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BACKGROUND AND AIMS The negative logarithmic relationship between orthodox seed longevity and moisture content in hermetic storage is subject to a low-moisture-content limit (m(c)), but is m(c) affected by temperature?Red clover (Trifolium pratense) and alfalfa (Medicago sativa) seeds were stored hermetically at 12 moisture contents (2-15 %) and five temperatures (-20, 30, 40, 50 and 65 degrees C) for up to 14.5 years, and loss in viability was estimated.Viability did not change during 14.5 years hermetic storage at -20 degrees C with moisture contents from 2.2 to 14.9 % for red clover, or 2.0 to 12.0 % for alfalfa. Negative logarithmic relationships between longevity and moisture contentsm(c) were detected at 30-65 degrees C, with discontinuities at low moisture contents; m(c) varied between 4.0 and 5.4 % (red clover) or 4.2 and 5.5 % (alfalfa), depending upon storage temperature. Within the ranges investigated, a reduction in moisture content below m(c) at any one temperature had no effect on longevity. Estimates of m(c) were greater the cooler the temperature, the relationship (P0.01) being curvilinear. Above m(c), the estimates of C(H) and C(Q) (i.e. the temperature term of the seed viability equation) did not differ (P0.10) between species, whereas those of K(E) and C(W) did (P0.001).The low-moisture-content limit to negative logarithmic relationships between seed longevity and moisture content in hermetic storage increased the cooler the storage temperature, by approx. 1.5 % over 35 degrees C (4.0-4.2 % at 65 degrees C to 5.4-5.5 % at 30-40 degrees C) in these species. Further reduction in moisture content was not damaging. The variation in m(c) implies greater sensitivity of longevity to temperature above, compared with below, m(c). This was confirmed (P0.005).

Tomato seeds with a moisture content of 16.4% were stored hermetically at one of five constant temperatures (10, 20, 30, 40, 50 8C) or in one of nine alternating temperature (24 h/24 h) regimes (10/30, 10/40, 10/50, 20/30, 20/40, 20/50, 30/40, 30/50, 40/50 8C) for up to 224 d. In each regime, seed survival conformed to cumulative negative normal distributions and all 14 survival curves could be constrained to acommon origin. Estimates of the constantsC H andC Q of the viability equation determined at constant temperatures were 0.0346 (s.e. 0.0058) and 0.000401 (s.e. 0.000096), respectively. The eAective temperature for seed survival of each alternating temperature regimewas always much higher than the mean. Tomato seeds were also stored hermetically at 15.9% moisture content at 40 8C for 0, 7, 14, 21 or 28 d before transfer to 50 8C. This investigation showed that the standard deviation of the subsequent survival curves at 50 8C was unaAected by the duration of previous storage at 40 8C. The results of both investigations were consistent with the hypothesis that loss in probit viability is solely a function of the current storage environment, with no eAect of change in temperature per se. The application of the viability equation to seed survival in fluctuating environments was validated against independent observations for rice in uncontrolled storage conditions. # 2001 Annals of Botany Company

Data on the survival of pollen ofTypha latifoliaL. stored for up to 261 d over seven different saturated salt solutions (providing 0.5 to 66% relative humidity) and six different constant temperatures (from −5 to +45 °C) were analysed to quantify the effect of air-dry storage environment on pollen longevity. Pollen survival curves conformed much more closely to negative cumulative normal distributions than to negative exponential relations. Estimates ofp50(storage period required to reduce pollen viability to 50%), provided by negative cumulative normal distributions, were available from 37 different storage environments in which pollen viability was reduced below 50%. Once observations at 0.5% and 5.5% relative humidity were excluded from analysis, there was a negative logarithmic relation between these estimates of longevity and pollen moisture content (%, wet basis) and a curvilinear semi-logarithmic relation between longevity and temperature. When the negative logarithmic relation between longevity and moisture content was replaced by a negative semi-logarithmic relation between longevity and the relative humidity of the storage environment the resultant model was less satisfactory, principally because pollen longevity over saturated solutions of calcium nitrate (43–62% relative humidity) and sodium nitrite (60–66% relative humidity) were consistently greater and smaller, respectively, than fitted values. Notwithstanding these errors, comparison between the fitted relations and observations at the two lowest relative humidities provided estimates of the lower-relative-humidity limits to these relations. These provisional estimates varied with storage temperature being lowest at 25 °C (0.25): the mean estimate was 11.9 (s.e.=1.4)%. The considerable similarities among models of pollen longevity in air-dry storage, and their estimated lower limits, and those developed previously for orthodox seeds and spores are discussed.

