Interaction of Temperature and Light in Germination of Seeds

Abstract

Fully imbibed, viable seeds of many plants may fail to germinate. One limitation on germination may be a light requirement which can be removed by the correct poising of a reversible photoreaction (1, 2, 8). This reaction which can be repeatedly reversed by alternate exposures to red and far-red radiation also controls photoperiodic flowering responses, etiolation, bulbing, and a number of other growth responses (5). That the photocontrol is ubiquitous in all higher plants is shown by the etiolation response of seedlings (6). The temperature, too, must be in a favorable range for seed germination which, for many kinds of seeds, is well below that best suited for subsequent growth of the seedling. A change in temperature is often effective in promoting germination. For limited periods, the temperature may greatly exceed the favorable constant range. A response to a change in temperature is also evident in vernalization, tillering, and dormancies of plant buds (9) as well as in dia-pause of animals. It is a phenomenon with broad significance. Germination requirements are examined here with reference to the results of a few simple physiological experiments involving the effects of light and temperature conditions and to some extent the effects of nitrate solutions. The results suggest a pattern for the control of the germination process as well as for the growth of all organisms that must follow the temperature of the environment. MATERIALS AND METHODS The experiments reported here were chiefly with seeds of Lepidium virginicum L. collected from plants grown in the greenhouse. Other seeds were obtained from various sources and all were held in dry storage at-18° C or + 5° C. The seeds were placed in Petri dishes on two layers of standard germination blotters saturated with water or 0.2 % KNO3 solution. The dishes were immediately covered with two or more layers of black cloth throughout all tests except when irradiations were in progress. That the cloth excluded visible light was indicated by failure of exceedingly light-sensitive seeds to respond to visible radiant energy when so protected. Durations of germination experiments varied with the kinds of seeds and with the purposes of the experiments. They ranged mainly I Received June 15, 1955. from 2 to 5 days after the final experimental variable was introduced. Seeds with emerged radicles were counted as germinated. Radiation sources were incandescent and standard cool white fluorescent lamps. The incandescent-filament lamp, with a filter of two layers of red and two layers of dark-blue cellophane, either with or without a supplemental water filter, was an effective source of far-red radiant energy in the region of wavelengths greater than 7000 A for inhibiting germination. When provided with a two-layer red cellophane filter to eliminate the blue part of the visible spectrum the fluorescent lamp, because of its relatively low emission between 7000 and 8500 A, was an adequate source of red radiant energy in the region of 5800 to 7000 A for promotion of germination. The sources used each gave about 0.3 milliwatts/cm2 power in the wavelength regions of maximum effectiveness. Blue cellophane filters have been found to fade when subjected to the full intensity of 300-watt flood lamps at a distance of a foot or 18 inches, but only after several hundred hours of use. Care was accordingly exercised to replace them before significant change in transmission occurred. During germination tests in seed germination cabinets , temperatures were usually controlled within + 10 C. Some of the experiments involved several different constant temperatures; others made use of daily temperature alternations, an 8-hour period at one temperature being followed by 16 hours at a lower temperature. In still other experiments, a single change was made from the temperature employed during imbibition to one used for a short period or during the remainder of the germination period. Exposures to radiant energy and in some cases applications of KNO3 or other solutions were made concurrently with, before, or after the temperature change. In all experiments with Lepidium virginicum L. and Nicotiana tabacum L. four lots of 100 seeds each were used for each treatment. Results for Fragaria virginiana Duchesne and Verbascum thapsus L. in table II were based on single lots of 100 seeds and those used for other species on two lots each. Control lots of seeds were held in darkness in all experiments and under special conditions in some experiments as required by their particular designs. All seeds of L. virginicum in the dark control lots almost invariably failed to germinate. 473