Diurnal Photosynthesis and Metabolic Activity Year-round in Two Dendrobium Orchids Cultivars

Abstract

Dendrobium Sonia ‘BOM Jo’ and Den. ‘Khao Sanan’ are the most popular orchid cut-flower cultivars for exporting. Currently, orchid growers are facing climate variability that affect the quantity and quality of yields. Therefore, the objective of this research was to evaluate net CO2 uptake and analyze organic acids and sugars contents year-round in two cultivars of Dendrobium hybrid. The experiment was conducted at a commercial nursery of Thaiorchids Company Limited, Damnoen Saduak, Ratchaburi Province, Thailand, with data collected every two months from June 2010 to April 2011. The two-year-old mature plants of Den. Sonia ‘BOM Jo’ and Den. ‘Khao Sanan’ with 4-5 pseudobulbs were measured for diurnal net CO2 uptake, stomatal conductance (gs), transpiration rate (E), organic acids and soluble sugars. The results showed that under commercial shade net house conditions, the diurnal net CO2 uptake pattern of two cultivars was similar over the six measurement periods. Leaves exhibited a typical CAM photosynthesis, including dark CO2 fixation (18:00-6:00), with the maximum value reached between 2:00 – 6:00, with 4−7 μmol m-2 s-1. Net CO2 uptake showed positive values in the early morning after sunrise (6:00 – 8:00) then, gradually decreased to zero in the daytime (8.00-10:00) until 18:00. Both Den. Sonia ‘BOM Jo’ and Den. ‘Khao Sanan’ varieties showed more than 80% of their total CO2 uptake during the nighttime. Likewise, gs exhibited a significant diurnal fluctuation in which the stomatal opening essentially occurred at night until early morning and remained closed throughout the day. E value generally followed gs, with low values during the daytime, then gradually increased after sunset and reached two peaks after midnight and early morning. Only malic acid and sucrose were found in the leaves of the two Dendrobium cultivars, while citric acid, isocitric acid, glucose and fructose were not detected. The diurnal net CO2 uptake and malic acid patterns indicated that both of the cultivars were strong CAM, despite year-round climate fluctuations. Net CO2 uptake are intrinsically linked with the environment. Therefore, further studies should be conducted to determine enzyme activity and carbohydrate metabolism, with emphasis on the relationship between environment and photosynthetic parameters in CAM Dendrobium hybrids. All data have practical implications with regard to commercial cultivation in Thailand.