Effects of Soil Moisture Content on Germination and Physiological Characteristics of Rice Seeds with Different Specific Gravity

Soil relative water content and seed plumpness have been shown to be the key factors affecting seed germination and seedling growth of rice under direct drought cropping. It remains to be determined whether seed germination and seedling growth of water-saving and drought-resistant rice (WDR) and conventional rice with the same proportion of rice seed have the same response to soil moisture changes. The purpose of this study was to investigate the seed germination and physiological characteristics of the rice cultivars Guangliangyou 1813 (GLY-1813,indica hybrid rice) and Hanyou 73 ((HY-73), WDR) with four different specific gravities (T1, T2, T3, and T4; the rice seeds were divided into four specific gravity levels by weight using saline water, the representative specific gravities were 1.2 kg m−3, respectively), at five soil moisture content gradients (soil relative water contents of 10–20%, 20–40%, 40–60%, 60–80%, and 80–100%), under dry direct seeding conditions. The results showed that GLY-1813 had a higher germination potential, germination and seedling emergence rates, greater root dry weight, seedling dry weight, root oxidation activity, and chlorophyll content, and lower malondialdehyde (MDA) content when the soil relative water content was 20–40% or 40–60%. Cultivar HY-73 had the highest germination rate and seedling physiological activity at 20–40% relative water content; its growth vigor was better than that of GLY-1813 at the same soil moisture level. In conclusion, the soil relative water content for seed germination of HY-73 was 20–40%, which was less than that of GLY-1813. When soil relative water content was sufficient for seed germination and growth, the higher the plumpness of the rice seed, the easier it was to resist the negative effects of an adverse growth environment.