Differential stem reserve food mobilization and sink strength in rice cultivars grown under submerged and aerobic conditions

Mobilization of photosynthetic products from shoots to developing grains supports plant growth and yield. Herein, we have studied the effect of aerobic condition on stem reserve food mobilization and activities of sucrose metabolizing enzymes in four rice (Oryza sativa L.) varieties viz. MAS25, MAS26 (aerobic) and PAU201, HBC19 (lowland) raised in pots under conventional and aerobic conditions. Sucrose content declined from vegetative (9.19–19.56 mg g⁻¹ DW) to reproductive stage (5.32–7.06 mg g⁻¹ DW) in shoot tissues and in developing grains concomitant with the activities of sucrose synthesizing enzymes viz. sucrose phosphate synthase (SPS), sucrose phosphate phosphatase (SPP) and sucrose synthase (synthesis) (SSs) under conventional condition. Aerobic condition significantly induced sucrose content (30.62–68.27%) owing to increased activities of sucrose synthesizing enzyme and decreased sucrose synthase (cleavage) (SSc) activity in concurrence with decreased starch content (13.3–53.2%). Decreased SSc activity reduced starch content as sucrose to starch conversion came to halt which triggered stunted growth and reduced the yield under aerobic condition. However, all varieties sustained aerobic condition which might be attributed to assimilated sucrose acting as osmoprotectant. To further assess, semi-quantitative RT-PCR for SPS and SSc (SUS) genes was carried out. The observed increase in sucrose content under aerobic condition could be attributed to increased and decreased expression of SPS and SUS gene, respectively. Biochemical analysis and gene expression highlighted the role of various enzymes prioritized for assimilate translocation under aerobic condition and gave evidence of difference of aerobic and conventional conditions on aerobic and lowland rice varieties.