Common beans (Phaseolus vulgaris L.) originated in the New World and are the grain legume of greatest production for direct human consumption. Common bean production is subject to frequent droughts in highland Mexico, in the Pacific coast of Central America, in northeast Brazil, and in eastern and southern Africa from Ethiopia to South Africa. This article reviews efforts to improve common bean for drought tolerance, referring to genetic diversity for drought response, the physiology of drought tolerance mechanisms, and breeding strategies. Different races of common bean respond differently to drought, with race Durango of highland Mexico being a major source of genes. Sister species of P. vulgaris likewise have unique traits, especially P. acutifolius which is well adapted to dryland conditions. Diverse sources of tolerance may have different mechanisms of plant response, implying the need for different methods of phenotyping to recognize the relevant traits. Practical considerations of field management are discussed including: trial planning; water management; and field preparation.

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An estimated 60% of common bean (Phaseolus vulgaris L.) production worldwide is at risk of drought. A breeding program was developed at the International Center of Tropical Agriculture (CIAT) to create drought resistant breeding lines with varietal potential in the small red, small black, cream (mulatinho) and cream‐striped (carioca) grain classes. Breeding populations were created from triple or double crosses. Field screening under terminal drought was performed at Palmira, Colombia in the dry season in F2, F3:5, and F6:8 generations over two cycles of recurrent selection in the small red and small black classes, and one cycle in the mulatinho and carioca classes. Drought resistant lines yielded significantly more than commercial check cultivars under drought in all color classes. Some outyielded the respective checks by 15 to 25% (depending on color class and trial) in one or more of three favorable environments, or in the combined analysis across favorable environments, and were also earlier to mature. Drought resistant lines presented up to 36% greater yield d−1 in favorable environments. Some also expressed superior yields in a phosphorus‐limited environment. Thus, selection for drought resistance has improved yield potential and plant efficiency across different environments. It is suggested that selection under drought stress reveals genes that correct inefficiencies inherited from the wild Phaseolus vulgaris, and are key to yield improvement of common bean.

Soybean plants (Glycine max [L.] Merr var Amsoy 71) were grown in growth chambers with high-phosphorus (high-P) and low-phosphorus (low-P) culture solutions. Low-P treatment reduced shoot growth significantly 7 days after treatment began. Root growth was much less affected by low-P, there being no significant reduction in root growth rate until 17 days had elapsed. The results suggest that low-P treatment decreased soybean growth primarily through an effect on the expansion of the leaf surface which was diminished by 85%, the main effect of low-P being on the rate of expansion of individual leaves. Low-P had a lesser effect on photosynthesis; light saturated photosynthetic rates at ambient and saturating CO2 levels were lowered by 55 and 45%, respectively, after 19 days of low-P treatment. Low-P treatment increased starch concentrations in mature leaves, expanding leaves and fibrous roots; sucrose concentrations, however, were reduced by low-P in leaves and increased in roots. Foliar F-2,6-BP levels were not affected by P treatment in the light but in darkness they increased with high-P and decreased with low-P. The increase in the starch/sucrose ratio in low-P leaves was correlated primarily with changes in the total activities of enzymes of starch and sucrose metabolism.Suboptimal phosphorus supply diminishes photosynthetic C02-fixation rates (2,24) and the expansion of the photosynthetic leaf surface (5,22). It may also lead to decreased cytosolic orthophosphate levels (26). Orthophosphate (Pi) is thought to regulate the activities of several enzymes involved in starch and sucrose metabolism in vivo and the export of C out of the chloroplast via the Pi-translocator (3). However, much of the evidence in support of Pi as a key regulator of carbon partitioning has been obtained with in vitro systems. In this paper, we study the nutritional effects of Pi on growth, photosynthesis, and starch/sucrose metabolism in an intact plant system; soybean plants were treated with sufficient P for optimal plant growth (high-P) and with suboptimal P levels (low-P). The high-P and low-P solutions were similar except that the high-P solution contained 200 Mm KH2PO4 and the low-P solution contained 10 Mm KH2PO4. Both high-P and low-P culture solutions contained 9.125 mM N03-N, 0.625 mM NH4+-N, 4 mm K+, 1.0 mm S, 2.5 mm Ca2+, 1.0 MM Mg2+, 250 Mm Na+, 250 gM Si, 500 Mm C1-, 50 Mm FeHEDTA, 25 Mm B, 1.0 gM Mn, 1.0 gM Zn, 0.4 gM Mo, and 0.4 uM Cu. METHODS AND MATERIALSThe presence of Si in combination with a 10-fold reduction in the Mn concentration in solution ameliorated a putative Mn toxicity symptom in both high-P and low-P plants. Gas ExchangeGas exchange analyses were performed on randomly selected high-P and low-P plants at 18 to 20 d after transplant. Fully expanded trifoliates at the second or third node numbered basipetally from the first primary trifoliate were used for both irradiance and CO2 saturation curve determinations using steady state gas exchange equipment described previously (25). Co2 concentrations of...

