Anticipated Senescence as a Result of High Source Sink Ratio in Rice.

Grain yield in modern rice (Oryza sativa L.) cultivars is attained when high percentage of ripened grain and grain dry weight as a result of high leaf photosynthesis during reproductive stages. However, an optimal source-sink ratio is uncommon under field and, most of the time, grain yield is limited by source or sink. The objective of this work was to evaluate carbon exchange rate (CER) in rice during the heading-physiological maturity period under different source-sink ratio. IRGA417, a widely adopted cultivar, was growth at Paraná Research Station of INTA (31º 50’ S, 60º 31 W; 110 m above sea level) Argentina during 2009/10 growing season. Seeds were sown in 100-pots and four-leaf seedlings were thinned to four seedlings per pot. Pots were placed in a plastic bath in order to grow rice under flooded conditions. At full heading, sink size was reduced by cutting 50% of spikelets on a half of plants (high source-sink ratio), while the rest of the plant remained unaltered (control). From full heading to physiological maturity (heading, 10, 20, and 30 days after first heading) CER was measured on flag leaf of selected plants in both treatments by infrared gas analyzer (LICOR 6400). In addition, greenness of the flag leaf was estimated by using a chlorophyll meter SPAD 502. Total dry matter and grain weight was estimated by cutting, processing and drying rice plants once both CER and SPAD measurement were performed. Treatments showed a significant difference in CER from 10 to 20 days after first heading. Thus, CER in plants with high source sink ratio was 16 and 35 % lower than that in control plants. Reduced number of spikelets did not modify grain weight. A strong positive correlation (R2=0.94, p<0.05) between CER and SPAD values was founded. Our results revealed that, under increased source sink ratio, a feed back control on leaf photosynthesis operates during the grain filling. In addition, an early decline in SPAD values suggests that both functional and visual symptoms of leaf senescence are coupled under high source sink ratio in rice.