Morphological and physiological factors contributing to stronger early vigor in the elite rice cultivar 9311 compared to Huanghuazhan (HHZ)

Huanghuazhan (HHZ) and 9311 are two elite rice cultivars in China. They have achieved high yield through quite different mechanisms. One of the major features that gives high yield capacity to 9311 is its strong early vigor, i.e., faster establishment of its seedling as well as its better growth in its early stages. To understand the mechanistic basis of early vigor in 9311, as compared to HHZ the cultivar, we have examined, under controlled environmental conditions, different morphological and physiological traits that may contribute to its early vigor. Our results show that the fresh weight of the seeds, at germination, not only determined the seedling biomass at 10 days after germination (DAG), but was also responsible for ~ 80% of variations in plant biomass between the two cultivars even up to 30 DAG. Furthermore, the 9311 cultivar had a larger root system, which led to its higher nitrogen uptake capacity. Other noteworthy observations about 9311 being a better cultivar than HHZ are : (i) Ten out of 15 genes involved in nitrogen metabolism were much more highly expressed in its roots; (ii) it had a higher water uptake rate, promoting better root-to-shoot nitrogen transfer; and (iii) consistent with the above, it had higher leaf photosynthetic rate and stomatal conductance. All of the above identified features explain, to a large extent, why the 9311, as compared to HHZ, exhibits much more vigorous early growth. At 50 days after germination (DAG), plants were transferred to NH4NO3-free solution for 2 days. Then, plants were transferred back to normal culture solution (containing 1 mM NH4NO3) for 3 hours. Then roots were sampled and frozen in liquid nitrogen immediately. Three replications were used. We used the PureLink RNA Mini Kit (Life Technologies Corporation) for RNA extraction from roots. To do this, we firstly grinded the root sample with liquid nitrogen, then followed the manufacture’s protocol for RNA extraction.