Rice Yield Potential Research Articles

Reduction of unproductive tillers did not increase the grain yield of irrigated rice

Tillering ability is one of the most important traits of the rice plant because it plays a major role in determining grain yield. However, it is not clear whether rice yield potential could be further increased if unproductive tillers were reduced or eliminated. Two field experiments were conducted to determine the effects of tiller reduction on plant growth and grain yield in two varieties with different tillering ability. Two methods were used to reduce tillers: the first was the removal of tillers manually at different crop growth stages and the second was to grow rice plants through holes in styrofoam slats, which provided a physical restriction on tiller emergence. Tiller number declined by 12–57% with the first method and by 18–47% with the second method compared with the control. Despite the significant reduction in tiller number and unproductive tiller percentage, none of the treatments with manual tiller removal and physical restriction of tillering resulted in a significant increase in grain yield compared with the control. Decreases in leaf area and biomass at heading and panicle number at maturity were observed when tiller number declined significantly, but grain yield was not necessarily reduced because it was compensated for by more biomass production after heading and increased spikelet number per panicle. The two varieties with different tillering ability had similar responses to tiller reduction. Based on the two methods of tiller reduction used in this study, we conclude that a further increase in rice yield potential may not be possible by reducing or eliminating unproductive tillers.

Canopy temperature on clear and cloudy days can be used to estimate varietal differences in stomatal conductance in rice

Leaf photosynthetic potential, a major determinant of yield potential in rice, is mostly explained by stomatal conductance ( g s) as well as leaf N content (Rubisco protein content). Therefore, the rapid and precise evaluation of g s as well as leaf N content would be a valuable addition to rice breeding programs aimed at improved yield potential. We established a simple method based on infrared thermography to estimate varietal differences in g s. Seven rice varieties were cultivated in two-row plots in 2007, and canopy temperatures in three varieties, including a control variety, Koshihikari, were simultaneously measured to determine canopy temperature difference (CTd) between Koshihikari and the other varieties on clear days. The varieties Takanari, Habataki, and TUAT1-5-6a displayed significantly ( P < 0.05) lower leaf temperature and higher g s and leaf photosynthetic rate ( P n) than Koshihikari. CTd was closely correlated with g s and P n. These results indicate that CTd may be useful as a relative index to estimate varietal differences in g s. A significant CTd was still observed even under cloudy conditions in 2008. Takanari and TUAT1-5-6a had lower leaf temperature than Koshihikari on cloudy days as well as on clear days. Subsequent investigation of photosynthetic light response curves revealed that Takanari and TUAT1-5-6a had higher g s and P n than Koshihikari under lower irradiance. These results suggest that infrared thermography may be a simple method of evaluating varietal differences in g s through CTd, and that it may be practicable even under cloudy conditions.

Mapping quantitative trait loci influencing panicle‐related traits from Chinese common wild rice (Oryza rufipogon) using introgression lines

AbstractPanicle‐related traits are important agronomic traits which directly associated with grain yield. In this study, we investigated quantitative trait loci (QTLs) associated with panicle‐related traits using a set of 265 introgression lines (ILs) of common wild rice (Oryza rufipogon Griff.) in the background of Indica high‐yielding cultivar Guichao 2 (O. sativa L.). A total of 39 QTLs associated with panicle‐related traits including panicle length (PL), primary branch number (PBN), secondary branch number (SBN), spikelet number per panicle (SPP) and spikelet density (SD), were detected in the ILs with single‐point analysis. The alleles of 20 QTLs derived from wild rice showed positive effects, and some QTLs, such as, QPl1b for PL, QPbn8 for PBN, QSd4 and QSd11b for SD and QSpp4 for SPP showed larger positive effects, providing good candidates and useful information for marker‐aided improvement of yield potential of rice. Most of the QTLs controlling SPP, SBN and SD were located in cluster or closely linked on chromosomes, and the directions of their additive effects were consistent, which explained the genetic basis of significant correlations between their phenotypic characters.

