Studies On The Varietal Differences Of Photosynthetic Characteristics And Its Underlying Mechanisms In Rice Plants

With increasing population and decreasing arable land, improving crop yield, especially rice yield, is vital to maintain food security. Yield improvement of rice plants in the past few decades was mainly dependent on the increased light interception rate and harvest index, rather than on the energy use efficiency and leaf photosynthesis. In recent years, improving leaf photosynthesis is considered to be one of the most important approaches to further increase rice production in the future. Leaf N content, CO2 diffusion capacity, leaf anatomy and utilization of photosynthates can sufficiently affect leaf photosynthesis. Agronomists usually made great affort to improving crop nutrient status especially N nutrition, but payed little attention to CO2 diffusion capacity. Specific leaf weight(SLW) is an important leaf anatomical parameter, and can significantly affect leaf N content and CO2 diffusion capacity. However, little reseach was conducted on this topic in cereal crops, especially in rice. Non-structural carbohydrates(NSC) is important to improve crop yield and grain filling rate, and utilizaiton of NSC can efficiently reduce their accumulation in the leaves, and prevent feedback inhibition of photosynthesis. However, it is not clear whether varietal difference in NSC can affect leaf photosynthesis efficiency.The objectives of this study were(1) to compare varietal differences in photosynthetic characteristics, and to analysis the effects of leaf N content, CO2 diffusion capacity, leaf anatomy and utilization of NSC on leaf photosynthesis;(2) to elucidate the differences in photosynthetic characteristics between indicia and japonica subspecies and to exploit its underlying mechanisms. Twelve rice cultivars were pot grown under both low and high N treatments in 2013. These rice cultivars can be divided into four groups, inbred indica, hybrid indica, inbred japonica and hybrid japonica.The parameters, such as leaf photosynthetic rate(A), stomatal conductance(gs), SLW, and leaf nitrogen N content(Narea) were measured at both tillering and grain filling stages. One representative rice cultivar from each variety group was grown in 2014 under low and high N supplies. A, gs, mesophyll conductance(gm), Narea and Rubisco content were measured. The results obtained were as follows:(1) Under either low or high N supply, varietal difference in A was not related to Narea at either tillering or grain filling stages. Although it was significant when the data from two N treatments were pooled(tillering stage, r = 0.77***; grain filling stage, r = 0.62*). There were significant and positive correlations between A and gs as well as gm. Varietal difference in SLW was significantly related to Narea, but gs and gm were not related to either Narea or SLW. This would be one of the reasons why A was not related to Narea or SLW. SLW was negatively related to leaf photosynthetic N use efficiency(PNUE), and decrease in SLW under high N supply would alleviate the decline in PNUE.(2) There were significant varietal differences in A, gs, gm, and PNUE. A, gs, gm, and PNUE in indica rice cultivars were higher than those in japonica rice cultivars. SLW in indica rice cultivars was lower than that in japonica rice cultivars. It suggested that gs, gm, SLW may be the reasons for the differences in photosynthetic characteristics between indica and japonica rice cultivars. The differences in Rubisco content and the allocation of leaf N to Rubisco(Rubisco content/Narea) between indica and japonica rice cultivars were not consistent with that in A and PNUE. This suggested that Rubisco content and Rubisco content/Narea were not the major reasons for the difference in leaf photosynthesis. Furthermore, the accumulation of NSC in stem and leaf sheath at grain filling stage can sufficiently decrease PNUE.