The low-moisture-content limit to the negative logarithmic relation between seed longevity and moisture content in three subspecies of rice

The lower limit to the negative logarithmic relation between seed longevity and moisture content was determined in three subspecies of rice (Oryza sativa L.) by storing seeds of five cultivars at 65 degrees C with 11 different moisture contents (1.5-15.3% f. wt) for various periods up to 150 d and then testing for germination. The estimates of the low-moisture-content limit (mc) were 4.3% for subsp. indica, 4.4% for subsp. japonica, and 4.5% for subsp. javanicia. These moisture contents were in equilibrium with 10.5-12.0% r.h. No significant effect of moisture content between 1.5% and mc on longevity was detected (P >0.05), while between mc and 15.3% there were negative logarithmic relations between longevity and moisture content. There were no significant differences in the relations between longevity and moisture either above or below mc between the two japonica cultivars or between the two javanica cultivars (P > 0.10). There was also no significant difference in the slope of the negative logarithmic relation between longevity and moisture above mc among the three subspecies (P > 0.25). However, there were significant differences in the standard deviation of the frequency distribution of seed deaths in time at any one moisture content, both above and below mc; this is a measure of seed longevity which is independent of pre-storage environment, and the differences observed show that there are genetically determined differences in longevity among the three subspecies (P < 0.005), indica being the longest and japonica the shortest lived. The results provide no evidence for intra-specific variation in mc and support the view that the maximum seed storage moisture content which provides the maximum longevity is that which is in equilibrium with about 10-11% r.h. It is concluded that while the seed viability constant CW of the seed viability equation is species specific and therefore applies to most, if not all, cultivars of rice, variation in the value of KE is the source of the differences in potential longevity of the three subspecies.