SURVIVAL AND VIGOR 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 moistur e contents between 1.3 and 6.9 %, and between 1.3 and 7.1 % of fresh m ass, 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 en vironments followed by various humidification treatments (to avoid imb ibition injury). The range of seed storage moisture contents over whic h deterioration was minimized depended upon the criterion of deteriora tion used, and varied somewhat between species. Comparison of these ra nges for seeds stored for the longest durations showed that for some c riteria seed performance was poorer (P < 0.05) at both the lowest and highest moisture contents investigated than at certain of the intermed iate storage moisture contents (e.g. most rapid germination occurred i n sunflower following storage at 2.2-4.7% moisture content), whereas f or other criteria all the drier storage moisture contents were superio r to the more moist (e.g. greatest seedling growth occurred in sunflow er following storage at 1.3-5.1 % moisture content). But none of these results suggested that lettuce and sunflower seeds stored hermeticall y 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, t hese storage moisture contents are in equilibrium with about 8-10% rel ative humidity (r.h.) at 20 degrees C, which is similar to and indeed marginally less than the 10-13% r.h. recommended following earlier stu dies on the longevity of seeds in hermetic storage at much warmer temp eratures. Thus, these results show no evidence that the optimum seed m oisture 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 sh ould not be revised upwardly, and that in situations where refrigerati on cannot be provided storage at even lower moisture contents is worth y of further investigation for those seeds in which desiccation at 20 OC to equilibrium at 10% r.h. results in moisture contents well below 5%. (C) 1995 Annals of Botany Company