Regulating nitrification could be a key strategy in improving nitrogen (N) recovery and agronomic N-use efficiency in situations where the loss of N following nitrification is significant. A highly sensitive bioassay using recombinant luminescent Nitrosomonas europaea, has been developed that can detect and quantify the amount of nitrification inhibitors produced by plants (hereafter referred to as BNI activity). A number of species including tropical and temperate pastures, cereals and legumes were tested for BNI in their root exudate. There was a wide range in BNI capacity among the 18 species tested; specific BNI (AT units activity g -1 root dry wt) ranged from 0 (i.e. no detectable activity) to 18.3 AT units. Among the tested cereal and legume crops, sorghum [Sorghum bicolor (L.)], pearl millet [Pennisetum glaucum (L.) R. Br.], and groundnut [Arachis hypogaea (L.)] showed detectable BNI in root exudate. Among pasture grasses, Brachiaria humidicola (Rendle) Schweick, B. decumbens Stapf showed the highest BNI capacity. Several high-and low-BNI genotypes were identified within the B. humidicola species. Soil collected from field plots of 10 year-old high-BNI genotypes of B. humidicola, showed a near total suppression (>90%) of nitrification; most of the soil inorganic N remained in the NH 4 + form after 30 days of incubation. In contrast, soils collected from low-BNI genotypes did not show any inhibitory effect; most of the soil inorganic N was converted to NO 3 -after 30 days of incubation. In both the high-and low-BNI genotypes, BNI was detected in root exudate only when plants were grown with NH 4 + , but not when grown with NO 3 -as the sole source of N. BNI compounds when added to the soil inhibited nitrification and the relationship was linear (r 2 = 0.92 ** ; n = 12). The BNI from high-and low-BNI types when added to N. europaea in pure culture, blocked both the ammonia monooxygenase (AMO) and the hydroxylamine oxidoreductase (HAO) pathways. Our results indicated that BNI capacity varies widely among and within species; and that some degree of BNI capacity is likely a widespread phenomenon in tropical pasture grasses. We suggest that the BNI capacity could either be managed and/or introduced into pastures/crops with an expression of this phenomenon, via genetic improvement approaches that combine high productivity along with some capacity to regulate soil nitrification process.

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Common bean (Phaseolus vulgaris L.) is the most important food legume in the diet of poor people in the tropics. Drought causes severe yield loss in this crop. Identification of traits associated with drought resistance contributes to improving the process of generating bean genotypes adapted to these conditions. Field studies were conducted at the International Center for Tropical Agriculture (CIAT), Palmira, Colombia, to determine the relationship between grain yield and different parameters such as effective use of water (EUW), canopy biomass, and dry partitioning indices (pod partitioning index, harvest index, and pod harvest index) in elite lines selected for drought resistance over the past decade. Carbon isotope discrimination (CID) was used for estimation of water use efficiency (WUE). The main objectives were: (i) to identify specific morpho-physiological traits that contribute to improved resistance to drought in lines developed over several cycles of breeding and that could be useful as selection criteria in breeding; and (ii) to identify genotypes with desirable traits that could serve as parents in the corresponding breeding programs. A set of 36 bean genotypes belonging to the Middle American gene pool were evaluated under field conditions with two levels of water supply (irrigated and drought) over two seasons. Eight bean lines (NCB 280, NCB 226, SEN 56, SCR 2, SCR 16, SMC 141, RCB 593, and BFS 67) were identified as resistant to drought stress. Resistance to terminal drought stress was positively associated with EUW combined with increased dry matter partitioned to pod and seed production and negatively associated with days to flowering and days to physiological maturity. Differences in genotypic response were observed between grain CID and grain yield under irrigated and drought stress. Based on phenotypic differences in CID, leaf stomatal conductance, canopy biomass, and grain yield under drought stress, the lines tested were classified into two groups, water savers and water spenders. Pod harvest index could be a useful selection criterion in breeding programs to select for drought resistance in common bean.

Sugar beets (Beta vulgaris L. cv F58-554H1) were cultured hydroponically for 2 weeks in growth chambers with two levels of orthophosphate (Pi) supplied in half strength Hoagland solution. Low-P plants were supplied with 1/20th of the Pi supplied to control plants. With low-P treatment, the acid soluble leaf phosphate and total leaf P decreased by about 88%. Low-P treatment had a much greater effect on leaf area than on photosynthesis. Low-P decreased total leaf area by 76%, dry weight per plant by 60%, and the rate of photosynthesis per area at light saturation by 35%. Low-P treatment significantly decreased the total extractable activity of phosphoglycerate kinase (by 18%) and NADPglyceraldehyde-3-phosphate dehydrogenase (by 16%), but did not decrease the total activities of ribulose-1,5-bisphosphate (RuBP) carboxylase (RuBPCase) and ribulose-5-phosphate kinase. Low-P treatment decreased the initial activities of three rate-limiting Calvin cycle enzymes, but had no effect on the initial activity of RuBPCase. Furthermore, low-P treatment significantly increased the total extractable activities of fructose-1,6-bisphosphatase (by 61%), fructose-1,6-bisphosphate aldolase (by 53%), and transketolase (by 46%). The results suggest that low-P treatment affected photosynthetic rate through an effect on RuBP regeneration rather than through RuBPCase activity and that the changes in Calvin cycle enzymes with low-P resulted in an increased flow of carbon to starch.