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Discovering traits of interest for higher yield potential in rice implies to consider differences in growing conditions (dry vs. wet seasons) and in maturity groups. This issue was addressed through the performance analysis of 32 high-yielding but contrasted genotypes, including 20 hybrids from 4 distinct countries (Colombia, China, India, Philippines) grown during the two seasons in the Philippines. While grain yield of hybrids was significantly higher than that of inbreds (9.5 vs. 8.3 t ha-1) in the early maturity group in the dry season (slightly higher panicle number and panicle size), it was maintained or even reduced in the other cases, with the detrimental effect getting larger in the wet season and with longer crop duration. In these cases, biomass of hybrids was higher than that of inbreds but could not be converted into higher yield due to lower tillering efficiency. And the lower panicle number of hybrids was not compensated enough by any higher spikelet number per panicle. Grain size of hybrids of the late maturity group in the wet season was even lower than that of inbreds while it was not affected in the other cases. The gradual reduction in grain yield of hybrids compared to inbreds with increasing crop duration and when shifting from dry to wet season questions the mode of biomass partitioning of hybrids and will be discussed on the basis of the comparative behavior of component traits like plant architecture, panicle structure and stay-green ability.

The rate of leaf appearance is related to early vigor, crop duration, and its environment. This experiment aimed to measure the variability of the leaf appearance rate and its response to two watering systems within the rice indica race. The GRISP Global Rice Phenotyping Network assembles a multi-environment, multi-trait phenomics resource for rice diversity panels. Our team specifically examined the dynamics of leaf appearance of 304 indica accessions of the modified PRAY panel that were either transplanted and flooded or direct-seeded and grown in aerobic conditions. In rice, the leaf appearance rate is fast for the first 4-5 embryonic leaves, slows down for the next leaves until about leaf 12, and slows down again for the last leaves. This pattern was confirmed in all plants with crop duration over 110 days. In earlier varieties, a proportion of the plants showed only two distinct phases intermediated by leaf 5. The earlier the variety, the higher this proportion was. The rates of leaf appearance and the durations of the second and third phases were strongly correlated: all late varieties had slow rates of leaf appearance and late transition to third phase. Meanwhile, the rate of appearance of the embryonic leaves was independent from the other four parameters. Differences in the leaf appearance kinetics between aerobic and flooded conditions were also related to the variety's crop duration. In varieties with duration over 105 days, the second phase is characterized by faster leaf appearance rate in flooded conditions. On the third phase, the rates equalize for both conditions. On the contrary, earlier varieties showed a reversed pattern: equal and then faster rate in flooded conditions, before and after the second transition, respectively. The transition from the second to the third phase was earlier by 0 to 14 days in aerobic than in flooded conditions for varieties with 80 to 105-day duration, and equal to 14 days for later varieties. All parameters of the leaf appearance kinetics are thus strongly correlated with the crop duration of the indica rice varieties except the rate of appearance of the embryonic leaves that can be bred independently.

Increase in yield potential in rice has been lately converted in developing genotypes with larger sink size, i.e. bigger and more compact panicles. Incidentally, filling efficiency of such genotypes is lower than that of check varieties. Alternatively, better sink regulation has been recognized as a way to improve grain yield. Sink regulation can be defined as the timing at which the plant switch on/off sinks and reinforce/reduce active sinks, where main sinks are leaves and roots at vegetative, internodes, panicles and reserves at reproductive, and spikelets at grain filling. At vegetative, early tiller production is essential to establish quick leaf area coverage, and significant reduction in grain yield is measured in case tillering is delayed by late transplanting or transient stressed conditions. In addition, earlier cessation of tiller production has been reported as a key process to promote larger panicle size (qTSN4 QTL) and higher resource remobilisation during grain filling (hybrid rice). At later stage, positive correlations between grain yield and filling efficiency (as long as genotypes with the same potential sink size are compared) highlights that increasing filling efficiency is a promising option. The higher compactness of the panicles of newly bred genotypes exacerbates the internal competition between spikelets and reduces filling efficiency. Indeed, the dominant effect of the apical above basal spikelets is more detrimental in compact panicles where higher ethylene concentration in basal and secondary spikelets reduce starch formation. Would any diversity in panicle architecture or peduncle section reduce internal competition and increase filling efficiency?

