THE EFFECT OF MATURITY ON THE MOISTURE RELATIONS OF SEED LONGEVITY INFOXGLOVE (DIGITALIS-PURPUREA L.)

Controlled aging experiments were carried out in order to evaluate the changes in seed longevity occurring following the attainment of maxim um seed dry weight in foxglove (Digitalis purpurea L.). Seeds harveste d in three successive years were stored at similar to 5% moisture cont ent (fresh weight basis) and 50 degrees C. Seed longevity was greater the later the seeds were harvested. When the data were modelled using the predictive viability model of Ellis and Roberts, these increases w ere attributable to increases in both the theoretical initial viabilit y (in probits) of the seed-lot, K-i, and in the standard deviation of the normal distribution of seed deaths in time. a. Furthermore, there was a positive relationship between K-i and sigma which was independen t of the year of harvest (r = 0.9016, for 11 d.f., P < 0.001). These r esults do not support the predictive model which relies on the assumpt ion that a will not differ between seed lots of the same species store d under identical conditions. When seeds were stored at a range of moi sture contents (between 5 and 10%) at 50 degrees C, increases in sigma during seed development were manifest as changes in the negative loga rithmic relationship between sigma and moisture content; below an uppe r limit, the regressions for seed lots at different stages of maturity were significantly different (P < 0.05) but could be constrained to e ither a common intercept or to a common slope (P > 0.05). It is propos ed that it was the inherent variation in individual seed lifespans whi ch increased during seed development. These results raise important co ncerns regarding the use of predetermined 'species constants' to predi ct the longevity of seed lots during long-term storage in seed banks.