Abstract:

Rice is a most important food crop in China and its grain yield per hectare has been over 6 ton in recent years, which is 65% higher than that of the world, on average. Improvement in rice cultivars plays an important role in the increase of the grain yield. However, little is known about physiological and agronomical features responsible for the improvement. In this study, 16 typical mid-season indica rice cultivars (including hybrid combinations) applied in the production in Jiangsu Province during the last 60 years were used, and classified into six types of early tall (ET), improved tall (IT), dwarf cultivars (DC), semi-dwarf cultivars (SDC), semi-dwarf hybrid (SDH), and super rice (SR), according to their application times, plant types and genotypes. All the tested cultivars were grown in the field and the evolution characteristics of grain yield and its components, source-sink relationships and plant types were investigated. Results showed that grain yield was progressively increased with the evolution of the cultivars. Increase in grain yield was attributed mainly to the increase in total number of spikelets, which resulted mainly from large panicles. DC had the greatest, whereas SR had the least, percentage of filled grains. There was no significant difference in 1000-grain weight among the six types of cultivars. From ET to DC, plant height was reduced and biomass and harvest index were simultaneously increased, especially the harvest index increased more. From DC to SR, plant height was a little increased, and biomass dramatically increased, especially after heading, and harvest index a little changed. Improvement of rice cultivars significantly reduced the leaf angle of the top three leaves on stems and increased leaf area at heading. Increase in spikelets, however, was more than that in leaf area, which resulted in the increase of grain-leaf ratio (the ratio of total spikelet number to leaf area at heading). With the improvement of rice cultivars, root dry weight and the ratio of root to shoot were increased, but the amount of root bleeding sap was reduced. The speed of reduction in the concentration of zeatin and zeatin riboside in root bleeding sap and leaves from heading to maturity was SR>SDH>SDC. The results suggest that enlargement of sink resulted from large panicles, increase in biomass especially from heading to maturity and improvement of plant type contribute to the increase of grain yield for modern rice cultivars. Low root activity may account for the low percentage of filled grains for SR. Approaches to further increase in grain yield of rice were discussed based on the evolution characteristics of the cultivars.

Key words: Mid-season indica rice, Evolution, Grain yield, Plant type, Source-sink relationship