A model was developed to quantify the effect of temperature and moisture content on the longevity of conidia of the entomopathogenic fungus Metarhizium flavoviride . This model incorporated a negative semi-logarithmic relation between longevity and temperature and a negative logarithmic relation between longevity and moisture content. Replacing the latter with a negative semi-logarithmic relation between longevity and the equilibrium relative humidity of the conidia was also effective. The latter model was applied successfully to observations on the survival of conidia of a further four entomopathogenic fungi ( Metarhizium anisopliae , Beauveria bassiana , Beauveria brongniartii, and Paecilomyces farinosus ) and ascospores, conidia or uredospores of four phytopathogenic fungi ( Alternaria porri , Helminthosporium oryzae , Uromyces appendiculatus , and Sclerotinia sclerotiorum ) across a wide range of different temperatures and relative humidities. The sensitivity of spore longevity to both temperature and equilibrium relative humidity, and the upper and lower relative humidity limits to the application of the model, varied considerably between entomopathogenic and phytopathogenic fungi, among species within each group, and among different strains within certain species.

Seeds of lettuce (Lactuca sativa L.) and sunflower (Helianthus annuus L.) were stored hermetically at 35 °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

The longevity and desiccation tolerance of samples of seeds of a japonica rice ( Oryza sativa L.) harvested serially during development from plants grown in two temperature regimes, viz 28/20 °C and 32/24 °C (12/12 h) were determined. Mass maturity (defined as the end of the seed-filling phase) occurred 19·7 and 18·3 d after 50% anthesis, respectively. Longevity (determined at 40 °C with 15% moisture content and quantified by the value of the constant K i of the seed viability equation) improved during seed development and maturation until 17 and 14 d after mass maturity in the cooler and warmer regimes, respectively, but declined thereafter. Changes in K i with time were similar in the two environments until mass maturity, but the increase in K i values after mass maturity was much greater in the cooler regime. Tolerance of desiccation to low (4%) moisture contents improved until 22 and 14 d after mass maturity in the cooler and warmer regimes, respectively, when maturation drying had reduced seed moisture contents naturally to 24 and 32% moisture content, respectively. Further delays to seed harvest reduced desiccation tolerance, particularly in the warmer environment. Comparison among 15 samples of seeds harvested at different times in the two environments showed a strong correlation ( r = 0·947, P K i ) and desiccation tolerance (to 4% moisture content). Hence, it is suggested that the regulation of desiccation tolerance to low moisture contents and potential air-dry longevity during seed development and maturation determined here may have a common cause.

La determination de l'humidite critique, permettant la longevite maximale des graines, est etudiee chez la graine de haricot. Ces deux criteres sont effectivement lies par plusieurs types d'equations prenant en compte la temperature et la duree du stockage. Les resultats sont discutes en tenant compte du potentiel hydrique et compares a ceux d'autres plantes

Cette etude est une comparaison des valeurs de teneur en eau des semences de huit especes vegetales, au dessous desquelles la relation logorithmique entre la longevite des semences et leur teneur en eau n'est plus verifiee. L'etude est menee sur quatre especes de legumineux, deux especes de graminees, une liliacee et une chenopodiacee

Discontinuities at low moisture contents in the otherwise logarithmic relations between seed longevity and seed moisture content (%, f. wt basis) in hermetic storage at 65 °C were detected at 2–0% in groundnut (Arachis hypogaea L.), 3·5% in onion (Allium cepa L.), 4·5% in sugar beet (Beta vulgaris L.), 4·6% in barley (Hordeum vulgare L.), 5·3% in chickpea (Cicer arietinum L.) and wheat (Triticum aestinum L.), and 5·6% in cowpea [Vigna unguiculata (L.) Walp.]. In contrast, the equilibrium relative humidity of seeds at these values was similar, varying between 9·9% (onion and sugar beet) and 11·5% (wheat). The mean value was 10·5% r.h. (s.e. 0.2). There was no significant (P > 0·05) effect of further reduction in seed moisture content below these critical values on longevity, except in wheat (P 0·10) and was equivalent to a doubling of longevity for each 8·7% reduction in equilibrium relative humidity. Accordingly it is concluded that the relative effect of water potential on seed longevity can be considered to be the same for these and probably also for many other orthodox species.