Yield potential and radiation use efficiency of “super” hybrid rice grown under subtropical conditions

China's “super” hybrid rice breeding project has developed many new varieties using a combination of the ideotype approach and intersubspecific heterosis. It is controversial whether these “super” hybrid varieties have increased the yield potential of irrigated rice. This study was conducted to compare grain yield and yield attributes among “super” hybrid, ordinary hybrid, and inbred varieties. Field experiments were done in Liuyang (moderate-yielding site) and Guidong (high-yielding site) counties, Hunan Province, China, in 2007 and 2008. Two varieties from each varietal group were grown in each field experiment under moderate and high N rates. Grain yield, yield components, aboveground total dry weight, harvest index, total N uptake, and crop radiation use efficiency (RUE) were measured for each variety. A significant difference in grain yield was observed among the varieties and varietal groups but not between the two N rates. “Super” hybrid varieties have increased rice yield potential by 12% compared with ordinary hybrid and inbred varieties. The higher grain yield of “super” hybrid varieties was attributed to improvement in both source and sink. “Super” hybrid varieties produced more biomass than ordinary hybrid and inbred varieties. Long growth duration and high accumulated incident radiation were partially responsible for high biomass production for the “super” hybrid varieties. “Super” hybrid varieties had significantly larger panicle size (spikelets per panicle) than ordinary hybrid and inbred varieties, which resulted in larger sink size (spikelets per m 2). Crop RUE did not explain the yield superiority of “super” hybrid rice. Our study suggests that “super” hybrid rice varieties do not necessarily require more N fertilizer to produce high grain yield.

Yield potential and water use efficiency of aerobic rice ( Oryza sativa L.) in Japan

Intensive rice farming in aerobic soil, referred to herein as aerobic rice, can greatly reduce the water input compared to that of flooded rice cultivation. The objective of this study was to compare the potential productivity of aerobic rice and flooded rice using high-yielding varieties at two locations in Japan in two successive years. In aerobic fields, the total amount of water supplied (irrigation plus rainfall) was 800–1300 mm. The soil water potential at 20-cm depth averaged between −15 and −30 kPa each growing season, but frequently reached −60 kPa. The average yield under aerobic conditions was similar to or even higher than that achieved with flooded conditions (7.9 t ha −1 in 2007 and 9.4 t ha −1 in 2008 for aerobic versus 8.2 t ha −1 for flooded). The average water productivity under aerobic conditions was 0.8–1.0 kg grain m −3 water, slightly higher than common values in the literature. The super-high-yielding cultivar Takanari achieved yields greater than 10 t ha −1 with no yield penalty under aerobic conditions in 3 out of 4 experiments. The favorable agronomic characteristic of Takanari was its ample sink capacity (grain number × grain weight). In conclusion, high-productivity rice cultivation in aerobic soil is a promising technology for water conservation. With continued breeding, future aerobic rice varieties will possess large numbers of spikelets and sufficient adaptation to aerobic conditions such that they will consistently achieve yields comparable to the potential yield of flooded rice.

Higher crop performance of rice hybrids than of elite inbreds in the tropics: 2. Does sink regulation, rather than sink size, play a major role?

Higher harvest index, widely reported as a main trait supporting major improvements in rice yield potential, also supports the higher performance of hybrids over inbreds. Although higher sink size is generally claimed for being the driver of higher grain filling in hybrid rice, it is relevant to question whether efficient sink regulation, that is timing and magnitude of the successive sink activities over crop growth from early stage, plays a role in supporting higher harvest index and is a key feature ensuring higher performance of hybrid rice. The partitioning coefficient (PC), as the increase in dry matter of one single organ over that of shoot, was calculated over short and successive periods to quantify the dynamics of dry matter partitioning. Four hybrid and four inbred genotypes of similar crop duration were grown under the same cropping management on the IRRI farm during a wet and a dry seasons. While PC to blade and sheath was similar for both plant types in both seasons at the early stage, PC to culm of hybrids was higher than that of inbreds, and PC to blade was lower, in the late vegetative and early reproductive phases. During the late reproductive phase, PC to panicle of hybrids was higher than that of inbreds, and PC to culm was lower whereas culm elongation was similar for both plant types. During grain filling, PC to culm was significantly more negative with hybrids which indicated stronger remobilization with hybrids. Specific culm length before anthesis, as low as 40 cm g −1, accounted for storage ability, and at maturity, as high as 130 cm g −1, for remobilization ability. Sink strength index, as a better indicator of dry matter partitioning efficiency than harvest index, and unfilled grain size, as an indicator of assimilate wastage, were suggested as key indices to account for the better sink regulation in hybrid rice and to improve screening protocols for increasing yield potential and tolerance to lodging.

