INFLUENCES OF LIGHT ON THE GERMINATION AND FIRST GROWTH IN TWO VEGETABLE SPECIES : ALLIUM CEPA AND RAPHANUS SATIVUS
In Radish, light red ( = 660 nm) inhibits stricto sensu germination. The effect depends upon cultivar and temperature. At 20°C, it increases with lighting duration and, to some extent, with irradiance ; on the contrary, it decreases when a dark period is previously arranged.
In Onion, the main effect of light is also inhibitory, but essentially because of abnormal seedlings formation, many of them with a curled cotyledon "knee". As it was shown by experimenting on a thermogradient plate, it varies in a complex way according to the temperature. At 20°C the effect of continuous illumination (1) depends on the seed lot and type of abnormality considered (2) increases with irradiance, up to a maximum (3) depends upon the wave-length, with a minimum in light red and further intensification (at least as for curled cotyledons) from green to incandescence light, blue and yellow or "Incandia" white. Shortening the photoperiod to eight hours per days hardly modifies the germination capacity which goes to its maximum only in darkness … after previous illumination during three days.