EFFECT OF INTERMITTENT IRRADIATION ON PHOTOPERIODIC RESPONSES

Intermittent irradiation often has been used as a technique for evaluating the nature of the non-photochemical reactions associated with and limiting the rate of photochemical reactions. Several investigators (1, 4, 5, 6) have used intermittently applied radiant energy in studying photoperiodism, but the irradiances used have been far above the point of saturation of the photochemical reaction and little attempt has been made to control the energy relationships per cycle. These high irradiances and unbalanced energies have made it difficult to interpret the results in terms of the interrelationships between the photochemical and the non-photochemical reactions. This report deals with several series of experiments which were started in the winter of 1938 with the view of determining, by the use of intermittent irradiation, the general interrelationships between the photochemical and non-photochemical reactions associated with photoperiodism. In order to observe quantitatively the influence of the length of the dark period in relation to the effectiveness of the radiant energy in promoting photoperiodic reactions, two conditions were maintained. First, the total energy applied in comparable series was of constant value ; and secondly, the energy for the most effective treatment of the series was below the region where photochemical saturation begins. These conditions were used, except in the preliminary experiments where an endeavor was being made to find where saturation of the photochemical reaction begins as measured by phasic responses. Saturation of the photochemical reaction responsible for long photoperiod responses may be said to have occurred at that irradiance beyond which further increases fail to cause comparable increases in photoperiodic effect, i.e. vegetativeness in short-day plants or flowering in long-day plants. Below the region of saturation, the photoperiodic effect is approximately proportional to the irradiance (7). To date the only measure that we have of the photochemical reaction is the phasic response of the apical meristem and, to some extent, height responses. This is an indirect and unsatisfactory method, but will have to serve until more is known of the photochemical reaction.