Almost complete and uniform grain sterility in rice over large areas has become a serious problem in the warm and humid lowland region in Sri Lanka. In view of this, the influence of high temperature a t low and very high relative humidity (RH) levels and normal temperature at very low and normal RH levels on the spikelet surface temperature and grain sterility in rice at heading were studied under controlled environments. Almost complete grain sterility in rice was induced by high temperature (35OC day/30°C night) when coupled with high RH (85-90%) at heading. Reduction in RH by 30% at high temperature resulted in decrease in grain sterility but made no significant increase in the completely filled grain percentage which was negligibly low at both very high and low RH levels. Under the normal temperature of 30°C da~125~C night, about twofold increase in grain sterility at very low RH (35-40%) over the lowest recorded 13.8% grain sterility at normal RH (65-70%) was observed. Percent partially filled grains were not found to be influenced by both temperature and RH of the surrounding atmosphere. The spikelet surface temperature in rice was always several degrees less than the atmospheric temperature and increase in atmospheric temperature a t heading resulted in increase in spikelet surface temperature. At a given atmospheric temperature, higher the RH, higher the spikelet surface temperature at heading. Spikelet surface optimum and critical high temperatures for grain sterility appeared to be around 26OC and between 26OC and 31°C but close to 31°C, respectively.

Weedy rice possesses a high diversity in yield attributing characteristics and is an important resource for rice improvement programmes. The present investigation was carried out to estimate the genetic parameters and the correlations of yield attributing characteristics of weedy rice using 370 weedy rice accessions in Sri Lanka. Analysis of variance revealed significant differences (p < 0.05) among the weedy rice accessions for all the characteristics studied, implying the presence of a substantial amount of genetic variability and scope for selection. Shattering percentage, total number of spikelets per plant and the number of filled seeds per panicle exhibited high genotypic and phenotypic coefficients of variation. The degree of difference between phenotypic and genotypic coefficients of variation was relatively low for all the characteristics, except for the total number of spikelets per panicle, indicating comparatively less environmental influence. Very high heritability values coupled with very high genetic advance were observed for the shattering percentage, total number of spikelets per plant and the number of filled seeds per panicle. High heritability values coupled with high genetic advance were recorded for height at the booting stage, number of panicles per plant and tiller number at the seedling and booting stages suggesting the prevalence of additive gene action in the expression of these characteristics and the possible selection for effective improvement based on their phenotypic performance. Correlation studies showed that simply selecting comparatively taller plants with long seeds would identify high yielding weedy rice plants, which may be used in rice improvement programmes.

Farmers themselves are allowed to manage on-farm variety adaptability trials (VATs), to represent farmer management practices in preference to research management. In this approach, within-trial uniformity and thus the scientific value of VATs will be lost. A study was conducted using rice to ascertain whether representation of farmer management in VATs is necessary and to test an approach that represents farmer management without losing within-trial uniformity and the scientific value of VATs. In the proposed approach, both the representation of farmer management and within-trial uniformity are reasonably assured through implementation of site-specific farmer practices by research personnel. This approach was compared with complete research management using recommended practices in a VAT conducted with five rice varieties in six farmers' fields over two seasons. When farmer management was implemented by research personnel, popular varieties among farmers were judged most adaptable. However, when VATs were managed using recommended practices, popular varieties among farmers were judged least adaptable. This provided direct evidence to show the usefulness of representation of farmer management and the validity and practical feasibility of the proposed approach in managing VATs. †

ABSTRACT

Rice (Oryza sativa L.), wheat (Triticum aestivum L.) and maize (Zea mays L.) are the three main cereals that account for the majority of calories in human diets and they will continue to remain the main sources of human nutrition. In order to meet the food demand of increasing population the only option left to increase production of these crops is to increase crop productivity per unit land area in the existing cultivable lands. Grain production of cereals is largely influenced by their yield potential. However, the terms 'yield potential', 'potential yield' and 'maximum potential yield' are synonymously used in the literature to describe different yield ceilings of crop plants without making a clear distinction among them. These terms have been defined in relation to the realized yield at farmer level. This review discusses the methods of estimating yield potential and potential yields of cereals, their physiological basis, and genetic improvement, with special reference to rice. The role of light interception, radiation use efficiency and harvest index as components of a varietal yield potential in relation to their genetic improvement and importance of improving varietal resistance to biotic and abiotic stresses in relation to improving potential yield have been discussed.