Effects of Exposures of Seeds to Various Physical Agents. I. Effects of Brief Exposures to Heat and Cold on Germination and Light-Sensitivity

Abstract

1. The effect on germination of brief conditioning at various temperatures of the grains or seeds of several species of plants was studied, and the effects of such conditioning on light-sensitivity, enzyme activity, and, in two instances, on subsequent growth were observed. Grains of crabgrass and Sporobolus airoides and seeds of flax and radish were conditioned at temperatures ranging from 10⚬ to 65⚬ or 75⚬ C. for 1 hour while possessing various moisture contents from air-dry (9-10%) to fully imbibed (40-44%). Air-dry grains of corn and wheat, seeds of radish and flax, and seeds of other species were conditioned at temperatures of 100⚬ C. for 20, 40, or 60 minutes, or exposed to higher temperatures. 2. Crabgrass and, to a lesser extent, Sporobolus exhibited what is termed a threefold response to light. After conditioning at 10⚬ C., germination percentage was higher in darkness than in light; after conditioning at higher temperatures (50⚬ C. or more), germination became relatively greater in light-in one case, light-obligate. Responses to light varied with differences in water content as well as with temperature of pretreatment. 3. After treatments which resulted in the suppression of germination, the activity of enzymes and the degree of autolysis were diminished. Loss of viability through heat treatments is not in all cases correlated with the loss of oxidase activity. 4. Treatments preventing germination of Sporobolus could be overcome in some proportion of the grains by the addition of glucose, sucrose, or hydroquinone; light, glucose, or light and glucose were similarly effective on crabgrass grains. 5. Grains of Sporobolus conditioned at 60⚬ C. for 1 hour and then planted in soil produced stunted plants; stunting was observed at the third and twelfth weeks after planting. 6. Many instances were observed in which temperature conditioning acting alone or with light promoted more germination than in the control groups. This is discussed in terms of increased chemical activity, the possible effects of light on enzyme activity, and the generally observed stimulatory effect of sudden changes in temperature. The possible relation of the latter point to changes in protoplasmic viscosity is mentioned. 7. In all cases a combination of high water content and high conditioning temperature was deleterious to germination. The upper limit of heat tolerance for seeds of a number of species heated dry is less than 135⚬ C. for 15 minutes. The most resistant species studied were wheat, flax, and radish; of these, wheat is most tolerant to high temperatures and is little affected by as much as 60 minutes' exposure to 100⚬ C. 8. The relative insensitivity of flax and radish seed over a wide range of conditions was found to be of interest. This may be related to the high oil content of these seeds. They are more resistant to treatment at 100⚬ C. than bean and corn, which are both low in oil. The responses of flax and radish to increasing time of exposure to this temperature are similar. Light is clearly inhibitory to these two species after the heat treatment, while it makes little or no difference in the germination of corn and bean. 9. Flax seeds heated 110⚬ C. for 65 minutes exhibit the following characteristics: (a) poor germination in Petri dishes with water, (b) much improved germination in soil or soil extract, (c) improved germination with ascorbic acid, (d) certain abnormalities in the roots of young seedlings, and (e) a stunting of plants grown in soil from seeds so treated. It is suggested that the efficacy of certain agents in overcoming partially the injurious effects of heat is related to their oxidation-reduction activity. 10. The heat resistance of flax and radish is discussed briefly in terms of a protective action of the oils present, possibly an antioxidant function of the unsaturated fatty acids. 11. The data here presented are of limited value in a strictly quantitative sense but demonstrate that brief heat and cold treatments can have numerous effects on the physiological system of the seed. Some of the factors limiting the reproducibility of results are mentioned, and the difficulty of interpretation of the results, owing to the complex and heterogeneous nature of the seed, is pointed out.