Physiological characteristics of a functional stay-green rice “SNU-SG1” during grain-filling period

Functional stay-green has been regarded as a promising characteristic to be introduced for improving rice yield potential. A functional stay-green rice “SNU-SG1” that was identified from japonica rice collections was compared with two regular high-yielding rice cultivars (HYVs) for the temporal change of leaf chlorophyll, soluble protein, and root activity, and nitrogen accumulation and remobilization during the grain-filling period. SNU-SG1 had slower decreasing rate and maintained higher concentration of chlorophyll and soluble protein in upper four leaves during the grain-filling period than HYVs “Suweon490” and “Andabyeo”, revealing a typical stay-green characteristic. Even though SNU-SG1 remobilized almost the same proportion of N accumulated before heading as HYVs to grain, it maintained much higher leaf N concentration due to the significantly higher N accumulation that is ascribable to the higher root activity sustenance during grain-filling period. The functional stay-green trait of SNU-SG1 seems to stem not only from the genetic control preventing chlorophyll degradation but also from the higher capacity to absorb N from soil due to the sustained strong root activity during grain-filling period. SNU-SG1 exhibited higher crop growth rate during late grain-filling period than HYVs, resulting in higher grain-filling percentage and non-structural carbohydrate re-accumulation in the stem at the final stage of grain filling. It is concluded that SNU-SG1 has a promising trait “functional stay-green” contributable to rice yield potential improvement through the improved grain filling.

Detection of a quantitative trait locus controlling carbon isotope discrimination and its contribution to stomatal conductance in japonica rice

Increasing leaf photosynthesis offers a possible way to improve yield potential in rice (Oryza sativa L.). Carbon isotope discrimination (Delta(13)C) has potential as an indirect selection criterion. In this study, we searched for quantitative trait loci (QTLs) controlling Delta(13)C, and assessed their association with leaf photosynthesis. Substitution mapping by using chromosome segment substitution lines (CSSLs), that carry segments from the indica cultivar Kasalath in the genetic background of the japonica cultivar Koshihikari, identified genomic regions affecting Delta(13)C on chromosomes (Chr.) 2, 3, 6, 7, and 12. One of the CSSLs, SL208, in which most regions on Chr. 3 were substituted with Kasalath segments, showed higher leaf stomatal conductance for CO(2) (g (s)) and Delta(13)C than Koshihikari during the vegetative stage although leaf photosynthetic rate did not differ between them. These results suggest an association between Delta(13)C and g (s). To test this association, we performed a QTL analysis for Delta(13)C at vegetative and heading stages in an F(2) population derived from a cross between SL208 and Koshihikari. The results confirmed a QTL controlling Delta(13)C on the long arm of Chr. 3. By using a near-isogenic line specific to Hd6, we ruled out the possibility that variation in Delta(13)C was generated through the pleiotropic effect of heading date.

C4 rice: a challenge for plant phenomics.

There is now strong evidence that yield potential in rice (Oryza sativa L.) is becoming limited by 'source' capacity, i.e. photosynthetic capacity or efficiency, and hence the ability to fill the large number of grain 'sinks' produced in modern varieties. One solution to this problem is to introduce a more efficient, higher capacity photosynthetic mechanism to rice, the C4 pathway. A major challenge is identifying and engineering the genes necessary to install C4 photosynthesis in rice. Recently, an international research consortium was established to achieve this aim. Central to the aims of this project is phenotyping large populations of rice and sorghum (Sorghum bicolor L.) mutants for 'C4-ness' to identify C3 plants that have acquired C4 characteristics or revertant C4 plants that have lost them. This paper describes a variety of plant phenomics approaches to identify these plants and the genes responsible, based on our detailed physiological knowledge of C4 photosynthesis. Strategies to asses the physiological effects of the installation of components of the C4 pathway in rice are also presented.

