MIDDAY PHOTOSYNTHETIC DEPRESSION IN ONCIDIUM

Abstract

Oncidium is an important orchid for cut flower and potted plant production. The net photosynthesis (Pn) of Oncidium on hot summer days exhibited a pattern of midday depression (MD), in which gas exchange rate decreased during midday and then recovered to some extent in the afternoon. The Fv/Fm value, reflecting quantum efficiency of PSII, also dropped to its minimum in the midday hours. These phenomena were observed in O. Gower Ramsey `Sunkist', O. Gower Ramsey 'Volcano Queen' and Colmanara Wildcat 'Bobcat'. Further experiments were conducted to investigate the relationship between environmental factors and MD. In growth chamber studies, the Pn of O. Gower Ramsey decreased when photosynthetic photon flux density (PPFD) exceeded 500 μmol·m -2 ·s -1 or when temperature exceeded a critical value, which was 23.5°C in April and 30°C in November. In multiple-site studies, the average Pn of O. Gower Ramsey grown in a rain shelter was lower than that in a phytotron or in a growth chamber. The plants had the highest Pn and the least MD when grown in a growth chamber, in which PPFD was controlled at 250 μmol·m -2 ·s -1 and temperature at a constant 25°C. In field studies, the MD in O. Gower Ramsey was more severe on a sunny day than on a sunny day with afternoon rain. In phytotron studies, the Pn of O. Gower Ramsey decreased markedly when plants were moved from 25 to 35°C, but increased from 35 to 25°C. Data revealed that high temperature was the main factor that resulted in the MD in Oncidium.