The possible involvement of membranes and water viscosity in the temperature effects on imbibition and solute leakage of radish (Raphauus sativa var. Early Scarlet Globe) seeds and excised sugar pine (Pinus Imbertiama Dougl.) embryos was evaluated. In these two seed materials, the temperature effect on initial rates of imbibition and solute leakage could be accounted for primarily by changes in water viscosity, the relationship being approximatel linear. It appears that membranes are involved both in water uptake and solute leakage. Heat-killed radish seeds and sugar pine embryos exhibited significantly higher rates of imbibition and solute leakage than did viable ones. In addtion, sugar pine embryos exhibited an abrupt change in rates of imbibition and solute leakage between 15 and 20°C, resulting in abnormally high water uptake and solute leakage above this temperature. logical nature, explained by the chemical activity of water expressed as the proportion of active "hydrone" molecules (2). Rate effects in peas, corn, and Xanthium, on the other hand (observed Qlo < 2.0), were suggested to be of a physical nature, with no "chemical" change of the water involved (12, 13). Rates of water uptake in wheat, pea, and rape seeds were suggested to be influenced strongly by the binding of water in the colloids and micellary structure of the seeds, but no temperature effect could be attributed to water viscosity (1). However, imbibition is more likely a complex phenomenon, depending both on properties of the water as well as the seed colloidal material (7). Similar temperature effects on imbibition of heat-killed seeds has been presented as evidence that water uptake of seeds is a physical, nonmetabolic process (7), and not appreciably effect by membranes (5).A clearer understanding of the temperature effects on seed imbibition appears to have been hampered by a variety of interfering features of the seeds themselves: partially impermeable seed Membranes of dry seeds are in a disorganized, possibly nonlamellar state and, as such, are inefficient barriers to movement of water and solutes (15). Conditions during the initial phase of seed imbibition, when the hydrating membranes are becoming reorganized into a continuous bilayer configuration exhibiting efficient semipermeable properties, are critical to survival and successful germination. Excessive rates ofwater uptake before reorganization is accomplished may lead to displacement of membrane components, deleterious mixing of cellular compounds, and loss of cellular components through leakage into the aqueous medium (15).Temperature is one of the more prominent environmental factors affecting seed germination. The majority ofnondormant seeds show an optimal temperature range for germination (commonly between 15 and 30°C), with drastic reductions in germination above and below this range (3,4,16). Both high and low temperature-related reductions in germinability seem to have a common cause, the inability of seed membranes to function as efficient semipermeable memb...