Physiological analysis of responses of lateral branching of the primary root for tolerance to phosphorus stress and drought in the common bean (Phaseolus vulgaris L.).

Abstract

Common bean (Phaseolus vulgaris L.) is essential to the food security of millions of people in developing nations. However, inadequate precipitation and low soil fertility, mainly phosphorus (P) deficiency, tend to limit its production in smallholder systems. Drought stress severely restricts root growth, binding the capacity of soil water exploration in deep horizons, while phosphorus (P) limitation increases the root capacity of foraging for nutrients in the top soil. Therefore, the development of traits associated with drought resistance and phosphorus stress will contribute to common bean improvement for lines suited for these environments. Thus, the present study aimed to evaluate the physiological response of lateral branching of the primary roots for tolerance to low phosphorus and drought stress in the common bean. To achieve this, pot and field experiments were established at the Agricultural Research Institute of Mozambique (IIAM) - Chókwè Research Station to: (1) determine the effect of drought stress on crop performance at different bean growth stages in the field and pot trials, (2) evaluate phosphorus use efficiency on grain yield efficiency index (GYEI) and P concentration in the plant tissues of selected genotypes in a pot study, and (3) assess the contribution of root phenes to shoot biomass and grain yield under combined stress (drought and low P) in the field and pot study. Eight genotypes were tested for shoot biomass, root biomass, shoot P concentration and uptake assessment in the pot study. For the field experiments, a total of 24 bean lines were used to assess phenology, yield components and total yield per unit. All pot studies were set using a randomized complete block design in a factorial arrangement with four replications. The first study had two levels of water (water stress and irrigated- no water stress); while the second study had two treatments combining phosphorus levels x genotypes, grown in a system of stratified phosphorus, 0.025 g P kg−1of soil as low rate and 0.2 g P kg−1 of soil as high P rate. The last pot experiment combined both water and phosphorus levels and, in all cases, eight genotypes

were tested. The field experiment was set as a split plot design with four replications for the drought and phosphorus studies, while for the combined study it was set as a splitsplit plot. Water stress lowered substantially the leaf relative water content, leaf water potential and all growth parameters in the pot experiment, decreasing shoot biomass by 47%, leaflet growth rates by 49% and number of leaflets by 53%. In spite of significant reduction, these variables were highly and positively correlated, and can be recommended for early selection of genotypes grown under limited water conditions. Phosphorus levels also significantly affected shoot and root biomass, shoot phosphorus concentration, phosphorus uptake and phosphorus leachate under high P levels. However, genotypes responded differently to phosphorus levels in term of root biomass, shoot biomass and P uptake; genotypes BFS 81, SEQ342-87 and IBC 301-204 performing better in terms of root and shoot biomass as well as P uptake. These genotypes can be recommended for early selection under low soil fertility, especially in soils with P deficiency. Under field conditions, drought stress had a significant effect (p<0.05) on yield components. Among all the genotypes evaluated, SEF 16, SX 14825- 7-1, TARS MST-1, SEN 52, BRT103- 182, FBN1211-66, IBC 301-204, SER 125 and MHR 311-17 were the most adapted and showed the best yield performances under drought stress, and therefore can be incorporated in breeding programs particularly in drought prone areas. Meanwhile, high phosphorus treatment significantly increased all yield components (pods per plant, 100- seed weight, and grain yield), and variability among genotypes was also observed for yield and yield components. Based on the grain yield efficiency index (GYEI), 10 bean lines out of the 24 were categorized as P use efficient genotypes and therefore, they can be used in phosphorus deficient soils as well as crop improvement program. Grain yield from field data under drought stress and low P were positively correlated with the pot data on root traits. Response of root phene to drought and phosphorus stress

appeared to be related to the phenotype traits of water stress and P use efficiency (that is, deep and shallow rooted systems, respectively). Deeper rooted genotypes produced more total root biomass, high taproot lateral branching density, which resulted in high total root length under drought and low P stress. On the other hand, shallow rooted genotypes allocated relatively low total root biomass and less allocation of taproot lateral branching. Increase in shoot biomass and seed yield in drought and low P stress was associated with higher mean values of taproot lateral branching density and total taproot length. Genotypes SER 125, BFS 81, FBN12111-66 and MER 22 11-28 showed greater score of taproots branching density in the pot study with the highest grain yield in the field under low P and drought stress. Therefore, these genotypes can be used in phosphorus deficient soils and drought stress environments or serve as parents for improving phosphorus use efficiency and drought tolerance in common bean. An increased total taproot lateral branching and length observed under stressful conditions (drought stress), contributed for plant performance, and could be considered as an important trait for selecting cultivars. Although no previous study has explicitly explored the utility of taproot lateral length and lateral branching for plant performance under edaphic stress, results from the present study suggest that an increased taproot lateral branching and length provide benefits under multiple environments. Genotypes that exhibited higher scores of taproot lateral branching in the field such as SER 125, BFS 81, FBN12111- 66 and MER 22 11-28, combining with higher grain yield under low P and drought were ranked as deeper rooted and suited to environments where water is limiting. In contrast, genotypes INB 814, SEN52, BIOF 2-06 and SEQ342-87 had relatively low scores of tap root branching density, but with better yield under low P and drought, and were classified as shallow rooted and suited to environments where P is limiting. Keywords: common bean, taproot lateral branching, drought, phosphorus use efficient