A study was conducted during Boro and T. Aman seasons of 2002 at Bangladesh Rice Research Institute (BRRI), Gazipur to see the relationship of SPAD (Soil plant analysis development) reading with chlorophyll and N contents of leaves and to determine the critical LCC value for rice crops. Hybrid varieties Sonarbangla-1 and BRRI hybrid dhan0l were used for both rice crops and BRRI dhan29 and BRRI dhan31 were used as checks for Boro and T. Aman crops, respectively. Sonarbangla-1, BRRI hybrid dhan0l and BRRI dhan29 had similar leaf chlorophyll contents in Boro season. The maximum chlorophyll content (1.6-1.8 mg/g leaf) was observed at 39-42 soil plant analysis development (SPAD) value. In T. Aman season, the inbred BRRI dhan3l showed lower amount of chlorophyll (1.2-1.4 mg/g leaf) at 39-42 SPAD value compared to the hybrids Sonarbangla-1 and BRRI hybrid dhan 0l. Seasonal variation in chlorophyll content between Sonarbangla-1 and BRRI hybrid dhan0l was not large. Relationship between SPAD value and chlorophyll content was very close (R 2 =>0.8) at panicle initiation and flower initiation stages for all the varieties. Similar relationship was also observed in case of SPAD value and nitrogen content in leaves. The results indicated that the rice leaves showing higher SPAD readings (>35) had higher chlorophyll and nitrogen contents. The adjusted critical LCC values were 3.0 for Boro and 3.5 for T. Aman seasons for all rice varieties.

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Water and nitrogen (N) play vital roles in rice (Oryza sativa L.) production. Water-saving management decisions can influence soil conditions (oxidation and reduction), and these soil conditions affect the fate of N in paddy soil. We designed three irrigation regimes, namely, conventional irrigation (Flooding), shallow water depth (SWD), and a Non-flooding treatment, for our study. For the Flooding treatment, a ponding water depth of 0.05-0.06 m was maintained for 99 days after transplanting (DAT), and the water was drained 20 d before harvesting. For the SWD treatment, a ponding water depth of 0.01-0.02 m was maintained from 20 DAT to 99 DAT, and water was drained 20 d before harvesting. For the Non-flooding treatment, the plots were saturated (not flooded) from 20 to 57 DAT. After 57 DAT, a ponded water depth of 0.01-0.02 m was maintained until 99 DAT, and the water was drained 20 d before harvesting. The N fate and rice growth did not differ between the treatments during the early growth stages in response to the differences in water management. Rice root activity during the middle growth stage was higher for SWD, which was possibly because of the above-ground biomass and other factors during the middle growth stage of the rice. We found that the recovery efficiencies for N fertilizer, N uptake and above-ground biomass at the heading stage were higher for SWD than for the other two treatments. Although the soil status (reduction or aerobic conditions) was the same for the different water treatments up to the maximum tiller number stage, either the SWD or Non-flooding option could be used to conserve water. In addition, the fate of the N fertilizer, the N use efficiency, the rice growth and the rice yield were the same or higher with these two treatments than with the Flooding water management treatment.

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