Continuous rice (Oryza sativa L.) cropping in Latin America and the Caribbean has resulted in serious weed problems and herbicide overuse. Competitive rice cultivars could help reduce herbicide dependence. A study was conducted during 1994 and 1995 at Palmira, Colombia, to (i) assess the competitiveness of semidwarf irrigated rice plant types adapted to Latin America and the Caribbean's direct‐seeding systems, (ii) identify plant traits responsible for such competitiveness, and (iii) detect adverse effects of competitiveness on rice yield potential. Pregerminated seed of 10 and 14 semidwarf rice cultivars was sown on drained puddled soil in 1994 and 1995, respectively. Cultivars were grown weed‐free or with junglerice [Echinochloa colona (L.) Link] (40 viable seeds m−2, broadcast immediately after seeding rice), and were intermittently irrigated to keep the soil near saturation. Rice and junglerice biomass, leaf area index, tiller number, and height were recorded at 20, 40, 60, 90, and 120 days after emergence (DAE). Rice cultivars differed in their competitiveness against junglerice. Average yield losses ranged from 27 to 62% under saturating junglerice infestations of up to 5.9 Mg DM ha−1. Leaf area index, tiller number, and canopy light interception recorded in competition, and not much before 40 DAE, correlated positively with rice competitiveness. Competitive semidwarf cultivars can substantially reduce the number of herbicide applications in systems where suboptimal water control does not allow weed suppression by flooding. Breeding to enhance rice competitiveness appears as a valid objective, since competitive and also highly productive cultivars were identified in this study.

Upland rice varieties tolerant to acid soil can be sown in the acid, well-drained savannah soils of Latin Amer-When savannas in Latin America are brought into cultivation, rice ica for the profitable production of grain while grazing (Oryza sativa L.) can be sown with the perennial grasses palisadegrass land is renovated (Cadavid and Smith, 1994; Friesen, [Brachiaria brizantha (Hochst. ex A. Rich) Stapf] and signalgrass (B. decumbens Stapf) to harvest a grain crop while establishing a 1994; Guimaraes, 1993). Signalgrass and palisadegrass pasture to suppress weeds and provide grazing in subsequent years. are competitive perennial tropical grasses often un-However, these Brachiaria spp. can reduce upland rice yields. Rice dersown in rice in the savannas of Colombia, Venezuela, cultivars need to be competitive with Brachiaria spp. to maintain and Brazil (CIAT, 1990; Yokohama et al., 1999) to acyields but must allow Brachiaria spp. sufficient growth for pasture complish this goal. For the system to succeed, rice cultiestablishment. Field studies were conducted during 1994 and 1995 on vars must be sufficiently competitive to suppress the a Typic Haplustox oxisol soil in the Eastern Plains of Colombia to vigorous initial growth of the pasture, and yields must evaluate the competitiveness of upland rice cultivars and to identify be high enough to cover the cost of pasture renovation. rice traits for competitiveness. Ten (1994) and 14 (1995) upland riceRice cultivars must also be competitive enough to supcultivars were grown with and without signalgrass in 1994 and palisadegrass in 1995. Rice cultivars differed substantially in their competi-

Wild Phaseolus vulgaris L. accessions containing arcelin codominant alleles 1 through 5 were reconfirmed and characterized for resistance to the Mexican bean weevil, Zabrotes subfasciatus (Boheman) (Coleoptera: Bruchidae). Accession G 02771 (arcelin 5) had the highest level of antibiosis resistance, followed by G 12952 (arcelin 4), G 12882 (arcelin 1) and G 12866 (arcelin 2). Arcelin 3 accessions conferred the lowest levels of resistance. As the presence of arcelin is inherited as a single dominant gene, a backcross breeding program has been used to transfer resistance to the Mexican bean weevil from wild beans to bean cultivars using serological techniques to detect the presence of arcelin and replicated insect feeding tests to measure resistance levels. Progeny containing arcelin 1 showed resistance equal or superior to that of the resistant check. Arcelin 2-derived lines had intermediate levels of resistance while no resistant progenies were obtained from crosses with arcelin 3 and 4 sources. Results are discussed in relation to the deployment of arcelin alleles in bean cultivars.

and Costa Rica (Fischer et al., 1993; Valverde, 1996). density [weed/(weed ϩ rice)], relative leaf area (RLA), and a visualIncreasingly frequent and complex herbicide use to con- estimate of relative ground cover (RC v ). With early weed emergencetrol herbicide-resistant weed biotypes has led to the

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INDEX WORDSPhaseolus vulgaris, field beans, storage proteins, acid PAGE, SDS-PAGE, cultivar identification. SUMMARY Electrophoretic procedures were developed for seed proteins which can discriminate cultivars of field beans. Proteins were extracted from seven varieties and the extracts were analysed using acid and SDS polyacrylamide gel electrophoresis. Electrophoregrams are presented to illustrate the results that can be obtained with the methods described. Results indicate that sufficient variation is present among the seven cultivars examined to afford unambiguous discrimation and identification of the cultivars. Banding patterns were stable for each genotype.

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