Molecular Genetics of Rice Root Development

Abstract Plant roots have a large range of functions, including acquisition of water and nutrients, as well as structural support. Dissecting the genetic and molecular mechanisms controlling rice root development is critical for the development of new rice ideotypes that are better adapted to adverse conditions and for the production of sustainably achieved rice yield potential. Most knowledge regarding the gene networks involved in root development has been accumulated in the model dicotyledon plant species Arabidopsis thaliana. Rice, the model monocotyledon species, presents several singularities compared to A. thaliana, including a root architecture characterized by a fibrous root system comprising five types of embryonic and postembryonic roots. The anatomy and morphology of the rice root system, which is typical for a cereal, differs from that of A. thaliana, for instance, by the presence of a lysigenous cortex and additional cell layers compared to the dicotyledon model. Moreover, the structure and functions of the root apical meristem (RAM) of rice are distinct from those of A. thaliana. Recently, several rice root mutants have been identified via forward or reverse genetics, and these will aid in forming hypothesis to characterize either the divergence or conservation of genetic pathways relative to A. thaliana. Furthermore, these mutants will help to identify key genes in rice roots that may be missing in A. thaliana. This review summarizes both classical and recent data concerning the molecular genetics of rice root development, including root anatomy and morphology, RAM structure, RAM patterning, and root mutants.

Combining Ability of Yield-Component Traits for Indica Chromosome Substituted Segments in Japonica Hybrid Rice

To study the heterosis of inter-subspecies hybrid in rice ( Oryza sativa L.), 10 chromosome segment substitution lines (CSSLs) with Asominori ( japonica) background derived from Asominori × IR24 ( indica) and the parent Asominori were crossed with 6 japonica tester lines (as male parents). A total of 66 combinations were planted in the field with NCII design. The 10 CSSLs had higher general combining abilities (GCAs) than the parent Asominori in the main yield-component traits, excluding the thousand-grain weight. Five CSSLs, AIS84, AIS27, AIS3, AIS80, and AIS76 with chromosome substitution segments from chromosomes 12, 4, 1, 11, and 11, respectively, ranked as the top 5 in the GCA of yield-related characteristics. The results showed stronger heterosis in the indica/japonica combinations at the chromosome segment level than in the typical japonica hybrid rice. Intentionally substituting japonica chromosome segments with indica counterparts, such as, X24–2– R367 in chromosome 12, R1854– R288 in chromosome 4, C970– C955 in chromosome 1, C1350– R257 in chromosome 11, and X52– R2913 in chromosome 11, could significantly promote the yield potential of japonica hybrid rice. Different combinations with one identical parent or different traits in the same combination had significant differences in the specific combining ability (SCA). Combinations with the highest and lowest SCA values were derived from different parental GCA groups, indicating no causal relations between SCA and parental GCA. High GCA combined with strong SCA is a basic principle of parental selection in the breeding programs of super high-yielding hybrid rice.

Natural variation in Ghd7 is an important regulator of heading date and yield potential in rice

Yield potential, plant height and heading date are three classes of traits that determine the productivity of many crop plants. Here we show that the quantitative trait locus (QTL) Ghd7, isolated from an elite rice hybrid and encoding a CCT domain protein, has major effects on an array of traits in rice, including number of grains per panicle, plant height and heading date. Enhanced expression of Ghd7 under long-day conditions delays heading and increases plant height and panicle size. Natural mutants with reduced function enable rice to be cultivated in temperate and cooler regions. Thus, Ghd7 has played crucial roles for increasing productivity and adaptability of rice globally.

Hormonal regulation of tiller dynamics in differentially-tillering rice cultivars

Tiller number can contribute significantly to yield potential of rice, but little knowledge is available on hormonal regulation of tillering and tiller dynamics. In the present study, Indole-3-acetic acid (IAA), kinetin (6-furfuryl amino purine) and Gibberellic acid (GA3) treatments have been applied at the early tillering stage to two rice cultivars that contrast for tiller number. The responses of the hormones were studied on growth, development, grain yield, senescence patterns, assimilate concentration of the panicle and ethylene production in different classes of tillers. The leaf area, panicle grain number, fertility percentage and grain yield of tillers were higher in the low-tillering cultivar than that of high-tillering cultivar; the treatment of kinetin was more effective in the latter than in the former. High ethylene production was responsible for reduction of growth duration and grain yield of the tillers. Kinetin application reduced ethylene production of the late-tillers significantly for the benefit of grain yield.