The germination of cassava seed in response to various constant and alternating temperature regimes within the range 19-40 °C was investigated using a two-dimensional temperature gradient plate. It was found that almost all seeds were incapable of germination unless the temperature for part of the day exceeded 30 °C and the mean temperature was at least 24 °C. However, dormant seeds required environments where the temperature for part of the day exceeded 36 °C, the mean temperature was at least 33 °C, and the amplitude of the diurnal temperature alteration was within the range 3-18 °C. Providing these conditions were met, the times spent at the upper and lower temperatures within a diurnal cycle were not critical. Hermetic storage of the seed for 77 days at 40 °C with 7-9 per cent moisture content did not influence the pattern of germination in response to constant and alternating temperatures. It is suggested that an alternating temperature regime of 30 °C for 8 h/38 °C for 16 h applied for a minimum of 21 days is appropriate for cassava seed viability tests.

The longevity of seeds of the majority of crop species in storage is extended by reducing temperature, or moisture content, or both. Seeds which conform to this pattern have been described as orthodox (Roberts, 1973). Consequently it is possible to maintain the viability of orthodox seed in storage at low temperatures and moisture contents. However, a minority of crop species produce seed which are killed by desiccation and therefore cannot at present be stored for long periods: these species' seeds have been described as recalcitrant (Roberts, 1973). Cassava seeds, Manihot esculenta Crantz, have been reported to be recalcitrant because after 6 months storage at laboratory temperature and between 5-9 and 1*9 per cent MC (moisture content, fresh weight basis) germination was reduced from 80 to 28 per cent (Mumford and Grout, 1978). However, from a preliminary examination of cassava seed germination after storage at 6-7 per cent MC, Ellis and Roberts (1979) suggested this classification is doubtful, and this opinion is now supported by a report where cassava seed stored at 5 °C and 60 per cent r.h. (approximately 6-8 per cent MC) showed no difference in seed viability between the ages of 0 and 7 years (International Institute of Tropical Agriculture, 1979). The purpose of the investigation reported here was to determine whether cassava seed germination and longevity in storage are affected by desiccation. Cassava seeds were obtained from the International Institute of Tropical Agriculture, Ibadan, Nigeria, and hermetically stored at 3-5 °C between receipt and starting this work. Six samples were taken from the lot: a control sample (no desiccation, 7-9 per cent MC, s.e. 0-24), and five samples taken after different periods of desiccation in a drier using unheated air (at 20 °C) passed through silica gel for 5 h (5-9 per cent MC, s.e. 0 02), 9 h (5-5 per cent MC, s.e. 0 004), 22 h (4-6 per cent MC, s.e. 0 09), 33 h (3-8 per cent MC, s.e. 0 05), and 51 h (3-2 per cent MC, s.e. 0 04). Seed moisture contents were determined by drying two 5 g samples of ground seed in a ventilated oven at 103-105 °C for 16 h. Seeds at each moisture content were divided into eight samples and each sealed in polythene lined aluminium foil packets. The storage treatments comprised a factorial combination of the six moisture contents and the eight following storage periods in a heated water bath maintained at 70 °C± 1 °C; 0, 1-5, 3, 6, 12, 24, 48 and 96 h. After storage seeds were tested for germination between moist rolled paper towels, four replicates of 25 seeds per treatment, and placed in an alternating temperature of 25/35 °C the lower temperature being maintained for 16 h of a 24 h cycle -as recommended from an earlier study (Ellis and Roberts, 1979). Germinated seeds were counted and removed twice weekly and the tests concluded after 42 days. The results of all the germination tests and calculations of the mean time taken by viable seed to germinate in the tests are shown in Fig. 1 . Although the percentage germination before storage of seeds dried to the six moisture contents varied between 50 and 70 per