This study examined the responses of green leaves (GL) and green pseudobulbs (GPSB) of CAM orchid Cattleya laeliocattleya Aloha Case to drought stress. After being subjected to drought stress, the decrease in water content (WC) was much greater in GPSB than in GL, indicating that GPSB facilitated a slow reduction in the WC of GL. This finding was further supported by the result of relative water content (RWC) of GL, which started to decrease only after 3 weeks of drought stress. Decreases of midday ratios of GL occurred in all plants. However, the degrees of decrease were much greater in drought-stressed GL than in well-watered GL. Reduced ratio (<0.8) at early morning was observed in drought-stressed GL after 3 weeks of treatments. Decreases in total chlorophyll (Chl) content, electron transport rate (ETR), photochemical quenching, qP, and nonphotochemical quenching, qN, were severer in GPSB than in GL after drought treatment. CAM acidity was significantly lower in both GL and GPSB after 2 weeks of drought treatment compared to well-watered plants. However, decrease of CAM acidity was smaller in GL than in GPSB. These results suggest that both GL and GPSB of CAM orchid Cattleya plantswere susceptible to drought stress. 1. Introduction Epiphytic orchids found in tropical environments are directly or indirectly exposed to natural air currents and solar radiation and receive only intermittent rains. In addition to coping with rapid changes in natural air current and light intensity, there is also the need for epiphytic orchids to adapt to periodic drought [1]. Many epiphytic orchids develop morphological features to conserve and/or store water to cope with drought. A morphological feature includes the presence of swollen stems called pseudobulbs that serve as a reserve for water and carbohydrates [2]. Among the many abiotic factors involved in the survival of epiphytes, water availability is probably the most important environmental factor limiting growth and survival of epiphytes [3]. Thus, tolerance to water deficit is a decisive factor in their survival. The presence of the pseudobulb may facilitate a slow reduction in the leaf water content and decline in water potential during a period of drought [1, 4, 5]. The pseudobulb is characterized by the presence of very thick cuticle, absence of stomata, and the abundance of water-storing cells [6]. This makes the pseudobulb an integral organ in the survival and growth of orchids [7, 8]. This is even more so for green pseudobulbs (GPSB) which can photosynthesise and hence contribute positively to carbon
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