Search

Irrigated rice consumes two to three times more water than other cereals. The availability of water is, however, decreasing and this prompted researchers to find ways in saving water in irrigated rice fields where high yield is critical to ensure food security. The alternate wetting and drying (AWD) technology has been implemented successfully in farmer's fields. What is now needed is to fine-tune this technology in a site-specific manner with regard to genotype characteristics and soil type suitability. Nine genotypes were evaluated in similar growing conditions under AWD30 (irrigation whenever soil water potential reaches -30 kPa). Water productivity increased for all genotypes and a few were identified as adapted through their efficient sink regulation and deep rooting system. Two of the nine classified as promising genotypes were grown in contrasting soil types from sandy loam to clay soil under AWD30 and continuous flooding. Grain yield reduction was higher with the hybrid (37-57% in light soil and 0-7% in heavy soil) than with the inbred (25-45% in light soil and no reduction in heavy soil). Water input under AWD30 was reduced by 29-55% in both genotypes in light soil and by 6-26% in clay soil. Water productivity was higher in heavy soil and reduction in shoot biomass at physiological maturity was stronger in light soil. Stronger reduction in harvest index and sink size was observed with the hybrid. Selecting adapted genotypes and adjusting water management with respect to soil type will further improve the AWD irrigation technology.

The irrigated rice system faces great challenges in terms of productivity and sustainability with respect to food shortage and environmental issues. This study was done to address concerns with regards to the soil resources availability, methane emissions, water scarcity and slow gain in grain yield. Comparison of soil properties between cultivated and uncultivated soils in farmer's field is a promising approach to evaluate soil fertility and to adjust existing practices to ensure that nutrient outflow will be in balanced with the nutrient inflow. In this study, cultivated soils had significantly lower fertility when compared with the uncultivated soils as indicated by the soil properties measured. The amount of fertilizer being added by the farmers was not sufficient. Moreover, soil management practices should be well monitored as methane emissions increased dramatically upon straw incorporation and methane was emitted mostly during the early growth phase of the rice crop. But, drainage during mid-tillering abruptly reduced methane emissions by 15-80% as compared with continuous flooding and without a significant effect on grain yield. The choice of inorganic fertilizer and cultivar also influenced the amount of methane emission rates. Ammonium sulfate plus urea significantly reduced emission by 50- 66% than urea alone. Drying the field during midtillering mitigated methane emission and also saved water. Alternate wetting and drying (AWD) practice in irrigated rice fields starting from mid-tillering with a threshold of -30 kPa appeared as a more effective water-saving technology than field drying at mid-tillering: water productivity was significantly increased for all the contrasted genotypes evaluated and the amount of water input (WI) was significantly reduced by 33?41% in -30 kPa depending on the genotypes. However, the delay in tiller cessation reported under AWD, where higher and prolonged duration of tiller emergence was observed compared with continuous flooding, was probably not favorable for maximizing grain yield (GY). The higher GY of rice hybrids compared with inbreds (by 14-18% average) was due to the higher biomass accumulation and efficient biomass partitioning among plant organs, related with earlier cessation of tiller emergence. This earlier cessation is associated with better partitioning of biomass within the plant and ultimately to the panicle. This then indicates that breeding programs targeted for high yield potential will be different with those for genotypes under water saving conditions.

Water saving technologies without reduction on grain yield, like alternate wetting and drying (AWD), have been implemented successfully in irrigated rice fields. It is now needed to identify key crop traits under more pronounced water constraint for genetic improvement and further increase in water productivity. Five contrasted genotypes were grown under three water treatments in 2006 (AWD30 with irrigation set up whenever soil water potential reached -30 kPa at 15 cm deep, AWD60, and CF as continuous flooding) while nine genotypes were grown under two water treatments in 2008 (AWD30 and CF). While water use decreased significantly for all genotypes by 29% to 37% under AWD30 and by 22% to 34% under AWD60 in 2006DS, and by 17% to 25% in 2008DS, grain yield was maintained only in rare cases which expressed high contrast among genotypes' responses: grain yield of PSBRc80 was maintained in all situations while that of IR64 was significantly and systematically affected. The response of yield components to AWD was, however, not consistent with regard to the performance: cases with stable grain yield included some with unaffected yield components (three genotypes) and others with compensation between panicle number and filled grain number per panicle (PSBRc80). Cases with reduced grain yield included some with reduced grain size (two genotypes including IR64) and others with reduced filled grain number per panicle (two genotypes). Tiller emergence rate and maximum tillering increased under AWD for all genotypes, however, effect on LAI and biomass accumulation was inconsistent. Partitioning to culm was favored under AWD to the detriment of blade during the reproductive phase of the most adapted genotypes in 2008 although this was not reported in 2006. Under CF, the root/shoot ratio, total root dry matter and root dry matter below 25 cm deep were consistently higher, but for the most three adapted genotypes only. As a response to AWD, the root/shoot ratio and total root dry matter of these genotypes were maintained (in 2006) or even increased (in 2008) while those of a genotype bred for aerobic conditions were rather low under CF, but its partitioning to root dry matter at deep layers increased strongly under AWD. Although the plant type and traits responsible to AWD adaptation were not clearly consistent among the promising genotypes under study, the size, distribution and adaptation of the rooting system were clearly identified as key crop traits for adaptation to alternate wetting and drying. (Texte intégral)

Improving yield potential in rice implies the characterization of particular crop traits that may be used by breeders in their breeding programs. The rice crop is known to initiate and develop many tillers; a significant part of this may be higher than 50%, do not produce any grain. Recent breeding programs for yield potential have selected genotypes with high tillering efficiency (low tiller mortality) such as new plant types to reduce dry matter loss (Schnier et al 1990) and respiration cost (Dingkuhn et al 1990), but they did not produce the expected high yield in experimental fields. A positive correlation of grain yield with tillering efficiency would also imply a detrimental effect of non-productive tillers. We reviewed here a number of our recent field works to characterize the correlation between grain yield and tillering efficiency across genotypes of the same crop duration grown in similar favorable conditions and across crop managements for the same genotype.

