Ma. Rebecca C. Laza

Chlorophyll meter estimates leaf area‐based nitrogen concentration of rice

Abstract The chlorophyll meter (SPAD‐502) can be used to diagnose the nitrogen (N) status of rice (Oryza sativa L) plants to determine the need for fertilizer‐N topdressing. However, accurate estimation of leaf N concentration on a dry weight basis (Ndw) by SPAD requires the adjustment of SPAD values for specific leaf weight (SLW). The objective of this study was to determine if SPAD estimates of leaf N concentration on a leaf area basis (Na) without correcting for SLW. SPAD values were measured from different genotypes at various growth stages with the chlorophyll meter. Leaf N concentration was determined by micro‐Kjeldahl procedure and expressed based on dry weight and leaf area. Specific leaf weight was calculated as the ratio of leaf dry weight to leaf area. At each growth stage, SPAD estimated Na better than Ndw, especially at the middle and later growth stages. When data for all stages were pooled, SPAD values also correlated with Na better than with Ndw (r value of 0.81 versus 0.43). This finding ...

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 (Δ13C) has potential as an indirect selection criterion. In this study, we searched for quantitative trait loci (QTLs) controlling Δ13C, 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 Δ13C 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 CO2 (gs) and Δ13C than Koshihikari during the vegetative stage although leaf photosynthetic rate did not differ between them. These results suggest an association between Δ13C and gs. To test this association, we performed a QTL analysis for Δ13C at vegetative and heading stages in an F2 population derived from a cross between SL208 and Koshihikari. The results confirmed a QTL controlling Δ13C on the long arm of Chr. 3. By using a near-isogenic line specific to Hd6, we ruled out the possibility that variation in Δ13C was generated through the pleiotropic effect of heading date.

Contribution of Biomass Partitioning and Translocation to Grain Yield under Sub-Optimum Growing Conditions in Irrigated Rice

Abstract The International Rice Research Institute (IRRI) has developed a new plant type (NPT) and Ft hybrids to further increase rice yield potential. In this study we compared yield and yield-related traits among four genotypic groups : indica inbreds, F[ hybrids, NPT and NPT X indica lines; and determined the contribution of biomass partitioning and translocation to grain yield under sub-optimum growing conditions. Field experiments were conduct¬ed in 1998 wet season (WS) and 1999 dry seasons (DS) in the Philippines. Forty-seven genotypes in the WS and 46 genotypes in the DS were studied. Growth analyses were done at flowering and physiological maturity and yield, and yield components were measured at physiological maturity. Among the genotypic groups, average grain yield of the Fj hybrids was the highest and that of the NPT lines was the lowest. Grain yield was highly associated with harvest index (HI) with an r2 of 0.73-0.84 in both seasons. The relationship between grain yield and biomass production was relatively weak. A negative relationship was observed between T,the amount of biomass accumulated before flowering and translocated to the grains during grain filling and Wr the biomass accumulation from flowering to physiological maturity. The NPT lines had the highest average Wrbut had the lowest Tamong the genotypic groups, which was opposite of that of the Fj hybrids. Compared to Wr, Twas more closely related to HI and grain yield. Results suggest that under sub-optimum growing conditions such as low total solar radiation increasing Tand HI is vital for achieving high actual grain yield in irrigated rice

Higher Leaf Area Growth Rate Contributes to Greater Vegetative Growth of F1 Rice Hybrids in the Tropics.

Abstract The early vigor of hybrid rice (Oryza sativa L.) developed in temperate areas has been mainly attributed to its higher tillering rate. The objective of this study is to identify factors that contribute to greater vegetative growth of Fi hybrid rice recently developed in the tropics. We analyzed data from field experiments conducted from 1994 to1997 in both dry and wet seasons at the International Rice Research Institute farm. Each experiment had one to four pairs of comparisons between best hybrids and best conventional varieties. Parameters used for comparison were tillering rate, leaf area growth rate (LAGR), and crop growth rate (CGR) between transplanting (TR) and midtillering (MT) and also between MT and panicle initiation (PI). In most cases, the hybrids exhibited greater CGR than the conventional varieties, either between TR and MT or between MT and PI or both. The difference in CGR was associated with the difference in LAGR between hybrids and conventional varieties. However, the tillering rate of hybrids was significantly lower than or equal to that of conventional varieties. Therefore, the higher LAGR, independent from tillering, contributed to greater CGR of recently developed F1 hybrid rice in the tropics. The difference in LAGR and CGR between hybrids and conventional varietieswas not associated with tillering. The higher LAGR of the Fl hybrids than that of the conventional varieties could be attributed to their thinner leaf blades.

Effect of Panicle Size on Grain Yield of IRRI-Released Indica Rice Cultivars in the Wet Season

Abstract Grain yield under wet season (WS) conditions has gradually received more attention due to looming scarcity of irrigation water, which limits the area for planting to flooded lowland rice in the dry season in the tropics. This study was conducted to determine (1) grain yield of IRRI (International Rice Research Institute)-released rice cultivars under WS conditions and (2) if panicle size (spikelet number per panicle) is an important trait that influences grain yield in the WS. Field experiments were conducted at the IRRI farm in the 2000 WS and 2001 WS using 14 IRRI-released conventional and two F1 hybrid cultivars under irrigated lowland conditions. Grain yield and yield-related traits were measured at maturity. Grain yield of tested cultivars ranged from 4.5 to 7.0 t ha−1 in the 2000 WS and from 4.1 to 5.6 t ha−1 in the 2001 WS. Large differences in panicle number and panicle size were observed among cultivars. All cultivars had small to intermediate panicle size ranging from 63 to 114 spikelets per panicle. Among all the measured yield-related traits, panicle size had the most consistent and closest positive correlation with grain yield. These results suggest that it is possible to improve maximum attainable yield in WS by breeding cultivars with larger panicle size. However, whether other cultivar groups such as the tropical japonica with large panicles (150-200 spikelets per panicle) would confer high yield in WS remains to be studied.

Microwave‐oven drying of rice leaves for rapid determination of dry weight and nitrogen concentration

Abstract Predicting the need for fertilizer‐nitrogen (N) topdressing based on plant N status is an important N management strategy for increasing both grain yield and N‐use efficiency of irrigated rice. Plant N analysis by conventional‐oven drying and micro‐Kjeldahl procedure generally requires several days. Accurate estimation of leaf N content per unit dry weight (Ndw) at different growth stages by the chlorophyll meter requires that meter‐reading values (SPAD values) be adjusted for specific leaf weight (SLW). This study demonstrated that a microwave oven can be used for drying rice leaves for quick estimation of leaf dry weight without significantly influencing the estimation of Ndw by micro‐Kjeldahl procedure. Microwave oven drying of 0.5 g (dry weight) leaf samples required only two minutes. Extended exposure duration up to six minutes did not alter the measured Kjeldahl N concentration. Specific leaf weight determined with microwave‐oven drying improved the prediction of Ndw by SPAD. The chlorophyl...