Changes In The Yield And Nitrogen Use Effieieney Of Mid-season Indica Rice Cultivars During The Improvement Process And Their Physiological Bases

To understand changing characteristics of grain yield and nitrogen (N) use efficiency during the improvement of rice cultivars and their physiological mechanism would have great significance in providing information to the high-yielding and high-efficiency breeding and cultivation. In this study,12typical mid-season indica rice cultivars adopted in the production in Jiangsu Province during the last sixty years were used. They were divided into early tall cultivar (ET), dwarf cultivar (DC), semi-dwarf cultivar (SDC) and super rice cultivar (SR) according to the time of use and plant types of cultivars. Three levels N fertilizer treatments, ON (no N application),210N (210kg hm-2N) and300N (300kg hm-2N), were conducted. The changes in grain yield and N use efficiency of mid-season indica rice cultivars during the improvement process and their physiological mechanism were investigated. The main results are as follows:1. Grain yield was progressively increased with the improvement of cultivars. Increase in grain yield was attributed mainly to the increase in total number of spikelets, which resulted mainly from a large panicle (more number of spikelets per panicle). The SR had lower percentage of filled grains, which limited its high-yielding potential. From ET to DC, the biomass and harvest index were simultaneously increased, especially the increase of harvest index was more significant. From DC to SR, the harvest index fell slightly, increase in grain yield was resulted mainly from large biomass. Grain quality was improved with the improvement of cultivars.2. There were significant differences in responding to N rates among the cultivars. Grain yield was increased for all the cultivars when application rates ranged form ON to210N, with more increase for modern SDC and SR than for ET and DC. When the N rates from210N to300N, the yield of super rice cultivars was continuously increased, while it was decreased for ET, DC and SR. With increases in N rates, the biomass of each cultivar was increased, companied with the decrease in harvest index.3. The leaf area index (LAT) at heading stage was significantly increased with the improvement of cultivars. The total number of spikelets was also markedly increased. There was no significant difference in grain-leaf ratio (the ratio of total number of spikelets to leaf area at heading) for all the cultivars under the ON and210N treatments, but the SR had a significantly higher grain-leaf ratio than the other cultivars. Compared with other cultivars, the SD and SR had higher root oxidation activity and leaf photo synthetic rate during the early and middle growth stages. However, senescence in leaves and roots at the late grain filling stage was earlier for SR than for other cultivars, which may be the important reason for a lower percentage of filled grains in SR.4. The root dry weight during the growing season was significantly increased with the improvement of cultivars. The root-shoot ratio was progressively increased during the improvement of cultivars at the mid-tillering stage and panicle initiation stage, and it showed was no significant difference among the cultivars at the late growth stage. The root dry weight of each cultivar was increased firstly and then decreased with the increase in N rates, but the decrease was small. At the mid-tillering stage and panicle initiation stage, the root-shoot ratio of each cultivar was decreased with the increase in N rates. There was no significant difference for a variety in the root-shoot ratio among the three N rates.5. The leaf angle of the top three leaves on stems was significantly reduced with improvement of cultivars. From ET to DC, the plant height was decreased significantly, wherase from DC to SR, the plant height was increased progressively. With the increase in N rates, the leaf angle of the top three leaves on stems was increased gradually, but there was no significant change in the plant height of a cultivar.6. The translocation ratio of non-structural carbohydrates (NSC) stored in stems and sheaths from heading to maturity and the contribution of NSC to grains showed SDC>SR>DC>ET, The NSC translocation ratio was decreased with the increase in N rates.7. The partial factor productivity of applied N (PFP), agronomic efficiency of applied N (AE), N physiological efficiency (PE) and grain production efficiency were significantly increased with the improvement of cultivars. There was no significant change in N recovery efficiency (RE) during the improvement process of cultivars. The N dry matter production efficiency (DMPE) varied with cultivars and N rates. The DMPE was SR>ET>SDC>DC Under the ON treatment, and it was higher for SR and SDC than for ET and DC under210N and300N treatments. The PFP, AE, PE, RE, NPGE and DMPE were decreased with the increase of N rates.8. Activities of N metabolic enzymes, glutamine synthetase (GS), nitrate reductase (NR) and glutamate syntheses (GOGAT) were significantly increased with the improvement of cultivars and with the increase of N rates.In summary, grain yield and N use efficiency were increased with the improvement of mid-season indica cultivars, which were mainly attributed to the increase in number of spikelets per panicle, improvement in plant types, enhancement in root and shoot activities, and high use of pre-anthesis NSC stored in stems. Super rice cultivars showed a lower percentage of filled grains which was associated with an early senescence. It would be an important approach to further increase grain yield of super rice through an increase in root activity during grain filling.