ABSTRACT
Transpiration rhythmicity and intensity were investigated in the chlorophyll-deficient mutant XL18 of Pisum sativum L. and in the phytochrome-deficient mutant aurea of Lycopersicon esculentum Mill. A custom-built psychrometer was used. In the XL18 mutant an acute transpiration response to monochromatic irradiation was observed such that red (R) light increased and far-red (FR) decreased transpiration rate, with equal rates of change. This result indicates that phytochrome is involved in regulation of transpiration. In wild type pea the chlorophyll-dependent component of transpiration was also shown to involve phytochrome. Monochromatic irradiation by red or far-red light induced an increase in transpiration with acceleration dependent on time of day. The response was irreversible by light of either wavelength. We conclude that both photoreceptors are involved in the acute response. Investigation of the daily course of transpiration revealed rhythmic changes in wild type pea and tomato under natural light conditions and in constant darkness. The rhythm was not apparent in the XL18 mutant in constant darkness, or in the aurea mutant under natural illumination. The latter results show that phytochrome, as a photoreceptor, is essential for maintaining the rhythm upon irradiation, while the photosynthetic component is crucial in darkness.