Overexpression of a NAC‐domain protein promotes shoot branching in rice

For a better understanding of shoot branching in rice (Oryza sativa), a rice activation-tagging library was screened for mutations in tiller development. Here, an activation-tagging mutant Ostil1 (Oryza sativa tillering1) was characterized, which showed increased tillers, enlarged tiller angle and semidwarf phenotype. Flanking sequence was obtained by plasmid rescue. RNA-interfering and overexpression transgenic rice plants were produced using Agrobacterium-mediated transformation. The mutant phenotype was cosegregated with the reallocation of Ds element, and the flanking region of the reallocated Ds element was identified as part of the OsNAC2 gene. Northern analysis showed that expression of OsNAC2 was greatly induced in the mutant plants. Transgenic rice overexpressing the OsNAC2 resulted in recapture of the mutant phenotype, while downregulation of OsNAC2 in the Ostil1 mutant through RNA interfering (RNAi) complemented the mutant phenotype, confirming that the Ostil1 was caused by overexpression of OsNAC2. Overexpression of OsNAC2 regulates shoot branching in rice. Overexpression of OsNAC2 contributes tiller bud outgrowth, but does not affect tiller bud initiation. This suggests that OsNAC2 has potential utility for improving plant structure for higher light-use efficiency and higher yield potential in rice.

Yield Physiology of Rice

ABSTRACT Rice (Oryza sativa L.) is a staple food for more than 50% of the world's population, including regions of high population density and rapid growth. Rice is produced under both upland and lowland ecosystems with about 76% of the global rice produced from irrigated lowland rice systems. The objective of this article is to discuss growth and formation of yield components in rice during crop growth cycles. The yield components of rice are the number of panicles per unit area, number of spikelets per panicle, weight of spikelet and spikelet sterility or filled spikelet. In addition, shoot dry weight, grain harvest index, and nitrogen (N) harvest index are also positively associated with grain yield. These yield components and yield associated parameters are formed during crop growth cycle. Growth cycle of the rice plant is divided into three stages. These stages are designated as vegetative, reproductive and spikelet filling or ripening. Yield potential of rice is formed or defined during these growth stages. Plant height, tillering (associated with panicle number), root growth, leaf area, and morphology are the main features of vegetative growth stage. In the reproductive growth stage panicle development takes place. Booting and flowering are part of the reproductive growth stage. Panicle size or spikelets per panicle are determined in the reproductive growth stage. Spikelet size or weight is determined during the spikelet filling growth stage. The reproductive growth stage is the most sensitive to biotic and abiotic stresses, followed by spikelet filling and vegetative growth stage. Recent advances in molecular linkage maps of rice and other developments of molecular biology offer new opportunities for improving rice yield components in favor of higher yield.

A Mitochondrial Repeat Specific Marker for Distinguishing Wild Abortive Type Cytoplasmic Male Sterile Rice Lines from their Cognate Isogenic Maintainer Lines

ABSTRACTThe realization of yield potential of rice (Oryza sativa L.) hybrids developed through the “three‐line system” largely depends on genetic purity of parental lines. Cytoplasmic male sterile (CMS) lines often get contaminated with cognate isonuclear maintainer lines during multiplication. We report identification of a rice mitochondrial DNA sequence, possessing a repeat motif that is polymorphic between the wild abortive (WA)‐CMS lines and their cognate isonuclear maintainer lines. We have designed a primer pair flanking the unique repeat and two fragments of different size were amplified in all WA‐CMS and their maintainer lines through PCR. The PCR assay was able to precisely detect the impurities in a commercial seed lot of a popular CMS line, IR58025A. The possibility of utilizing this marker as a replacement for the morphology‐based (grow‐out test) GOT in accurate detection of contaminants in CMS seed stocks is discussed.

Genetic dissection of developmental behavior of crop growth rate and its relationships with yield and yield related traits in rice

Crop growth rate is a dynamic character that determines the final yield in cereal crops. The objective of the current study is to reveal its genetic basis and to genetically understand its relationships with grain yield and yield related traits like heading date, grain-filling duration, 1000-grain weight and plant height in rice ( Oryza sativa L.). With 258 recombinant inbred lines (RILs) derived from a cross between cv. Lemont and cv. Teqing, time-related QTL mapping was conducted. Eight QTL for crop growth rate were identified at four consecutive growth periods. These QTL individually explained 5–29% of the trait variation. The Teqing alleles increased the trait values at all the four QTL detected before heading, while Lemont contributed alleles to increase the trait values at three of the other four QTL mapped after heading, which was in agreement with the observation that Teqing was the high-value parent before heading, while Lemont became the high-value parent after heading. No common QTL was observed at different growth periods. These results implied that crop growth rate of rice might be controlled by different genes (QTL) during different time intervals. A total of 22 QTL were identified for the other five traits, and the amount of variation explained by individual QTL ranged from 3% to 26%. The interval RM25–CSU754 on chromosome 8 was identified to produce a major effect on multiple traits such as crop growth rate IV, grain yield, heading date, grain-filling duration, plant heights II and IV, which provided a genetic explanation for the close correlations among the traits. The implications of the results for further improvement in rice yield potential were discussed.

