H.R. Lafitte

Physiological mechanisms contributing to the increased N stress tolerance of tropical maize selected for drought tolerance

Abstract An improved response of crop varieties to various stress factors may be associated to constitutive stress tolerance mechanisms that increase yield and yield stability. Increased leaf longevity, increased water and nutrient uptake, greater assimilate supply during grain filling, and increased grain and ear set have been associated with constitutive stress tolerance mechanisms in maize ( Zea mays L.). We examined tropical maize for adaptive changes associated with drought tolerance that are sustained under N stress and therefore may indicate constitutive stress tolerance mechanisms. Original and drought-tolerant selection cycles of four populations were evaluated in five experiments differing in N supply at Poza Rica, Mexico between 1992 and 1994. Selection for tolerance to mid-season drought stress consistently increased grain yield across N levels due to an increase in both the number of ears per plant and kernel weight. The number of ears per plant was associated with a shorter anthesis–silking interval (ASI) of drought-tolerant cycles. Reduction in ASI due to selection was greater under N stress as compared to well-fertilized conditions, however, it was not associated with either biomass or N accumulation of plants and ears around flowering. The N content of individual kernels did not change with selection and grain N concentration decreased. Greater kernel weights were likely the result of delayed leaf senescence and increased assimilate supply during grain filling. We conclude that decreased ear abortion and increased assimilate supply during grain filling of maize selected for tolerance to mid-season drought also provide tolerance to N stress and therefore may contribute to increased yield and yield stability.

Genetic improvement of rice in aerobic systems: progress from yield to genes

Abstract Genetic improvement of rice for aerobic (non-flooded) environments has received less attention than breeding for lowland production systems. Aerobic rice has traditionally been grown in low-input systems, but as fresh water for irrigation becomes increasingly scarce, aerobic rice cultivation is expected to expand into regions with more intensive cropping. The primary yield constraints for the low-input aerobic crop include water deficit, acid and infertile soils, weed competition, and disease. Yield potential has been improved through traditional breeding approaches, and some improved upland rice cultivars show a similar pattern of interactions with environments as traditional cultivars. Critical environmental factors that interact with genotype are the distribution of rainfall during the season, the amount of solar radiation received in the period just prior to flowering, and disease pressure. In systems where adequate inputs are applied, aerobic rice tends to yield less than lowland rice, and yield reductions are dramatic when water deficit occurs. The poor adaptation of the lowland cultivar IR72 to aerobic soils is associated with reduced height and harvest index in aerobic conditions, but IR72 had similar biomass production by anthesis as better-adapted upland cultivars. The best-yielding upland lines had very stable pre-anthesis biomass production across five contrasting environments. The physiological and molecular dissection of aerobic rice yield is expected to identify opportunities to accelerate progress in the areas of aerobic adaptation, tolerance to water deficit, and improved weed competitiveness. QTLs have been reported for a number of traits potentially related to performance under water deficit, such as improved root morphology and osmotic adjustment. In an upland-by-lowland mapping population, alleles from the lowland cultivar contributed significantly to improved yield in aerobic environments.

Rice root morphological traits are related to isozyme group and adaptation

Abstract Rice accessions from the International Rice Research Institute (IRRI) germplasm bank were evaluated for root traits of 40-day-old plants grown in soil in the greenhouse. The 136 accessions represented six groups defined on the basis of isozyme classification, with isozyme group six further subdivided on the basis of origin and morphology. An additional 28 rice cultivars were evaluated for seminal root xylem vessel diameter when grown in pots in a growth chamber. Rice groups differed in root thickness, root xylem vessel diameter, root:shoot ratio, and patterns of root distribution. Isozyme group 1, which corresponds generally to the indica subspecies, had thin, superficial roots with narrow vessels and a low root:shoot ratio. The other major isozyme group, group 6, comprising japonica types, was characterized by thick roots with wider vessels, a greater proportion of the root weight below 15xa0cm, and a larger root:shoot ratio. On an average, the bulu and temperate group 6 accessions were similar to the non- bulu types except that their root:shoot ratios and proportion of root weight above 15xa0cm were more similar to group 1. Group 2, with aus types from South Asia, was characterized by intermediate root thickness, but vertical root distribution and root:shoot ratio were more similar to group 6. The minor isozyme groups 3–5 were represented by few accessions, and in general, they had root thickness and root distribution profiles more similar to group 1 than to group 6. While significant differences were observed among isozyme groups for all the traits under study, there was significant variation within groups and groups overlapped for all traits measured. This study highlights the wide range of variability for constitutive root traits in rice. For example, root thickness ranged from 0.68 to 1.04xa0mm, seminal root xylem vessel diameters from 30 to 58xa0μm, root:shoot ratios from 0.05 to 0.21, and accessions had from 44 to 73% of the total root weight concentrated in the surface 15xa0cm of soil. For the 28 cultivars evaluated, root xylem vessel diameter was highly correlated with reported values of leaf epicuticular wax content ( r =0.89). These values indicate the range of genetic variation within the rice genome for root morphological traits.

Research opportunities to improve nutrient-use efficiency in rice cropping systems

Abstract This is a summary of discussions and conclusions from a workshop on nutrient-use efficiency of rice cropping systems, held 13–15 December 1995 at the International Rice Research Institute, Los Banos, Philippines [Field Crops Research, Special Issue, 1998]. Workshop participants reviewed current research in crop management and genetic improvement of rice, and concluded that increased rice production will depend on explicit considerations of nutrient supply. For intensive irrigated rice systems, research priorities include understanding how soil nutrient supply is linked to cropping intensity, what pattern of nutrient supply is required to achieve the high yield levels needed to meet the needs of rice consumers in the next century, and how to improve congruence of nutrient supply and crop demand through management. In rainfed environments, nutrient demand is closely linked to water availability, and participants saw a need to characterize environments accoring to the different patterns of resource limitations. Soil and climate databases will be important in this effort. An explicit conceptual framework must underlie field work in these variable environments, and simple simulation models were identified as essential tools for targeting experiments and pre-testing improved technologies. Cultivar improvement for efficient nutrient use can complement agronomic approaches. The potential gains in improved nutrient acquisition and/or improved physiological efficiency are not clear for all nutrients, and these need to be estimated in order to set breeding priorities. Improved screening techniques related to specific mechanisms of nutrient efficiency will facilitate genetic improvement. Promising research approaches are described to address each of these constraints to improving nutrient-use efficiency in rice.