Partitioning of dry matter during drought stress in rainfed lowland rice
Introduction
Rainfed rice is grown on approximately 59 million hectares worldwide, representing 45% of the total area planted to rice (IRRI Rice Almanac, 1993–1995). Water deficit commonly occurs during the growing season, and the intensity of stress depends on the duration and frequency of water deficit. The drought pattern is complex in rainfed lowlands, as drought may occur early in the growing season or any time from flowering to grain filling, and may follow a period when soils were flooded and anaerobic (Wade, 1999). Drought stress suppresses leaf expansion, tillering and midday photosynthesis (Kramer and Boyer, 1995), and reduces photosynthetic rates and leaf area due to early senescence (Nooden, 1988). All of these factors are responsible for a reduction in dry matter accumulation and grain yield under drought.
In India, 47 million hectares are under rice production, of which 45% is irrigated, 33% is rainfed lowland, 15% is rainfed upland and 7% is flood-prone. The rainfed lowland area (17 million hectares) is mostly in Eastern India (Singh and Dwivedi, 1997). In Chattisgarh state, 80% of the total rice area is under rainfed conditions. Drought stress is a recurrent phenomenon in this region and is the single most important constraint for achieving higher and more stable yields. The average productivity of this region is still only 1.5 t/ha. Apart from drought, other constraints are insect pests and diseases and the use of low yielding, taller traditional cultivars, which still occupy a significant area due to the lack of suitable modern high-yielding cultivars adapted to drought conditions.
Selection of improved cultivars for the rainfed lowland ecosystem is difficult, with success being dependent on a number of factors including defining target environments, identifying traits which confer improved adaptation to the key constraints, developing effective selection methods for useful traits and understanding the performance of modern cultivars in different environments (Cooper, 1999). In order to evaluate the performance of a cultivar, its adaptive behavior should be tested over a set of sites and years, which reasonably represent the range of environments likely to be encountered in the long term.
The productivity of cereals depends not only on the accumulation of dry matter, but also on its effective partitioning to plant parts of economic importance, and this is a key to yield stability particularly under drought stress. Remobilization of reserves to grain is critical for grain yield if the plants are subjected to water stress during grain filling (Nicolas et al., 1985, Palta et al., 1994, Ehdaie and Waines, 1996). Among cereals and particularly in wheat, pre-anthesis assimilates help in yield stability during terminal drought stress (Blum et al., 1983). In wheat, pre-anthesis assimilate reserves from stem and sheaths contribute 25–33% of the final grain weight (Rawson and Evans, 1971, Gallagher et al., 1976, Hans, 1993, Gebbing and Schnyder, 1999). In cereals, grains are the most important sink for carbon and nitrogen after anthesis. In rice, available carbon assimilates for grain production are determined by carbon assimilation during the grain-filling period plus assimilate reserves stored in the straw (Cook and Yoshida, 1972). Pre-anthesis storage may contribute 20–40% of the final crop yield depending on cultivar, reflecting its importance for attaining higher grain yields (Yoshida, 1972, Murata and Matsushima, 1975, Ntanos and Koutroubas, 2002). The early senescence induced by a moderate water deficit during grain filling can enhance the remobilization of stored assimilates and accelerate grain filling of rice (Yang et al., 2001).
This study examined variation in the production of dry matter before and after flowering and its partitioning to reproductive organs, using five diverse rice cultivars grown in contrasting rainfed lowland environments in four consecutive years at Raipur, India. Our objectives were to quantify the production and redistribution of dry matter in grain filling and how this varied among maturity groups and cultivars. The relationship between maintenance of plant water status, delay in flowering and dry matter partitioning under drought stress was examined, in order to consider traits that may be associated with a greater ability to redistribute dry matter to grain in terminal drought, and their implications for yield stability.
Section snippets
Description of the locations and plant establishment
The experiments were conducted in eight locations during the period of 1995–1998 at Indira Gandhi Agricultural University in Raipur, Chattisgarh, India (21°16′N, 81°36′E, 300 m above sea level) during the monsoon season. The locations were chosen within a few kilometers of each other with soils differing in texture and hydrology (Table 1). Meteorological data were collected from a nearby weather station (Table 2). Basal nutrients (N:P:K) were applied at 40:50:30 kg ha−1, as urea, superphosphate
Characterization of sites
The soils in sites 95-1 and 97-2 were alfisol, those in 95-3 and 96-2 were vertisol, while the rest were inceptisol (Table 1). Across locations, sand content, clay content and soil pH varied from 12 to 30%, 35 to 53% and 6.8 to 7.5%, respectively. Rainfall of 1038, 1087, 1116 and 908 mm was received from June to December 1995 to 1998, respectively, but with considerable within-season variation (Table 2). The sites also varied in depth of ponded water during the flowering and post-flowering
Differences among sites in dry matter production
The sites used from 1995 to 1998 experienced conditions ranging from favorable to severe drought and thus provided contrasting conditions to study the responses in dry matter production and partitioning pattern of the five diverse rice cultivars under different pre- and post-flowering hydrological conditions. Some sites were subjected to severe water stress after flowering (95-3, 96-1 and 96-2), while other sites (95-2, 97-2 and 98-1) were more favorable, and sites 97-1 and 95-1 were
Conclusions
This study showed that dry matter redistribution from leaves and stems contributed significantly to grain yield in rainfed lowland rice when subjected to drought during reproductive development. NSG-19, with an enhanced capacity for leaf senescence during grain filling, had more stable yields in terminal drought than IR-20 of similar maturity. These results help to explain the patterns of adaptation reported by Wade et al. (1999) in studies of genotype by environment interactions in the rainfed
Acknowledgements
The Rainfed Lowland Rice Research Consortium received support from the Asian Development Bank, Philippines and the Directorate-General for International Cooperation, Netherlands, with additional support to IRRI from the Department for International Development, United Kingdom.
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