Susceptibility of CAM Dendrobium leaves and flowers to high light and high temperature under natural tropical conditions

Responses of CAM Dendrobium Sonia leaves and flowers to high light and high temperature were studied in shade-grown plants after exposure to intermediate and full sunlight under natural conditions. Photosynthetic O₂ evolution decreased in leaves after exposure to full sunlight for 2 weeks while leaves exposed to intermediate sunlight showed an increase in photosynthesis as compared to those leaves maintained in the shade. On the first day of treatment, the changes of F_v/F_m in both leaves and petals grown in the shade were negligible during the day. However, there was a steep decrease in F_v/F_m in both leaves and petals with an increase in incident light during midday after exposure to full sunlight. When exposed to intermediate sunlight, there were no significant changes in F_v/F_m in leaves. The F_v/F_m values of petals, however, decreased during midday. Temperature of thin petals was higher than that of thick leaves during midday under full and intermediate sunlight while that of petals and leaves were similar when grown in the shade. Over the 2-week treatment period, lowered chlorophyll and sustained decreases in F_v/F_m were observed in both leaves and flowers (sepals and petals) when exposed to full sunlight, indicative of ‘chronic photoinhibition'. Photoinhibition was prevented in leaves but occurred in flowers when exposed to intermediate sunlight. It was assumed that photodamage to both leaves and flowers were partially due to the higher temperature. The higher susceptibility of flowers to high light as compared to that of leaves was due to its higher temperature during midday. This was further supported by the findings that more severe damage occurred in flowers at higher temperature of 38°C than 28°C under a higher PFD of 1500 μmol m⁻² s⁻¹.