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A greenhouse experiment was conducted using twelve Japonica rice genotypes differing in grain Cd concentration, to study the genotypic difference in germination and early seedling growth in response to Cd stress and its relation to Cd accumulation. The results showed that under low Cd level (0.5 mM), germination was significantly enhanced for most rice genotypes, but higher Cd levels (2.5 and 10.0 mM) inhibited germination dramatically. When exposed to 10.0 mM Cd, a few genotypes could still germinate, but the seedlings of all genotypes would die in early stage. Shoot height and dry weight, and root length and dry weight of seedling were significantly inhibited at 0.5 and 2.5 mM Cd levels, irrespectively of genotypes, and the inhibition became more severe under higher Cd level. There was a significant genotypic difference in response of germination, early seedling growth and shoot Cd concentration to Cd stress. The difference was the largest for germination, followed by root growth, the least for shoot growth. A cluster analysis of tolerance indices, including germination rate, shoot height and weight, root length and weight of the plants exposed to 2.5 mM Cd level showed that 12 rice genotypes may be grouped into three clusters, i.e. high-, intermediate- and weak Cd-tolerance. No significant correlation was found between grain Cd concentration and Cd-tolerance in terms of germination and seedling growth, and shoot Cd concentrations, suggesting that it is possible to develop rice cultivars with high Cd-tolerance and low grain Cd accumulation. [ABSTRACT FROM AUTHOR]

The majority of investigations on mechanisms underlying yield difference between superior and conventional cultivars has focused on the grain-filling stage after anthesis. However, the physiological process before anthesis has been less explored. We aimed to quantify the contribution of canopy photosynthesis at pre-anthesis stage to grain yield formation and clarify the mechanism underlying the yield advantage of the superior cultivars. This study selected representative cultivars of indica and japonica rice, each type having a pair of cultivars with ideal plant architecture (IPA) and conventional architecture. Field experiments over four years (2019–2022) were conducted to compare the relevance of canopy photosynthesis to yield formation between the phases before and after anthesis. The two IPA cultivars, JYZK-6 and ZZY-1, produced higher grain yield than the two conventional cultivars, NJ-5055 and HHZ. JYZK-6 and ZZY-1 also had good distribution of light interception inside the canopy, with the middle and lower part receiving more light than NJ-5055 and HHZ. At leaf level, there was no significant difference in photosynthetic rate between the two types of cultivars. At canopy level, the IPA cultivars had stronger photosynthetic activities before anthesis, outperforming NJ-5055 by 35.56% in averaged canopy photosynthesis, and by 46.65% in averaged daily net photosynthetic accumulation in 2021–2022. By contrast, canopy photosynthesis after anthesis was not significantly different between the two types. The IPA cultivars stored heavier biomass in internodes and sheaths before anthesis and remobilized more reserves to the developing grains, with the remobilization rate being 16.42% (internodes) and 35.34% (sheaths) higher than NJ-5055 and HHZ. The higher canopy productivity before anthesis is one of the main reasons accounting for the superior IPA cultivars, suggesting the necessity of optimizing plant growth and development during the vegetative and reproductive phases. • Canopy photosynthesis was compared between IPA and conventional cultivars. • IPA cultivars had a good distribution of light interception inside the canopy. • IPA cultivars had stronger canopy photosynthetic activities before anthesis. • We suggest the necessity of optimizing canopy structure development before anthesis. [ABSTRACT FROM AUTHOR]