Grain Yield And Nitrogen Use Efficiency Of The Mid-season Indica Rice Cultivars Applied At Different Decades

To understand characteristics of grain yield and nitrogen use efficiency of the mid-season indica rice cultivars applied at different decades and their physiological mechanism would have great significance in providing the guidance to the high-yielding and high-efficiency breeding and cultivation. Improvement in rice cultivars plays an important role in the increase of the grain yield. However, little information is available on changes in grain yield and nitrogen use efficiency of mid-season indica rice cultivars during the improvement process. In this study,12typical cultivars (including hybrid combinations) applied in the production in Jiangsu province during the last seventy years were used, and classified into four types of early tall (ET), dwarf cultivar (DC), semi-dwarf cultivar (SDC) and super rice (SR), according to their application times and plant types. Three nitrogen (N) rates, ON (no nitrogen application),210kg N/ha (mid amount of N) and300kg N/ha (high amount of N), were conducted. The changes in grain yield and nitrogen 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 under each nitrogen rate. 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), especially in super rice. From ET to SDC, the filled-grain percentage was dramatically increased. But from SDC to SR, the filled grain percentage was a little decreased. The SR had lower filled-grain percentage; the average filled-grain percentage of three tested super rice cultivars was low than80%, which limited their high-yielding potential.2. Changes in tiller numbers of the mid-season indica rice cultivars showed the "S" curve as the growth process, and tiller numbers were increased with the increase in nitrogen rates. With the improvement of cultivars and increases in N rates, biomasses during the whole growth period and from heading to maturity were increased, and the harvest index was increased first and then decreased. From ET to DC, the harvest index and biomass were simultaneously increased, with more increase in harvest index. From DC to SR, the increase in grain yield was resulted mainly from the increase in biomass.3. Root dry weight of the mid-season indica rice cultivars was increased significantly as the growth process. AT the main growth stages, root dry weight was significantly increased with the improvement of cultivars, with more increase at the heading stage. During the whole growth period, the root-shoot ratio and root oxidation activity were decreased with the growth process. The root oxidation activity of super rice was the largest at the heading stage among the four types of cultivars, but the decline of root oxidation activity was faster for super rice than for semi-dwarf cultivars from heading to maturity, which could account for an important physiological reason for a lower filled-grain percentage of super rice.4. During the improvement of cultivars, the plant height was reduced first and then increased under each nitrogen rate, and the angles of top three leaves on stems were decreased, and the leaf area index was gradually increased under the MN and HN treatments. The super rice cultivars had a smaller leaf angle of the top three leaves on stems and a larger leaf area index when compared with any other types of cultivars, which would result in a higher leaf area index and higher biomass for super rice cultivars.5. The peaks of nitrogen uptake of the mid-season indica rice cultivars appeared from the panicle initiation to heading. The ratio of nitrogen uptake during this period to the total nitrogen uptake during the whole growth stage was30-45%. The peak of nitrogen uptake for super rice was from the panicle initiation to maturity, and it was higher at HN than at MN. The nitrogen accumulation of the mid-season indica rice cultivars was increased with the increase in nitrogen rate at each growth stage, and it was also increased significantly with the improvement of cultivars.6. At the same nitrogen rate, the agronomic efficiency, recovery efficiency, physiological efficiency and partial factor productivity of nitrogen and internal nitrogen use efficiency were increased with the improvement of mid-season indica rice cultivars. Except the agronomic efficiency of super rice, the recovery efficiency, physiological efficiency and partial factor productivity of nitrogen and internal nitrogen use efficiency were decreased with the increase in nitrogen rates for the same type of cultivars.7. Activities of nitrogen metabolic enzymes, glutamine synthetase (GS), nitrate reductase (NR) and glutamate syntheses (GOGAT) were significantly increased with the improvement of cultivars and the increase of nitrogen rates.8. Grain quality was improved during the improvement of mid-season indica rice cultivars. Compared with those of old cultivars (early tall and dwarf cultivars), the milled rice rate, ratio of length to width, gel consistency and breakdown values of modern cultivars were significantly increased, whereas the chalkiness, protein content, amylose content, setback values were decreased, thereby taste quality of the rice was improved with the improvement of cultivars. Generally, application of nitrogen increased protein content in grains, gel consistency, chalkiness, and decreased setback values. The effect of nitrogen application on other indexes of rice quality varied with cultivars.In summary, grain yield, grain quality and nitrogen 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, and to the enhancement in root and shoot photosynthetic activities. Super rice cultivars showed a lower filled-grain percentage 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 and leaf photosynthetic rate during grain filling.