Seasonal Variation In Diurnal Photosynthetic Pattern of Oncidium Gower Ramsey Over a One-Year Period

Previously we reported that Oncidium Gower Ramsey exhibited midday photosynthetic depression in the summer; high temperature was speculated to be the major cause of the reduction of photosynthesis. In the current study, we further investigated how monthly variations in environmental factors affected photosynthetic rate throughout the entire year. Our results showed that the diurnal pattern of photosynthetic rate of Oncidium Gower Ramsey fell into three types. The diurnal photosynthetic rate curve had a single peak in January and December, with highest photosynthetic rate occurring at midday. In this pattern, light intensity was the main factor that determined photosynthetic rate. The second type of diurnal pattern showed two peaks, which was observed from February to June and from September to November. In this type, photosynthetic rate decreased at midday but recovered in the afternoon, with good recovery observed in February and March. In particular, net photosynthetic rate was significantly reduced in March when leaf temperature exceeded 33 ^oC and vapor pressure deficit (VPD) exceeded 3 kPa. The third type of diurnal pattern exhibited a severe midday depression without recovery in the afternoon, and was observed in July and August. Specifically, net photosynthetic rate decreased and approached 0 µmol^.m^-2.s^-1 in July when leaf temperature was above 40 ^oC and VPD was higher than 3 kPa. The highest net photosynthetic rate was observed in February. The critical leaf temperature beyond which photosynthetic rate reduction took place increased from 26.2 ^oC in February to 42.7 ^oC in July, indicating that the photosynthetic temperature response of Oncidium Gower Ramsey changes with season and that the plants can acclimate to changing temperatures with its heat tolerance increases gradually from early spring to summer.