Common bean (Phaseolus vulgaris L.) yield, root growth, and nitrogen fixation response to moisture deficits

Common bean (Phaseolus vulgaris L.) is grown on more than 12 million hectares and constitutes the most important food legume for more than 500 million people in Latin America, the Caribbean, and Africa, where it is often grown under moisture deficits in soils with non-optimal pH. The objectives of this study utilized limiting and non-limiting moisture regimes to determine (i) if selected genotypes of common bean exhibited differences in drought resistance as measured by yield, (ii) if drought resistant genotypes had differing root growth, and (iii) if genotypes differed for N fixation. Field studies were conducted at the Agricultural Experiment Station in St. Croix, USVI in 1999 and 2000 to evaluate the effect of moisture deficits on seed yield. Yield of the nine genotypes ranged from 142 to 1508 kg ha−1 in 1999 and 568 to 896 kg ha −1 in 2000. In both years, yield was affected by infestations of common bacterial blight (Xanthomonas campestris pv. phaseoli), Cercospora (Cercospora canescens), and N-deficiency. Geometric mean ranked PR9603-22 and the nodulated (nod) and non-nodulated (nn) isolines of DOR 364, among the top four genotypes with regard to drought resistance in 1999 and 2000. Root length was quantified for 10 root width classes with diameters ranging from 0.01–4.5 mm. Plants in growth pouches (25.4 x 35.6 cm) were grown in the growth chamber containing half-strength Hoagland's nutrient solution (control) or half-strength Hoagland's nutrient solution + 10−6 M (abscisic acid) ABA. The ABA treatment significantly increased total root length (TRL), root length of various root width classes, and root and shoot dry weight. Generally, XAN 176 and SEA5 had a higher TRL than the other genotypes and both had the highest root and shoot dry weight. For plants grown in polyvinyl chloride tubes [(PVC) 0.35 x 0.92 m], water deficit significantly reduced root length in root width classes at all depths except 30.6–45.7 cm and reduced TRL by approximately 75, 38, and 38% at depths of 0–15, 15.1–30.5, and 0–92 cm, respectively. The genotypes XAN 176 and SEA5 were consistently among the lines producing the greatest root length in both stress and non-stress environments. Approximately 97 and 93% of all roots were in root classes ≤1 mm in diameter in plants grown in growth pouches and PVC tubes, respectively. N fixation was estimated via the N difference method, using non-nodulating (nn) isolines of BAT 477 and DOR 364 as the reference crops. Total N-fixed among the genotypes was low, ranging from no fixation (−34.3 kg ha−1) to 19.9 kg ha−1. DOR 364 (nn) gave a higher estimate of N-fixation than did BAT 477 (nn). BAT 477 (nodulated) was one of the genotypes with the highest root-N concentrations as were the higher yielding genotypes XAN 176 and PR9603-22. Nitrogen harvest index values among genotypes ranged from 7 to 76%. Nitrogen use efficiency did not differ among irrigated and rainfed treatments in 1999 but was greater in the irrigated treatment in 2000. Genotypes varied for yield, TRL, NUE, NHI, and N fixation. Growth pouch and PVC studies identified XAN 176 and SEA5 as having high TRL, suggesting that growth pouches may be a viable method for assessing root growth of differing lines.