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Seed storage, temperature and relative humidity: response

Recently, we introduced the idea of using thermodynamic principles to model the nature and kinetics of seed aging (Vertucci and Roos, 1990), which has stimulated considerable interest (Ellis et al., 1991; Vertucci and Roos, 1991; Smith, 1992). The recent correspondence by Smith (1992) deserves some further comments regarding the thermodynamic principles and their implications for seed storage protocols. Smith (1992) makes a distinction between the 'threecomponent' system (seed, air and a saturated salt solution) that we use in our experiments (Vertucci and Roos, 1990) and the 'two-component' system (seed and air) used by others (for example, Tompsett, 1986; Ellis et al., 1989, 1990), and questions the validity of comparing the two systems. He suggests that there is no information about the partition of water in seeds versus the surrounding air. This is, in fact, not the case. That partition is described by moisture isotherms: curves that describe the unique relationship between water content, relative humidity and temperature when the system is at equilibrium. Moisture sorption isotherms for a variety of seeds at various temperatures have been described previously (for example, Vertucci and Leopold, 1987; Vertucci and Roos, 1990, 1991). Moisture isotherms of air at various temperatures are described by psychrometric charts available in humidity control manuals (e.g. Harriman, 1990). Smith (1992) suggests that the initial moisture content and temperature of seeds affect the subsequent shape of moisture sorption isotherms. However, because isotherms are representative of the equilibrium condition, we know that the temperature/relative humidity/moisture content relationships are independent of seed or air volume or the initial conditions (Atkins, 1982). Smith's comments were made in the context of what happens when temperature is changed. Le Chatelier's Principle tells us that if a system at equilibrium is perturbed (i.e. temperature is changed), the system will change to restore equilibrium: in other words, the equilibrium conditions will change (Atkins, 1982). In a three-component system, the relative humidity of the air is buffered by the salt solution, and so it changes only slightly with temperature, and these changes are available in published tables. Moisture sorption isotherms document that when temperature is lowered and relative humidity is constant, the moisture content of seeds increases (Vertucci and Leopold, 1987; Vertucci and Roos, 1991). This is the opposite of what is expected for air according to psychometric charts. The difference between the response of seed moisture content and air moisture content to temperature change is because vapour condensation (air to seed) is an exothermic process and vaporization (seed to air) is an endothermic process. According to Le Chatelier's Principle, a reduction in temperature favours the products for exothermic reactions and the reactants for endothermic reactions (Atkins, 1982). We disagree with the statement by Smith (1992) that "the 'two-component' system is more appropriate [than a buffered system] for modelling of current seed-bank procedures." Since seeds will be in equilibrium with their environment during long-term storage, it is irrelevant whether or not equilibrium was achieved using a buffered system. The equilibrium conditions can be deduced in a three-component system from moisture sorption isotherms and calculated in a two-component system from moisture sorption isotherms, a psychometric chart and the volume of the seed and air space. If a two-component system is used and the package has limited headspace, the moisture content of the seeds will not change perceptibly. According to moisture sorption isotherms, if seeds at a given moisture content are cooled, the relative humidity decreases. This is what happens when typical seed storage practices are used (Cromarty et al., 1985). Notice that exactly the opposite effect occurs in many studies designed to determine the optimum moisture content for storage: the temperature of seeds at a given moisture content is raised resulting in a higher relatively humidity. We believe that caution must be exercised when extrapolating data obtained from accelerated aging studies to actual seed storage conditions because the thermodynamic conditions of the two environments may be quite different. There is no support for the contention made by Smith (1992) that sorption isotherms at different temperatures will approach a single isotherm 'influenced by the temperature at which the seeds were [originally] equilibrated' if a package with limited air space is used and the air is unbuffered. If this were true, then the slope of the van't Hoff analysis would equal 0 (as Smith (1992) implies), meaning that the enthalpy of water sorption also equals 0 (since the slope of the /«(RH/100) vs 1/T plot is equal to the enthalpy of sorption (AH) divided by the gas constant R).