| Generally, assimilates for filling grains in cereals are supplied concurrently from two sources:current assimilates in photosynthetic leaves during the grain filling duration and reserves in stems (leaf sheaths and culms) accumulated before heading. In rice, the apparent contribution of non-structural carbohydrates (NSC) in stems to final grain yield has been estimated at 30% under normal condition. A large amount of carbohydrate in stems at heading stage can play an important role in raising sink activity, improving lodging resistance and buffering the yield loss under unfavorable conditions. Nitrogen is considered to be one of the most important macronutrient for rice production, and an amount of nitrogen application can affect markedly characters of source-sink-flow in rice. However, it is poorly understood that genotypes differences and genetics analysis of non-structural carbohydrates accumulation and recombination in rice under different nitrogen conditions.The objectives of this study were:1) to study relationships of non-structural carbohydrates accumulation and translocation with yield formation and effects of source-sink-flow characters in rice recombinant inbred lines under two nitrogen levels,2) to examine a difference in NSC accumulation and translocation in rice lines with different sink content under two nitrogen conditions; 3) to examine the genotypic variations in NSC accumulation and translocation in rice lines with different plant height under two nitrogen conditions; 4) to compare the effects of nitrogen fertilizer on the contents of non-structural carbohydrate at different internodes in different rice verities,5) to understand the genetic basis of relationships of non-structural carbohydrates accumulation and translocation with yield formation in rice under the two nitrogen applications, and 6) to identify two new locus responsible for increased grain weight and NSC translocation and to elucidate its physiological function to understand how to improve potential yield in rice. In this study, field and pot experiments were conducted. The following results were obtained:(1) Stem non-structural carbohydrates (NSC) and its relationships with yield formation were investigated for two years under low (LN) and normal nitrogen (NN), using 46 recombinant inbred lines from Zhenshan 97 x Minghui 63. Concentration of NSC (CNSC) and total mass of NSC in stem (TMNSC) at heading, apparent transferred mass of NSC (ATMNSC) and apparent contribution of transferred NSC to grain yield (ACNSC) were larger under LN compared with those under NN, respectively, however, there was no significant difference in apparent ratio of transferred NSC from stems to grains (ARNSC). ATMNSC positively correlated with grain yield,1000-grain weight, and ACNSC under both nitrogen levels, while ARNSC markedly correlated with harvest index and ACNSC.Leaf area had more contributions to grain yield compared with ATMNSC under LN and NN. ATMNSC had larger direct effects grain yield under LN than that under NN. For ATMNSC, TMNSC at heading, small vascular bundles (SVB) and spikelets per m2 under LN had positive direct effects. For ARNSC, SVB and spikelets per m2 under LN had larger and positive direct effects, and large vascular bundles had negative direct effects. For ACNSC, TMNSC at heading and SVB under LN had positive direct effects. In brief, low nitrogen supply increased stem NSC accumulation and translocation to developing grains. For example, apparent contribution of transferred NSC to grain yield (ACNSC) was higher under LN, ranging from 0.71% to 28.44% with mean of 13.09%, versus from 0.96% to 15.30% with mean of 9.53% under NN. Characters of source-sink-flow system had different effects on NSC translocation and contribution to yield formation, depending on genotypes and nitrogen levels.(2) In this study, sink content (SC) showed an abundant variation in RI lines, and 6 SC types of rice were clustered by SPSS soft ware. SC minimum A-type and maximum F-type was 388g m-2 and 887g m-2 under normal nitrogen condition, respectively. Effect of nitrogen levels on SC was significant. SC was significantly decreased under low nitrogen condition. RI lines with larger SC had higher NSC accumulation in stems at heading, higher ratio of NSC translocation and higher transferred mass of NSC during grain filling. The pattern of change in dry matter showed a similar trend with NSC in stem. As sink increased, dry matter and NSC translocation in stem increased. It indicted that the activity of NSC metabolism in stem can be regulated by SC. However, NSC residues in stem at maturity had no significant differences under all SC types. Apparent contribution of transferred NSC to grain yield was increased under low nitrogen condition. Apparent contribution of transferred NSC to grain yield showed decreasing, when SC was less than 730g m-2 under normal nitrogen condition. ACNSC show increasing trend, when SC was over 730g m-2. Under normal nitrogen condition, apparent contribution of transferred NSC and dry matter to grain yield in A-type had higher than that in C-type by about 3.73% and 8.69%, respectively. When SC was over 730g m-2, F-type had higher than that in C-type by about 2.13% and 6.59%, respectively.(3) To understand the phenotypic variation of nonstructural carbohydrate (NSC) accumulation in rice culms and leaf sheaths and its relationship with yield formation during grain filling period under two nitrogen conditions; Three rice verities: Liangyoupeijiu (indica hybrid variety), Yangdao 6 (indica inbred variety) and IR64 (tropical indica variety), were grown in pots under two nitrogen treatments. In this study, upper three internodes on the main stem per plant were divided into six parts of the peduncle, the penultimate internode, the third internode, flag leaf sheath,-2 leaf sheath and -3 leaf sheath. NSC concentration was measured at different parts in stems. The NSC concentration was always less than 135mg g-1 at the peduncle, and it is no sensitive for nitrogen treatments. The results showed that the peduncle maybe play an important role in transporting of assimilates. The penultimate internodes, the third internodes and corresponding sheaths, as mainly NSC storage organ before heading, were affected by nitrogen treatments in NSC concentration, especially the top value of NSC concentration at different organs. The NSC concentration of third internodes was most sensitive for nitrogen treatments among of different organs. Otherwise, Liangyoupeijiu had the higher NSC concentration than inbred varieties in this study, but it is more sensitive than inbred varieties. On the other hand, NSC concentration is not sensitive for nitrogen treatments in IR64, but this part of NSC in stems is very important to grain yield development, especially to maintaining grain filling percentage. The results indicate that varieties in different genetic backgrounds showed different changes under different nitrogen conditions, and it is the main reason that genotypic variation for stem NSC in response to nitrogen supply. Carbon remobilization in the penultimate internode and -2 leaf sheath played an important role in grain yield formation.