The calculated N balance where only grain was removed ranged from 44 kg N ha‐' net gain to 44 kg N ha−1 loss. Where both seed and straw were removed, nearly all calculations were negative, with loss of up to 70 kg N ha−1 from soil.

With development of new cultivars for winter sowing, production of Kabuli chickpea (Cicer arietinum L.) has expanded into drier areas of the Mediterranean region where low or less effective populations of indigenous rhizobia may limit N2 fixation. This study was conducted to quantify field N2 fixation using 15N for eight chickpea cultivars as affected by native rhizobial populations and three introduced rhizobial strains, and to determine the extent of strain‐cultivar interactions for N2 fixation and yield. Host‐by‐strain interactions for N2 fixation were highly significant in a greenhouse experiment utilizing a N‐free aseptic hydroponic system. Inoculation significantly increased total above ground dry matter (AGDM) and seed yields over uninoculated control in field trials conducted at two locations at Tel Hadya, Syria, on a Vertic Chromoxeralf soil during two seasons (1987–1989). All yield and N2 fixation parameters differed significantly among chickpea cultivars and rhizobial strains in the wetter (1987–1988) season where cultivar‐strain interactions were significant for seed yield, N yield, and N2 fixation; differences in N yield and N2 fixation were not significant in the drier season. Average quantities of N2 fixed were 68 and 27 kg N ha−1 in inoculated treatments for 1987–1988 and 1988–1989 trials, respectively. Inoculation with the best strain treatment in 1987–1988 increased the average proportion of N derived from fixation (%Ndfa) from 52% to 72%; maximum %Ndfa for this trial was 81%. Maximum %Ndfa was 67% in the 1988–1989 trial. Significant strain‐by‐cultivar interactions and the yield response to selected rhizobial strains suggest an important role for field inoculation of chickpea for increased N2 fixation.

The limitation of symbiotic nitrogen fixation due to P deficiency restricts the development of a sustainable agriculture, particularly in Mediterrancan and tropical soils. Common bean genotypes, APN18, BAT271, PVA846, POT51, G2633 and G12168, were grown in an aerated N‐free nutrient solution at low (72 μmol plant‐1 week‐1) and control P supplies (360 μmol plant‐1 week‐1). Nitrogenase activity was estimated by in situ measurements of acetylene reduction activity (ARA) in a flow‐through system. During the assays, maximum values of ARA (peak ARA) were reached between 20 and 30 min after exposure to C2H2, depending on P treatment and growth stage. Thereafter, a decline in C2H4 evolution was observed. This decline was most pronounced in low‐P plants and there was a significant genotypic effect. ARA per plant was decreased by P deficiency, mostly because nodulation was delayed and the number and mass of nodules were reduced. The ARA decrease during pod filling was also activated by P deficiency. ARA per g dry weight nodule was increased by P deficiency in G2633 and G12168, unchanged in APN18, BAT271 and POT51 and decreased in PVA846. Except for the climbing type IV G2633, total N at harvest for all P treatments was correlated with the cumulative value of peak ARA and with peak ARA at early pod‐filling which was the highest peak ARA throughout the growth cycle of type III bushy genotypes. We conclude that if phenology and growth habit are carefully considered, peak ARA is a reliable screen of genotypes for N2 fixation tolerance to P deficiency. Selection of lines with early nodulation under P deficiency is also advisable, and the effect of P deficiency on the nodule functioning needs to be considered.