Many studies report that rice crops grown in aerobic irrigated conditions were shorter, with smaller leaf size and smaller crop leaf area, than those grown in flooded conditions. In rice plants, there are two transitions between stable phases in the rate of leaf appearance. The first five leaves of the main stem appear quickly, then the following leaves until approximatively leaf 12 appear at an intermediate rate, and the last leaves appear even more slowly. Previous observations showed that in aerobic conditions, the rate of appearance is slower and the second transition in the leaf appearance kinetics occurs earlier than in flooded conditions. Experiments were carried out to better understand the mechanisms underlying the rate of leaf appearance and to search for possible relationships between this rate and the leaf size. Neither change in the maximum rate of leaf elongation nor change in the maximum length of the leaf meristematic division zone was synchronized with the second change of the rate of leaf appearance. A linear relationship between the final length and the maximum length of the division zone (reached after 3.5 phyllochrons) was confirmed until leaf 11. Thus, the number of divisions per cell line in the division zone increased with leaf rank during the fixed interval of 3.5 phyllochrons, from 2.5 to 3 division cycles per phyllochron, and induced through exponential amplification the ascending part of the well-known bell profile of the leaf length. Under less favorable conditions, the leaf length profile and the related division zone length profile stayed at lower values only due to a small reduction in the rate of cell division in the leaf meristem. No variation in the leaf developmental processes accompanied the change of the phyllochron whose origin must strictly come from a change in the apical meristem. Small changes in the rate of cell division could be responsible for the significant and sustained size reduction observed in plants in aerobic or unfavorable conditions. This result opens an exciting field for future researches on the possibility of switching-off this detrimental reaction of plants.

Discovering traits of interest for higher yield potential in rice can be addressed by comparing high yielding genotypes of diverse genetic background and geographical origin. Overall, 32 entries developed by distinct institutions and companies, including 20 hybrids from four countries (Philippines, China, Columbia, and India) were analyzed during contrasted seasons at thë IRRI farm, Philippines. While grain yield was significantly higher in the dry (9.0 t ha-1) than in the wet (6.7 t ha-1) season, the performance of hybrids in favorable conditions (dry season) was confirmed to be significantly higher than that of inbreds, 9.2 vs 8.6 t ha-1. When looking at individuals regardless of origin and genetic background, two types of component traits supporting higher performance during both seasons were identified: those related to (i) high light interception capacity during grain filling (erect leaves and tillers, substantial green leaf area index maintained a few days before maturity) and (ii) a compact structure of the panicle (short panicle with minimal rachis and high ratio of primary to secondary spikelets, within a range of variation of panicle length from 23 to 28 cm). The abovementioned traits were confirmed as having great value to support the difference in final grain yield when comparing two by two genotypes with the same potential sink size. Interestingly, high light interception capacity and culm dry weight at around maximum tillering, and high tillering efficiency, were also favorable traits. These major findings for irrigated rice are discussed in light of previous results published on the dynamics of biomass accumulation and sink regulation.

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.

Des expérimentations menées sur la comparaison de variétés de riz hybride et de lignées consanguines à haut rendement, permettent de mettre en évidence des caractéristiques végétales sous-jacentes expliquant que le riz hybride produise jusqu'à 15% de plus que les variétés consanguines classiques. L'élongation internodale plus rapide, un arrangement spatial des feuilles plus efficient après la fermeture du canopée et un remplissage plus efficace des grains sont autant de caractéristiques proposées et jugées plus pertinentes que la plus grande vigueur des jeunes pousses, pour expliquer la supériorité du riz hybride.

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.