(4) As plant height increased. NSC accumulation at heading and at maturity show the increasing at first then decreasing, and C distribution in structural carbohydrates increased according to vascular bundle characters in stem. As plant height increased, ratio of NSC translocation and dry matter showed the decreasing trend, while apparent transferred mass of NSC showed that increasing at first then decreasing. The change of leaf area at heading showed a similar trend with apparent transferred mass of NSC as plant height increased. When plant height was 115cm, apparent transferred mass of NSC arrived at the top value. However, apparent contribution of transferred NSC to grain yield showed decreasing as plant height increased. It suggested that leaf source, stem source and assimalition matter (dry matter and NSC) translocation were concordant at 115cm (plant height).(5) To understand the genetic basis of relationships of non-structural carbohydrates accumulation and translocation in rice culms and leaf sheaths with yield formation under two nitrogen levels, quantitative trait loci (QTLs) for 9 related traits were mapped with a RIL (recombinant inbred line) population derived from a cross between Zhenshan97 (indica var.) and Minghui63 (indica var.) under field environment. Six QTLs affecting NSC accumulation at heading were detected on chromosome 1,6 (2 locus),7 (2 locus) and 11. The three QTLs were identified under normal nitrogen condition, accounting for explaining 54.29% of the total phenotypic variation. The Zhenshan97 alleles increased the trait value at all. Another three QTLs were detected under low nitrogen condition, accounting for 50.35% of the total variation. The numbers of QTLs for ARNSC under normal and low nitrogen conditions were two and three four, respectively. The amounts of total variation explained by the ARNSC QTL were 20.37% and 42.49% under normal and low nitrogen conditions, respectively. Under normal nitrogen condition, a total of five QTLs were detected for, jointly explaining 47.79% of the total variation, while those explained by the three QTL were 19.37%,14.42% and 12.59% under low nitrogen condition. The Zhenshan97 allele increased NSC accumulation at heading stage, ATMNSC and at the region near R753 on chromosome 1. Another QTL at the region RG360-R3166 on chromosome 5 increased ARNSC, ATMNSC and 1000-grain weight. Zhenshan97 contributed the allele at QTLs. In all, the two regions, i.e., G359-R753-C161 and RG360-R3166 provide a genetic explanation for the close correlations between NSC translocation and grain yield traits.(6) Two pleiotropic QTLs for grain weight and NSC translocation in stem were localized on a rice genetic map under different nitrogen conditions. QTLs were detected on chromosome 1 (G359-R753-C161) and 5 (RG360-R3166) respectively, and two loci all explained large phenotypic variation from Zhenshan 97. Based on the above results, we selected two lines from RILs including plelotropic QTLs (R91 and R156), and a line didn’t include two loci (R201). QTLs functions were further confirmed in a plot experiment. There were no significant differences in leaf area at heading, total biomass at maturity and sink capability among of three lines. The concentration of soluble sugars and starch in stems showed increasing trends among of three lines early reproductive stage, but there were significant differences in changes of from 10 days after heading to maturity. For example, the soluble sugars and starch maintain increasing trend in R201, but the others all (R91 and R156) showed significantly decreasing. These results suggest that NSC translocation from stems to grains is baffled during late grain filling stage in R201. Further studying found that there were significant differences in soluble sugars concentration in branches among three lines, and showed higher soluble sugars concentration in R201 than that in others, suggesting that NSC remobilized from stems to rachis branches is normal to R201. However, R201 re-accumulate soluble sugars and starch in stems at late grain filling stage. It suggests that R201 can’t normally unload NSC from branches to grains, consequently resulting in NSC re-accumulation in stems. We forejudge that either G359-R753-C161 on chromosome 1 or RG360-R3166 chromosome 5 may include sucrose synthase gene which can regulate sugar unloading in grains after heading 10 days. So the two chromosomal regions will show remarkable value in map-based cloning and breeding programs using a marker assisted approach. Two loci can be transferred to other large sink breeding materials, it may significantly improve grain yield.The results of this study showed that stored NSC in stem play an important role in yield formation, especially in super rice varieties. So studying relationships of non-structural carbohydrates accumulation and translocation with yield formation, and its genetic mechanisms under different nitrogen conditions will help to understating yield formation mechanism, especially under low nitrogen stress. As the additive effects are harmonic among QTLs affecting yield trait and NSC translocation, the favored allele (from ZS97 in this study) will be transferred to other breeding materials to validate the effect of molecular assisted selection (MAS), targeting on the large chromosomal segment, as a candidate MAS strategy. The best way is to combine breeding programs and crop management strategies depending on the improvement of carbohydrate storage and translocation to grains. It will be an important significance for increasing rice yield potential. |
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