Crop management strategies need to be improved and integrated in irrigated rice fields in order to approach yield potential, improve crop efficiency and decrease labor requirement. Crop establishment techniques (direct-seeding, nursery management, sowing date), in relation to genotype characteristics, appear to provide the options with the highest impact on productivity. Several trials involving first, direct-seeding and transplanting with the same sowing date and seedling density, second a range in nursery management considering seed density, transplanting age and nursery types, and third contrasted cropping seasons and plant densities, together with distinct patterns of growth amongst genotypes, have been implemented in the IRRI farm from 2003 to 2006. Increase in grain yield up to 1.5 t ha-1 was obtained from broadcasting compared with transplanting for most genotypes during both wet and dry seasons as long as land leveling and irrigation were highly controlled. In these cases, higher panicle density at maturity was observed as a result of earlier tiller emergence and leaf area production per plant. Seed rate as low as 25 kg ha-1 for hybrids generated a highly productive crop. As early as early tillering, some genotypes were even capable of adapting their spatial plant structure in response to the distance of their neighbors in order to increase light interception and leaf area production. As in direct-seeding, earlier tiller emergence and leaf area production contributed to the increase in grain yield, up to 1 t ha-1, in early transplanting (7 days, i.e. 3-leaf stage) over late transplanting (21 and 35 days, i.e. 7 and 11-leaf stage) regardless of the genotype. In case of same transplanting date, the same observation was valid for low seed rate in the nursery (500 to 3 000 seed m-2) compared to high seed rate (10 000 to 40 000 seed m 2), for which tiller emergence was delayed and grain yield was reduced. No transplanting shock was observed in all these trials. Rather high seedling competition in the nursery, due to either late transplanting or high seed density, was reported to be critical. to achieve high leaf area production and high grain yield in irrigated rice fields. No significant impact of nursery type, when comparing seedling trays, wet-beds, dapogs and improved mat nurseries, was quantified on plant growth, as long as the same seed density and seedling age for transplanting were applied. Even if tiller mortality was higher in case of early tiller emergence, this was proven not to have affected grain yield in a trial when aboveground water depth was increased up to 8 cm during late tiller emergence: the maximum tiller number was then reduced but the final tiller number was not affected. Quicker leaf area coverage appears to be a critical factor for high performance despite its role in tiller mortality. Regardless of the genotype, crop management in irrigated fields for direct-seeding and transplanting should induce early tiller emergence per plant, with the required adjustment for nitrogen application, in order to increase grain yield. This is also critical to weed competitiveness.

Yield advantage of hybrid rice over conventional inbred rice in tropical favorable environments, up to 15%, is commonly and simply attributed to higher biomass production due to higher seedling vigor and higher harvest index. Addressing hybrid superiority with less integrated plant traits should contribute to the development of plant-type management and the design of improved phenotypes. Precise phenotyping of high-yielding hybrid rice (H1, H3, and H5) and improved lines (I1 and I10) with the same crop duration was performed in a series of field experiments in 2004 and 2005 in the Philippines in order to quantify dynamics in assimilate partitioning and architectural plasticity of the plant in response to canopy competition. As commonly observed, higher yield of hybrid varieties was associated at maturity with higher shoot dry weight (38.5 g m-2 for H5 and 31.9 g m-2 for I10 in the wet season) and with higher harvest index (0.51 for H5 and 0.46 for I10). Indeed, hybrid rice had the ability to support a heavier panicle with an even weaker stem (high specific stem length), but then was more susceptible to lodging. Early seedling vigor for H1 and I1 was similar in the dry and wet seasons: dynamics in shoot dry matter, leaf area index and specific leaf area (SLA) did not significantly differ between genotypes. In contrast, internode dry weight of H1 was significantly higher with time than that of Il, both in the dry and in the wet seasons: at 69 days after sowing (DAS), it was 5.3 g for H1 and only 2.7 g for I1, while, for H1, there was quicker increase in the partitioning coefficient for internode and quicker decrease in the partitioning coefficient for blade. Internode dry weight of H1 was also significantly higher with time compared to that of I10: 3.9 g at 69 DAS, despite slightly higher vigor (lower SLA in H1). Early seedling vigor was also similar between H5 and I10 when observed in the wet season, as were dynamics in internode dry weight. Increase in shoot dry weight during grain filling in H5 was, however, significantly higher than that of I10 and I1. During the reproductive phase, Il exhibited a slower stem elongation and a quicker increase in blade angle with the main tiller compared to that of a check hybrid (H3). Despite the increasing competition for access to light in the canopy, I1 also exhibited an increase in the ellipse area formed by the leaf area of the plant at the highest collar level, whereas this was relatively constant for the hybrid variety. At flowering, better light distribution in the hybrid canopy, due to lower mutual shading between leaves, was suspected to have enhanced dry matter accumulation and a larger number of fully elongated stems was suspected to have increased harvest index by maximizing assimilate partitioning to grain right after flowering. Faster internode elongation and more efficient leaf spatial arrangement after canopy closure are proposed as more relevant traits than higher seedling vigor to account for hybrid rice superiority.