Genotypic difference in canopy diffusive conductance measured by a new remote‐sensing method and its association with the difference in rice yield potential

There have been few practical ways of measuring physiological determinants of rice yield. Rapid evaluation of yield determination traits may expedite breeding of high-yielding rice. Here, we report a new remote-sensing technique for the evaluation of canopy ecophysiological status under field conditions developed based on simultaneous measurements of sunlit and suddenly shaded canopy temperatures. This technique has the advantage of instantaneous estimation of aerodynamic resistance (r(a)) and canopy diffusive resistance (r(c) without measuring wind velocity. Canopy diffusive conductance (1 / r(c)) estimated by the remote sensing method was closely related to leaf stomatal conductance (g(s)) measured with a portable gas exchange system. This result supported the validity of this new method for quantitative estimation of canopy physiological characteristics. Significant genotypic differences were obtained in canopy-air temperature difference (Tc-Ta), r(c) and 1 / r(c) during the 2-week period preceding full heading for two years, and 1 / r(c) was highly correlated with crop growth rate (CGR), which was closely related to the final yield. These results suggest that 1 / r(c) can be an effective criterion for the selection of high-yielding rice genotypes, and the remote sensing technique proposed here can be a powerful tool for the rapid evaluation of 1 / r(c) under field conditions.

Evaluation of Pressmud Cake as Source of Phosphorus for Rice-Wheat Rotation

ABSTRACT Efficient utilization of P from pressmud cake (PMC) in rice-wheat rotation is critical in reducing the costs of fertilizer P and improving soil fertility. The objective of this study was to determine the effects of variable rates of PMC and fertilizer P on yield and soil fertility under rice-wheat rotation on a silt loam (Aquic Ustorthent) soil in the flood plains of Punjab, India. Phosphorus responses in rice were obtained at 13 kg P ha−1. Application of PMC at 51 ha−1 was as efficient as 13 kg P ha−1 in increasing grain yield of rice. Wheat responded to direct application of 26 kg P ha−1 when rice received zero or 13 kg P ha−1. In the presence of PMC at 5 t ha−1 applied to rice, grain yield responses were obtained up to 13 kg P ha−1, but no response to P was observed when 10 t PMC ha−1 was applied. Application of PMC helped to raise the yield potential of rice and wheat when compared with application of chemical P fertilizer. Application of PMC increased the organic carbon and available P contents in soil.

Rice yield potential is closely related to crop growth rate during late reproductive period

Yield potential in rice has remained stagnant since the development of IR8 by International Rice Research Institute (IRRI), especially in tropical environments. We conducted field experiments in Kyoto, Japan (temperate climate) in 1999 and 2000 to determine the factors related to yield potential in rice. A rice cultivar Takanari showed the highest grain yield among the genotypes across the 2 years, and successfully produced over 11 t ha −1 of grain yield in 2000. The genotypic difference in grain yield was most closely related to that in crop growth rate (CGR) during the late reproductive period (14–0 days before full heading). Rice genotypes having higher CGR during this period produced a greater number of spikelets per unit land area. The higher CGR also led to larger accumulation of non-structural carbohydrate (NSC) in the culms and leaf sheaths during the period that was positively correlated with the rapid translocation of NSC to panicle in the initial period of grain filling. Solar radiation during grain filling was higher in 2000 than 1999. Therefore, Takanari appeared to have succeeded in over 11 t ha −1 of grain yield by achievement of both prerequisite biomass production during the late reproductive period and better grain filling. A large genotypic variability in CGR during the late reproductive period was mainly derived from that in radiation use efficiency. These results indicate that improvement of canopy photosynthesis during the late reproductive period may be essential as a first step toward increased yield potential in rice.