Studies in Stomatal Behaviour: IX. PHOTOPERIODIC EFFECTS ON RHYTHMIC PHENOMENA INXANTHIUM PENNSYLVANICUM

Abstract

An investigation of the effects of different day-length treatments on stomatal behaviour in Xanthium pennsylvanicum Wall. has shown that there are differences in long-and short-day treatments similar to those first reported by Schwabe (1952) for Chrysanthemum and Kalanchoe, viz. stomatal opening towards the end of the night in short days, but not in long days (short night preceded by a period of low intensity illumination). In Xanthium there were in addition very marked differences in rates of stomatal opening in the morning after different lengths of night. These effects were not persistent, there being an immediate reversal upon a change from long- to shrot-day treatment, or vice versa. Further investigation showed that there was an endogenous rhythm affecting the stomata in continuous darkness; rate of opening was slow after very short nights, but became greater with extension of the night to 14–16 hrs., this being the first ‘peak’ of an ‘opening ability’ rhythm. The rhythm died out rapidly and the second cycle was much reduced in amplitude. The period of the rhythm appeared to be approximately 24 hrs. The phase was set mainly by the time of onset of darkness, but the duration of the pretreatment with low intensity illumination was also important—prolonging this was found to reudce the time in darkness before the first peak. Each hour of light of 1,500 lux given before darkness was found to be equivalent ot approximately 0.3 hr. to darkness. The predominant effect of the length of the preceding night was on the slope of the opening curves rather than on the time of onset of opening following illumination. The first peak of the opening ability rhythm was often manifested even in continuous darkness by a period of ‘night opening’ of the stomata. The results are discussed and compared with those of other authors and attempts are made to relate them to theories of the stomatal mechanism.