development of drought-resistant cultivars using physiomorphological traits in rice
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Field Crops Research
ELSEVIER Field Crops Research 40 ( 1995) 67-86
Development of drought-resistant cultivars using physio- morphological traits in rice
S. Fukai *, M. Cooper Department of Agriculture, The University of Queensland, Brisbane, Qld. 4072, Australia
Received 17 May 1994; accepted 22 October 1994
Drought is a major problem for rice grown under rainfed lowland and upland conditions, but progress in breeding to improve drought resistance has been slow. This paper describes patterns of water-stress development in rice fields, reviews genetic variation in physio-morphological traits for drought resistance in rice, and suggests how knowledge of stress physiology can contribute to plant breeding programmes that aim to increase yield in water-limiting environments. To provide a basis for integrating physiological research with plant-breeding objectives we define drought resistance in terms of relative yield of genotypes. Therefore, a drought-resistant genotype will be one which has a higher grain yield than others when all genotypes are exposed to the same level of water stress.
A major reason for the slow progress in breeding for drought resistance in rice is the complexity of the drought environment, which often results in the lack of clear identification of the target environment(s). There is a need to identify the relative importance of the three common drought types; early-season drought which often causes delay in transplanting, mild intermittent stress which can have a severe cumulative effect, and late stress which affects particularly late-maturing genotypes. In addition, in rainfed lowland rice, flooded and non-flooded soil conditions may alternate during the growing season, and affect nutrient availability or cause toxicity.
Several drought-resistance mechanisms, and putative traits which contribute to them, have been identified for rice; important among these being drought escape via appropriate phenology, root characteristics, specific dehydration avoidance and tolerance mechanisms, and drought recovery. Some of these mechanisms/traits have been shown to confer drought resistance and others show potential to do so in rice. The most important is the appropriate phenology which matches crop growth and development with the water environment. A deep root system, with high root length density at depth is useful in extracting water thoroughly in upland conditions, but does not appear to offer much scope for improving drought resistance in rainfed lowland rice where the development of a hard pan may prevent deep root penetration. Under water-limiting environments, genotypes which maintain the highest leaf water potential generally grow best, but it is not known if genotypic variation in leaf water potential is solely caused by root factors. Osmotic adjustment is promising, because it can potentially counteract the effects of a rapid decline in leaf water potential and there is large genetic variation for this trait. There is genotypic variation in expression of green leaf retention which appears to be a useful character for prolonged droughts, but it is affected by plant size which complicates its use as a selection criterion for drought resistance.
There is a general lack of drought related research for rice in rainfed lowland conditions. This needs to be rectified, particularly considering their importance relative to upland conditions in Asian countries. We suggest that focussing physiological-genetic research efforts onto clearly defined, major target environments should provide a basis for increasing the relevance of stress physiology and the efficiency of breeding programmes for development of drought-resistant genotypes.
* Corresponding author.
0378-4290/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSD10378-4290(94)00096-4
68 S. F&i, h4. Cooper/Field Crops Research 40 (1995) 67-86
Keywords: Drought resistance; Genotype by environment interaction; Plant breeding; Orym S&XI; Rainfed lowland; Upland; Rice; Stress physiology
Rice is a semi-aquatic plant that is commonly grown under flooded conditions. However, about half of the rice area in the world does not have sufficient water to maintain flooded conditions and yield is reduced to some extent by drought, defined here as a period of no rainfall or no irrigation that affects crop growth (Han- son et al., 1990). While the total area of upland rice is much smaller than that of rainfed lowland rice, drought physiology research has been concentrated mostly on upland rice to identify responses of various growth processes to water stress. Recently a coordinated research effort for rainfed lowland rice commenced with the formation of the Rainfed Lowland Rice Research Consortium.
In the rainfed rice-growing areas, there is a wide range of water-stress environments, differing in both the timing and intensity of water stress. Within each rice-growing area which is targeted by a plant breeding programme, there may still be a mixture of different types of water-stress environments. Developing an understanding of the target environment is critical to the success of translating drought physiology research into successful crop improvement strategies. Physio- morphological traits which may confer drought resis- tance (i.e. putative traits) have been identified in some crops, for example for sorghum (Ludlow and Muchow, 1990). Similarly in rice, a large number of traits which may affect grain yield under drought conditions have been suggested (OToole, 1982). Some traits appear more important than others, but their usefulness in increasing grain yield has not been confirmed in most cases. Despite our increased understanding of stress physiology, the development of drought-resistant cul- tivars, i.e. cultivars which produce higher yield than others in drought conditions, has been slow in rice and other crops.
Plant breeding aims to produce cultivars suitable for a defined target population of environments. Generally, advanced lines from breeding programmes are tested at a number of locations within the target geographical region for a number of years before release as new cultivars. Consequently, traditional plant breeding
takes time, and new strategies are continually sought to improve the efficiency of plant breeding pro- grammes. A major reason for the slow progress in developing drought-resistant rice cultivars is the inci- dence of large genotype by environment (G X E) inter- actions, which result from a combination of differences in genotypic adaptation and the heterogeneous envi- ronments within the target areas. A consequence of G X E interactions is that particular lines do not perform well under all conditions encountered in the target pop- ulation of environments, complicating selection of new cultivars. The effect of environmental variation on gen- otypic performance and GX E interactions is com- monly examined in terms of year-to-year and site-to-site variation (Evenson et al., 1978). While use- ful information on the relative performance of geno- types can be obtained from such analyses, they rarely provide an explanation of the environmental factors which cause G X E interactions. Where water stress is a common occurrence in the target population of envi- ronments, environmental characterisation in terms of the way that water stress develops should provide a basis for interpreting G X E interactions.
This paper reviews the patterns of water-stress devel- opment in rainfed lowland and upland rice environ- ments, discusses a number of physio-morphological traits which might be exploited in improving drought resistance and some of the problems associated with G X E interactions in physiological studies of drought resistance. It goes on to suggest ways of improving the efficiency of rice breeding programmes by adopting physiological approaches that are appropriate for the target population of environments.
2. Water-stress development and adaptation mechanisms
2.1. Definition of drought environments
It is essential to define the types of drought environ- ment which are encountered in the target population of environments for each plant breeding programme. It is simply not sufficient in most cases to say that a breeding
S. F&i, M. Cooper/ Field Crops Research 40 (1995) 67-86 69
programme is designed to improve drought resistance in rice, because different types of drought may require different approaches in the programme. Commonly, the effects of water stress in plant breeding trials are quan- tified in terms of the reduction in mean yield of the trials relative to higher yielding or control trials. This form of environmental index provides little or no infor- mation on the way in which water stress developed in each trial and therefore limited scope for defining the types of drought environment encountered. Conse- quently, the majority of trials are conducted without knowledge of the type of stress which occurred. Given that different traits confer adaptation to different types of stress, there is a need to define the drought environ- ment to enable assessment of the relevance of individ- ual breeding trials and physiological experiments to the target population of environments.
Rice-growing environments where water stress is likely to occur can be grouped into two major catego- ries, upland and rainfed lowland. The latter may be further grouped on the presence or absence of a strongly developed hard pan in the soil. Opportunities exist to characterise the environment by measuring critical var- iables such as the depth of free water in the rainfed paddy (Maguling et al., 1982; OToole et al., 1983). Such physical characterisation is time consuming and so must be focussed on critical aspect