Eucalyptus Physiology. I. Photoperiodic Responses

Abstract

Seedlings of selected Eucalyptus species grown under factorial combinations of temperature and photoperiod showed greater response to temperature than to photoperiod. In a few cases maximum growth occurred at an optimum photoperiod of about 12 hr. Growth responses to an increase of low intensity light from 8 to 12 hr were usually of the quantitative, long-day type. The 12 hr optimum was associated with quantitative, short-day responses that may occur with an increase ;n photoperiod from 12 to 16 hr. These optima rarely occurred at more than one growing temperature for any one species or seed source. Such temperature dependence, combined with the small magnitude of the two types of response and with the variability among species, may explain many of the hitherto equivocal results on the effect of photoperiod in Eucalyptus.

Long-day responses were more common than short-day responses but whether associated with a photoperiod optimum or not, both response types appeared to be largely unrelated to the latitude and to altitude of the seed source. This contrasts with the behaviour of northern hemisphere vegetation.