No abstract
The trend to commercially exploit a reduced genetic base in most crops has raised concerns about the possibility of large yield reductions as a result of environmental disturbances. This study describes the genetic base of irrigated rice (Oryza sativa L.) from Latin America and the Caribbean. Schematic pedigrees of 143 cultivars released between 1971 and 1989 were constructed, and releases were grouped by the periods 1971 to 1975, 1976 to 1980, 1981 to 1985, and 1986 to 1989. The contribution of landraces was estimated by assuming they provided 50% of their genes in each single cross. Total contribution was calculated by summing within and across pedigrees and expressed as percentage. Genetic diversity for the 1987‖1988 crop was estimated by weighting contribution by area planted. Coefficients of parentage. (r) among cultivars and among the parents of cultivars developed the Colombian program were calculated. A total of 101 different landraces contributed to the genetic composition of the cultivars released. Number of landraces per period was proportional to the number of cultivars released. A group of 14 ancient cultivars contributed to at least one cultivar during the four periods considered, accounting for nearly 70% of the genes and contributing to an increased relationship among parents of cultivars. This group can be considered as the genetic core of irrigated rice in the region. The major landraces in the core were ‘Cina’, ‘Lati Sail’, and ‘Dee Geo Woo Gen’, mainly introduced to the region through ‘IR8’. An additional group of eight landraces was incorporated into the core after 1975, primarily to incorporate disease resistance. Mean coefficient of parentage within countries varied from r = 0.41 in Costa Rica to 0.00 in Chile. Cultivars from Argentina, Chile, Guyana, and Surinam often showed r < 0.05 with materials from other countries. Genetic variability has been broadened, combining different genes with the genetic core as needed. The identification of new genetic cores to reduce vulnerability while increasing yield potential should be considered.
Evaluation of milling performance of rice (Oryza sativa L.) cultivars often has resulted in a wide range of values depending on the growing environment, and harvesting and drying methods. A common stress affecting milling yield evaluations is the delay in harvesting. This experiment was designed to evaluate the performance of 16 rice cultivars under 2 wk of harvest delay. Cultivar plantings were staggered to obtain simultaneous maturity during three consecutive seasons at Palmira, Colombia. A combination of harvesting and threshing‐time treatments was used. Harvesting was done at maturity, and 1 and 2 wk later. Threshing was done immediately after harvest and with a delay of 4 d. Total milled‐ and head‐rice yields were measured. Cultivar response to delayed harvesting followed four patterns ranging from constant head‐rice yields throughout the period to significant weekly reductions. Threshing delays reduced head‐rice yields by an average of 5.8% for all cultivars. Total milled‐rice yields was reduced 3% across cultivars by a 2‐week delay in harvest or by a 4‐day delay in threshing. Cultivar performance patterns detected could be used to check the reaction of other cultivars for harvest delay effects.
The narrowing gap between commercial and experimental yields in irrigated rice (Oryza sativa L.) has renewed interest in genetic yieldimprovement. Evaluation of past gains is useful for identifying areas with potential for further enhancement. Tall rice cultivar Bluebonnet 50, introduced semidwarfs ‘IR8’ and ‘IR22’, and 13 semidwarfs developed locally from 1971 to 1989 were grown in Colombia under irrigation and rainfed conditions with and without blast (Pyricularia grisea, Sacc.) control during 1992 and 1993. The objective was to estimate progress from introduction and breeding of semidwarfs. Both types of semidwarfs yielded 80 and 122% more than Bluebonnet 50 under irrigation and rainfed conditions with blast control, respectively. Without blast control, the yield advantage (126%) was significant only in 1993. Tillering of semidwarfs was higher in all environments, ranging from 379 to 721 panicles per square meter versus 173 to 392 panicles per square meter for Bluebonnet 50. Panicles were smaller, ranging from 61 to 95 filled grains per panicle versus 118 to 187 filled grains per panicle for Bluebonnet 50. Mean grain yields of introduced and local semidwarfs were similar, but the latter were 10 to 15 cm taller. Linear regressions of means of local cultivars on year of release were nonsignificant (P > 0.05) for the irrigated and rainfed‐with‐blast control environments. For rainfed conditions without blast control, yield increased 0.13 ± 0.041 ba−1 yr−1; panicle size (1.1 ± 0.4 grains yr−1) and plant height (1.1 ± 0.2 cm yr−1) also increased. Breeding in Colombia has improved adaptation of semidwarf rice to rainfed environments.
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