Changes in Activities of Antioxidative Enzymes and Pigment Contents in Leaves of Cattleya and Cymbidium Plants Subjected to Different Light Intensities

Abstract

Sophrolaeliocattleya Estella Jewel (Cattleya) and Cymbidium Sazanami plants grown in a shaded greenhouse, kept between 18 and 28°C, were exposed to full sunlight (FS, 0.61kW·m-2) on 21st October, whereas others were grown under 70% (high radiant flux), 40% (control) and 10% (low radiant flux) full sunlight from May to October. 1. Activities of foliar antioxidative enzymes, superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT), were much more in a CAM plant, Cattleya than in a C3 plant, Cymbidium ; those enzyme activities increased slightly in the daytime in the former and at the beginning of night in the latter. FS reduced SOD activity but greatly enhanced CAT activity and slightly promoted APX activity in Cattleya leaves, whereas the same radiant flux on Cymbidium leaves resulted in a big decreased in SOD and CAT activities and a slight one in APX activity. The decrease is attributed to light stress. 2. One month after the onset of the shading treatment, SOD and CAT activities in Cattleya leaves at the high radiant flux decreased significantly compared with those of the control and remained at low levels ; CAT activity, however, recovered slightly with further cultivation. In Cymbidium leaves under a high radiant flux, SOD, APX and CAT activities decreased for three months from the onset and of the experiment. At the low radiant flux, APX and CAT activities in Cattleya and Cymbidium leaves increased and sustained their high activities during further cultivation with the exception of SOD of Cattleya. 3. Foliar chlorophyll contents in both orchid hybrids decreased with an increasing radiant flux, yielding high positive correlations to those three enzyme activities. Ratios of chlorophyll a to chlorophyll b and β-carotene contents in leaves of Cattleya plants grown at the high radiant flux were markedly low after one month from the onset of shading, but they increased to the same levels as the control after 3 months. In Cymbidium leaves, the ratios and β-carotene contents decreased slightly at the high radiant flux, while at the low radiant flux, both chlorophyll and β-carotene contents increased greatly. Thus, Cattleya plants have a greater potential to adapt to high radiant fluxes than do Cymbidium plants, although these two orchids hybrids do not easily acclimatize to high radiant radiant fluxes but they do readily to low fluxes.