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-

No abstract

No abstract

· ln ali the crops studied there was variation among genotypes in carbon isotope discrimination (A). ln trials of barley genotypes there were strong positive correlations between grain yield and A, though in trials which were irrigated or received abundant rainfa\1, this correlation was non-significant or negative. Sorne of the variation in yield and A was related ta date of heading, as early varieties in stressed trials had highest yields and had high A.ln an experiment with 8 early and 1 0 lata !lowe ring rioe genotypes the re were consistent positive correlations between yield and A in the early group, but in the late group the correlations tended to be negative. Among the early group the early genotypes were highest yielding, and may be presumed to have avoided the worst effects of drought and hence had higher A. ln the Jale group, there was no correlation between yield and date of flowering. ln one experiment with cowpeas, a weak positive correlation between grain yield and t. was observed, but in a second trial, in an abnormally wet season, no such correlation was observed.ln ali three crops further work is needed before the usefulness of carbon isotope discrimination for screening for high yield in water-limited environments can be assessed.Résumé.-Chez toutes les espèces cultivées étudiées, il a été observé des différences entre les génotypes en ce qui concerne la discrimination isotopique du carbone (t.). Chez l'Orge, il existe une corrélation positive très étroite entre le rendement en grains et A, bien que chez les plantes bien irriguées ou ayant reçu une pluviosité abondante cette corrélation ne soit pas significative ou même devient négative. Certaines variations dans le rendement et A sont reliées à la date de montaison, les variétés précoces ayant des rendements plus élevés et une valeur plus grande du A, dans les lots stressés. Chez le Riz (8 génotypes à floraison précoce et 10 à floraison tardive), on a observé une corrélation positive entre le rendement et A chez les génotypes précoces, mais chez les génotypes tardifs, les corrélations tendent à devenir négatives. Dans le groupe précoce, les génotypes précoces ont le rendement le plus élevé, et on suppose qu'ils ont évité les effets les plus néfastes da la sécheresse et donc on un t. plus élevé. Dans le groupe tardif, il n'y a pas de corrélation entre le rendement et la date de floraison. ©Société botanique de France 1990 -ISSN 0181 • 1789.Downloaded by [5.8.47.58] at 20:11 04 June 2016 22 ACTUALITES BOTANIQUES Chez le Niébé, dans une expérience, il a été observé une corrélation positive, mais faible entre la production en grains et li, mais dans une autre expérience qui a eu lieu au cours d'une saison anormalement humide, aucune corrélation n'a été mise en évidence.Chez les 3 espèces cultivées, des études plus poussées sont donc nécessaires avant de conclure à la possiblilité d'utiliser la discrimination isotopique du carbone pour le screening de variétés à hauts rendements et résistants aux environnements limitants en eau.

-Field experiments involving upland rice genotypes, sown in various dates in late season, were carried out to assess the relationship of carbon isotope discrimination with grain yield and drought resistance. In each one of the three years, one trial was kept under good water availability, while other suffered water shortage for a period of 18-23 days, encompassing panicle emergence and flowering. Drought stress reduced carbon isotope discrimination measured on soluble sugars (∆s) extracted from stem uppermost internode at the end of the imposition period, but had relatively less effect on bulk dry matter of leaves, sampled at the same period, or that of uppermost internodes and grains, sampled at harvest. The drought-induced reduction in ∆s was accompanied of reduced spikelet fertility and grain yield. In the three trials subjected to drought, genotypes with the highest yield and spikelet fertility had the lowest ∆s. However, this relationship was weak and it was concluded that ∆s is not a sufficiently reliable indicator of rice drought resistance to be useful as a screening test in breeding programs. On the other hand, grain yield and spikelet fertility of genotypes which were the soonest to reach 50% flowering within the drought imposition period, were the least adversely affected by drought. Then, timing of drought in relation to panicle emergence and to flowering appeared to be a more important cause of yield variation among genotypes than variation in ∆s.Index terms: Oryza sativa, internodes, spikelets fertility. DISCRIMINAÇÃO ISOTÓPICA DE CARBONO E PRODUTIVIDADE DO ARROZ DE SEQUEIROAFETADO POR SECA NO FLORESCIMENTO RESUMO -Experimentos de campo, envolvendo genótipos de arroz de sequeiro, em várias datas de semeadura, realizadas tardiamente na estação de cultivo, foram conduzidos para investigar a relação da discriminação isotópica de carbono com a produtividade de grãos e resistência à seca. Em cada ano, um experimento foi mantido em boas condições hídricas, enquanto o outro sofreu falta de água, por período de 18-23 dias, abrangendo a emergência da panícula e florescimento. A deficiência hídrica reduziu a discriminação isotópica de carbono dos açúcares solúveis (∆s) extraídos do último entrenó do colmo, ao final do período de estresse, causando um efeito relativamente menor sobre a matéria seca das folhas amostradas na mesma ocasião, ou dos entrenós superiores e grãos amostrados na colheita. A redução de ∆s induzida por deficiência hídrica foi acompanhada de redução da fertilidade das espiguetas e do rendimento dos grãos. Nos três experimentos submetidos a estresse hídrico, os genótipos de maior rendimento e fertilidade de espiguetas apresentaram os menores valores de ∆s. Esta relação, no entanto, foi fraca, concluindo-se, assim, que ∆s não é um indicador seguro da resistência à seca em arroz, para ser utilizado com eficiência em programas de melhoramento. Por outro lado, o rendimento de grãos e a fertilidade das espiguetas dos genótipos que apresentaram o florescimento mais precocemente no perío...

No abstract

No abstract