Survival and vigour of lettuce (Lactuca sativa L.) and sunflower (Helianthus annuus L.) seeds stored at low and very-low moisture contents

Seeds of lettuce (Lactuca sativa L.) and sunflower (Helianthus annuus L.) were stored hermetically at 35 degrees C with 11 different moisture contents between 1.3 and 6.9%, and between 1.3 and 7.1% of fresh mass, respectively. Germination and vigour (mean germination time, root length, seedling dry weight) were determined after storage for 0, 8, or 16 weeks (sunflower) or 0, 8, 16, or 48 weeks (lettuce) in these environments followed by various humidification treatments (to avoid imbibition injury). The range of seed storage moisture contents over which deterioration was minimized depended upon the criterion of deterioration used, and varied somewhat between species. Comparison of these ranges for seeds stored for the longest durations showed that for some criteria seed performance was poorer (P < 0.05) at both the lowest and highest moisture contents investigated than at certain of the intermediate storage moisture contents (e.g. most rapid germination occurred in sunflower following storage at 2.2-4.7% moisture content), whereas for other criteria all the drier storage moisture contents were superior to the more moist (e.g. greatest seedling growth occurred in sunflower following storage at 1.3-5.1% moisture content). But none of these results suggested that lettuce and sunflower seeds stored hermetically at 2.5-3.0% or 2.2-2.5% moisture content, respectively, were less vigorous than at any other moisture content tested. In both species, these storage moisture contents are in equilibrium with about 8-10% relative humidity (r.h.) at 20 degrees C, which is similar to and indeed marginally less than the 10-13% r.h. recommended following earlier studies on the longevity of seeds in hermetic storage at much warmer temperatures. Thus, these results show no evidence that the optimum seed moisture content for storage increases with decrease in temperature, at least over the range 35-65 degrees C, as has been suggested elsewhere. We conclude that the international recommendation for the long-term seed storage for genetic conservation at 5 +/- 1% moisture content should not be revised upwardly, and that in situations where refrigeration cannot be provided storage at even lower moisture contents is worthy of further investigation for those seeds in which desiccation at 20 degrees C to equilibrium at 10% r.h. results in moisture contents well below 5%.