The impact of projected global warming on crop yields has been evaluated by indirect methods using simulation models. Direct studies on the effects of observed climate change on crop growth and yield could provide more accurate information for assessing the impact of climate change on crop production. We analyzed weather data at the

Genetic improvement in grain yield has been intensively studied in wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), oat (Avena sativa L.), maize (Zea mays L.), and soybean [Glycine max (L.) Merr.]. Such information is limited in rice (Oryza sativa L.). The objective of this study was to determine the trend in the yield of rice cultivars–lines developed since 1966. Twelve cultivars–lines were grown at the International Rice Research Institute (IRRI) farm and the Philippine Rice Research Institute farm during the dry season of 1996. Seven cultivars–lines were grown at IRRI farm in the dry season of 1998. Growth analyses were performed at key growth stages, and yield and yield components were determined at physiological maturity. Regression analysis of yield versus year of release indicated an annual gain in rice yield of 75 to 81 kg ha−1, equivalent to 1% per year. The highest yields obtained with the most recently released cultivars was 9 to 10 Mg ha−1, which is equivalent to reported yields of IR8 and other early IRRI cultivars obtained in the late 1960s and early 1970s at these same sites. Therefore, the 1% annual increase in yield may not represent genetic gain in yield potential. The increasing trend in yield of cultivars released before 1980 was mainly due to the improvement in harvest index (HI), while an increase in total biomass was associated with yield trends for cultivars–lines developed after 1980. Results suggest that further increases in rice yield potential will likely occur through increasing biomass production rather than increasing HI.

The chlorophyll meter provides a simple, quick, and nondestructive method to estimate leaf N status of rice (Oryza sativa L.), but the linear relationship between leaf N concentration on a dry‐weight basis (Ndw) and the meter reading differs depending on developmental stage and genotype. The objective was to determine whether prediction of (Ndw) with the chlorophyll meter can be improved by a simple correction for specific leaf weight (SLW). Leaf N status was estimated by a chlorophyll meter (SPAD‐502) and measured directly by micro‐Kjel‐dahl procedure. Specific leaf weight was calculated as the ratio of dry weight to leaf area. In one field study ‘IR72’, measurements were taken at midtillering, panicle initiation, and flowering stages on the uppermost fully expanded leaves of both N‐deficient and N‐sufficient plants. There was a linear relationship between Ndw and SPAD values at each stage, but regression lines differed significantly between growth stages. Based on pooled data from all stages, the degree of linear fit was poor (r2 = 0.49). Adjusting SPAD values for SLW (SPAD/SLW) improved the prediction of Ndw (r2 = 0.93). For another set of measurements made on the flag leaves of five genotypes grown in the field and greenhouse, prediction of Ndw was also improved, from r2 = 0.51 based on SPAD values alone to r2 = 0.87 based on the SPAD/SLW ratio. These results demonstrate that SLW influences the prediction of Ndw by the chlorophyll meter, and that the adjustment of SPAD values for SLW greatly increases the accuracy of the prediction. However, when SPAD values are adjusted for SLW, the chlorophyll meter's estimate of Ndw is no longer as quick, simple, or nondestructive as the nonadjusted SPAD values.

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Development of new plant types (NPTs) and hybrids are two major approaches for improving the yield potential of irrigated rice (Oryza sativa L.). This study was conducted (i) to compare grain yield and yield attributes among three high‐yielding groups of rice, namely indica inbred, indica/indica F1 hybrid, and second‐generation NPT, and (ii) to identify the morphophysiological traits responsible for the yield difference among the three groups. Fifteen genotypes, five from each of the three groups, were grown in the dry (DS) and wet seasons (WS) of 2003 and 2004 at the International Rice Research Institute, Philippines. On average, hybrids produced 11 to 14% greater grain yield than indica inbreds and NPTs in the DS. In the WS, the difference in grain yield was relatively small among the three groups. High grain yield of hybrids in the DS was the result of high number of spikelets per square meter due to a large number of spikelets per panicle and high harvest index rather than biomass production. The NPTs did not show yield advantage over the indica inbreds and demonstrated significantly lower yield than hybrids, mainly because of fewer spikelets per panicle and per square meter. Spikelet production efficiency per unit of N uptake and per unit of aboveground biomass at physiological maturity was generally higher in hybrids than indica inbreds or NPTs. Increasing harvest index and spikelet production efficiency by developing NPTs with more spikelets per panicle should be emphasized for improving the grain yield of NPTs.

Rice (Oryza sativa L.) grain yield highly varies depending on cropping seasons under the tropical irrigated conditions. Th is study aimed to (i) compare the grain yield of rice in dry season (DS) and wet season (WS) and (ii) determine climatic and physiological factors critical to the yield gap between DS and WS. Six genotypes, two each for indica inbred, indica/indica F 1 hybrid, and the second-generation new plant type, were grown in DS and WS of 2003 and 2004. Signifi cantly higher grain yields were achieved in DS than in WS by 94% for 2003 and 35% for 2004. Mean daily radiation was higher in DS than WS, particularly during grain fi lling stage than before fl owering. Th e greater radiation during ripening in DS contributed to the higher grain yield. Major difference in biomass production between DS and WS occurred aft er fl owering. Greater biomass accumulation from fl owering to physiological maturity was associated with higher grain yield in DS than in WS, but not translocation of biomass accumulated before fl owering to grains. Higher grain yield in DS was partly the result of greater spikelets due to higher spikelet production effi ciency per unit biomass at fl owering. Aboveground total biomass at physiological maturity was a crucial physiological factor to the yield gap between DS and WS. Daily mean radiation and biomass accumulation during ripening, and sink production effi ciency per unit biomass were critical factors to the yield gap of rice between DS and WS under the high-yielding tropical irrigated conditions.Abbreviations: DS, dry season; WS, wet season; Wr, biomass accumulated during ripening; T, translocation of the biomass at fl owering to rice grains.

A large panicle with numerous florets is essential for improving rice ( Oryza sativa L.) yield. Rice panicle size is determined by such underlying morphogenetic processes as: (1) primary branch formation on the panicle axis; (2) floret formation on the primary branches (mainly determined by the secondary branch formation); and (3) pre-flowering abortion of florets in the panicle. We examined QTLs for these processes to understand how they are integrated into panicle size. We developed 106 backcross-inbred lines (BC1F4) from a cross between 'Akihikari' (a temperate japonica) and 'IRAT109' (a tropical japonica) and constructed a genetic map. One QTL detected on chromosome 2, with a large effect (R=0.30) on the number of florets per panicle, affected both primary branch formation on the panicle axis and floret formation on the primary branches. In addition, three QTLs that affect only one of these two processes were identified on chromosomes 4, 9, and 11, each having a subsidiary effect on the number of florets per panicle (R2=0.04-0.07). QTLs for pre-flowering floret abortion were detected at three different regions of the genome (chromosomes 1, 10, and 11). This is the first report on QTLs for pre-flowering floret abortion in grasses. The absence of a co-location between QTLs suggests that floret formation and abortion are not directly linked causally. These results demonstrate that studying the partitioning of panicle size into these underlying morphogenetic components would be helpful in understanding the complicated genetic control of panicle size.