What are backshoots good for? Seasonal changes in mineral, carbohydrate and water content of different organs of the epiphytic orchid, Dimerandra emarginata

The epiphytic orchid, Dimerandra emarginata (G. Meyer) Hoehne, keeps older stems ('backshoots') after the attached foliage dies. I investigated season al changes in the water, carbohydrate and nutrient content of these leafles s stems along with associated, connected organs for 1 year. Older stems sig nificantly increase the water storage capacity of a plant, allowing it to t olerate long periods of drought. Reduction in total non-structural carbohyd rates (TNC) at the beginning of the growing season indicated an important r ole of carbon stored in older stems for the re-initiation of vegetative gro wth after the dry season, while the lack of an increase in TNC before fruit ing suggests no such role for reproduction. While seasonal fluctuations in water contents and TNC were pronounced, concentrations of nutrients (N, P, K) showed only small seasonal variation in all vegetative organs. Allocatio n of carbon and nutrients to developing fruits was rather high, exceeding, e.g. 20 % of the total plant pool of potassium. This large investment in fr uits was derived from all vegetative organs. I conclude that persistent ste ms serve a crucial function in the water, carbon and nutrient relations of epiphytes such as D. emarginata, which live in habitats where access to wat er and key nutrients is often severely limited. (C) 1999 Annals of Botany C ompany.