Increasing the yield potential in irrigated rice - manipulating source and sinks.   P. Raghuveer Rao, S. Ravichandran, R. Mahender Kumar, D. Subrahmanyam, A. S. Ram Prasad and S. R. Voleti Directorate of Rice Research, Rajendranagar, Hyderabad, India. E-mail:prrao2005@yahoo.co.in Introduction : In crop plants, the physiological basis of dry matter production is dependant on the source-sink concept, where the source is the potential capacity to utilize the photosynthetic products. If the sink is small, the yield cannot be high; and even if the sink is large, the yield cannot be high if the source capacity is limited. Generally crop plants including rice and wheat have shown changes in source and sink during the course of domestications and evolution into modern cultivars. The modern plant type concept is essentially a blend of improved source and sink. The leaf (source) is short and erectofile to capture greater solar radiations and the sink is expanded by increased grain number and size by profuse tillering. Physiologists in rice have tried to optimize source (total dry matter production, 18 tonnes/ha) with an harvest index of 50% to attain an yield of a 9 tonnes/ha in medium duration varieties with a duration of 130-140 days. In the present study efforts were made to estimate source capacity sink capacity and assimilate portioning (HI) in irrigated rice for early, medium, late duration varieties and hybrids. Materials and methods: Six varities (Akshayadan, Varadhan, DRR dhan 38 (Kodachadri), KRH-2, PA-6201, PHB-71) were grown in split plot design with main plots as Nitrogen levels (0, 50,100 and 200 Kg N/ha ) and sub plots as varieties. Different nitrogen levels are given to manipulate the source and sink sizes and their influence on physiological, biomass, yield and yield attributes. Data was collected for physiological parameters for growth and yield attributes. Physiological parameters data was collected for photosynthesis, stomatal conductance, transpiration, water use efficiency and Corboxylation efficiency was measured using LICOR 6400. Source and sink sizes were computed along with growth analysis for estimating CGR, NAR, RGR etc. The data was further analyzed for making correlations for various parameters and path coefficient analysis along with heritability factor were derived. Source size was estimated=LAI×NAR×CGR Sink size was estimated by=Panicles/m2×spikelet number/ panicle×1000grain weight Growth analysis was done and CGR, NAR, RGR, AGR, SLW and SLA were estimated. Using LICOR 6400 to estimate Photosynthesis,Transpiration ,WUE, Ci, Leaf temp. , A/T ratio and Corboxylation efficiency (A/Ci) ratio. Results : NAR Leaf area CG duration LAD Canopy photosynthesis Biomass HI/Partitioning Yield Possible factors contributing to rice productivity Conclusions: Biomass of 20 tonnes/ha and Grain yield of 8- 9 tonnes/ha was achieved with following character: Rate of Photosynthesis (µmol CO2/m-2/sec) 22-25; Rate of Transpiration (mmol H2O /m-2/sec) 10-12; Internal CO2 250ppm; Leaf Temperature -1C; A/T Ratio of 1.9-2.0 and High Corboxylation (A/Ci ratio) of 0.09; CGR of 50-60 g/M2/Week; LAI of 6-7; HI 45-50%. Photosynthetic rate and transpiration rate were high in hybrids as compared to high yielding varieties, among hybrids PHB-71 had the highest photosynthetic rate and transpiration rate, among high yielding varieties Akshayadhan had high photosynthetic rate. Varadhan had high water use efficiency when measured by Licor 6400 in terms of A/T and IWEU ratios. A/Ci ratio i.e., Carboxylation efficiency or activity of RUBP carboxylase was found to be higher in hybrids as compared to high yielding varieties. Significant correlation was observed for yield with TDM and spikelets per panicle. It was found that yield levels can be improved in irrigated ecology by improving the Bio-mass (Total Dry Matter) and sink size (spikelets per panicle) in HYV and Hybrids. Source and Sink capacity were estimated in early, medium and late duration HYV along with Hybrids. Source was limiting in early duration HYV and can be improved for achieving higher yields. In Medium duration HYV both source and Sink need to be improved for achieving higher yields. In Long duration HYV source is optimum, but sink capacity can be further improved for achieving higher yields. In Hybrids both source and Sink are optimum in capacity and have a yield potential of 9 tonnes/ha. Acknowledgements: The authors wish to thank Dr B.C Virakthamath, PD, DRR and ICAR for supporting the project. We also wish to thank Indian society of plant physiology and staff of Crop physiology, ANGRAU for Organizing this seminar.