Effect of a drought period on the mobilisation of non-structural carbohydrates, photosynthetic efficiency and water status in an epiphytic orchid

Environmental stress induces modifications and adaptations, a marked characteristic of epiphytic orchids. The presence of the pseudobulb is associated with carbohydrate reserve functions and water maintenance in the plant. We evaluated the effects of drought on carbohydrate metabolism in plants subjected to drought for 0, 25 and 45 d by measuring the following parameters: Fv/Fm ratio, ethanol and water-soluble polysaccharides, acid invertase (β-D-fructofuranosidase; EC 3.2.1.25) activity, relative water content, concentration of osmotically active solutes and dry mass/fresh mass ratio. The carbohydrates present in leaf and pseudobulb were characterised by acid hydrolysis followed by paper chromatography and high performance anion exchange chromatography. Our results showed that a period of drought decreased the efficiency of the photochemical system (decrease in the ratio Fv/Fm) at the same time as the ratio of sucrose/monosaccharides was constant in the leaf, but increased in the pseudobulb. As a consequence of the changes in sucrose/monosaccharides ratio in the pseudobulb, plants under drought stress have decreased invertase activity and increased concentration of osmotically active solutes. The pseudobulb stores a glucomannan, thought to be a multifunctional carbohydrate involved in water stress and carbon reserve. Our results suggest that, under water stress, glucomannan in the pseudobulb is mobilised to moderate effects of drought.