Response Of The Mid-season Japonica Rice Cultivars Applied At Different Decades In Jiangsu To Nitrogen Rates And Its Physiological Mechanism

Jiangsu province is the main base of rice production in China. The development of Japonica rice production in Jiangsu Province has been established as one of the major agricultural projects since1990s, Jiangsu province has adjusted the structural of rice cultivars and the Japonica rice planting area accounts for more than80%of the total acreage. Implementation of strategy on Jiangsu "changing Indica rice to Japanica rice" has created enormous social and economic benefits for the food balance, sustainable and stable yield increase, and had a tremendous impact on the national food security. As the main province of Japonica rice production and consumption in China, the rice varieties in Jiangsu have undergone many improvements, and the grain yield has been greatly improved, which has played an important role in protecting our courtry’s food supply security and social stability. After joining the WTO, China will further open markets for agricultural products, rice quality is a key factor in the competitiveness of the market and has become the main contradiction of supply and demand. Nowadays, Japonica rice has become the most important major crop and rice consumption has occupied a principal position in Jiangsu province. Much work has been done on nitrogen use efficiency and yield improvement for different rice cultivars. However, the information on the effect of nitrogen rates on the mid-season Japonica rice cultivars applied at different decades in Jiangsu is very limited and its physiological mechanism is yet to be understood. To study the effect of nitrogen rates on rice yield, rice quality, mineral element concentrations in grains and on the use efficiency of nitrogen, phosphorus, potassium nutrient would have great significance in providing guidance to breeding and cultivation for high-yielding, good-qualty and high use-efficiency of resources in rice. In this study,12typical mid-season japonica rice cultivars (including super rice) applied in the production in Jiangsu province during the last60years were used, and were classified into six types of1950s,1960s,1970s,1980s, and1990s in the20th century and since the21th century according to their application decades. Three N rates, zero N (ON),240kg ha-1N (MN) and360kg ha-1N (HN), were applied. The objectives were to investigate the changes in rice yield, grain quality mineral element concentrations in grains and the use efficiency of nitrogen, phosphorus, potassium in Japonica rice in Jiangu Province, and to understand the machnism underlying the response of cultivar improvement to nitrogen rates. The main results were as follows:1. Whether or not nitrogen was applied, the grain yield was progressively increased with the improvement of cultivars. The response to N rates was more sensitive for the cultivars bred in the early decades than for the modern super rice cultivars. The grain yield was MN>HN>ON for the cultivars bred in the20th century, whereas it was HN>MN>ON for the super rice cultivars bred since the21th century. The total number of spikelets and spikelets per panicle were simultaneously increased with the the improvement of grain yield. The number of panicles was increased for cultivars from1950s to1980s in20th centry, whereas it was decreased for those at1990s in20th centry and since2000. The1000-grain weight was decreased for cultivars from1950s to1980s in20th centry, and it was increased for those at1990s in20th centry and since2000. The percentage of filled grains was increased for cultivars from1950s to1980s in20th centry, whereas it was decreased for those at1990s in20th centry and since2000. Under the condition of the total nitrogen rate at0-360kg hm-2, the number of effective panicles was MN>HN>ON for the cultivars bred before1990s, whereas it was HN>MN>ON for those at1990s in20th centry and since2000. The percentage of filled grains and1000-grain weight showed ON>MN>HN with the improvement of cultivars. The use efficiency of nitrogen, phosphorus, potassium nitrogen was progressively increased with the improvement of cultivars. The cultivars bred in the20th century showed a high harvest index of nitrogen, phosphorus, potassium at MN, whereas it was at HN for the super rice cultivars bred since the21th century. Compared to earlier japonica rice varieties, modern super rice cultivars grew better under the high amount of nitrogen application.2. A greater number of total spikelets and more biomass production contributed to the increase in grain yield during the improvement of rice cultivars and at the HN rate for the super rice. The biomass and harvest index were simultaneously increased, leading to an increase in the grain yield of modern mid-season japonica rice cultivars. Improvement in population quality (high grain-leaf ratio, high percentage of productive tillers, high leaf photosynthetic rate, high root oxidation activity and the decrease in leaf angle of the top three leaves on stems) contributed to the increase in biomass and grain yield during the improvement of rice cultivars and at the HN rate for the super rice. The cultivars bred in the20th century showed higher root and shoot physiological indicators at MN, such as root dry weight, root oxidination activity, root total absorbing service area, bleeding sap amount of roots, content of zeatin+zeatin riboside (Z+ZR) in roots, root-shoot ratio, leaf photosynthetic rate and chlorophyll fluorescence, activities of the enzymes involved in nitrogen metabolism (nitrate reductase, glutamine synthase and glutamate synthase) and reactive oxygen species scavenging capacity in the flag leaf and higher non-structural carbohydrates (NSC) in the stem and sheath, whereas the physiological indicators were better at HN for the super rice cultivars bred since the21th century. The results indicate that the improvement in physiological qualities of roots and shoots would be the important reason for the increase in grain yield during the improvement of rice cultivars and at the HN rate for the morden super rice. The percentage of filled grains and the yield potential of super rice could be increased by enhancing root oxidination activity at the late grain filling stage.3. Regardless of nitrogen rates, the milling quality and appearance quality of the mid-season japonica rice cultivars were improved during the improvement of rice cultivars. Compared with that of the early mid-season japonica rice cultivars, the taste of modern super rice varieties was improved. The chalk kernel percentage, chalk size and chalkiness were increased at MN and HN when compared with those at ON. Effects of nitrogen rates on the head rice rate varied with varieties, i.e., the modern varieties showed a higher head rice rate at ON, indicating that nitrogen fertilizer is not conducive to improve milling quality of mid-season japonica rice cultivars in Jiangsu Province. When nitrogen rate was ranged from0to360kg hm-2, the appearant and taste qualities were decreased with the increase in nitrogen rates for all the tested rice cultivars, implying that application of nitrogen is not conducive to increase quality for mid-season japonica rice cultivars in Jiangsu Province. Nitrogen is also not conducive to increase other mineral elements in rice grains except for nitrogen. Further research is needed to simultaneously increase grain yield and quality and nitrogen use efficiency through the improvement in nitrogen management in rice production.