Experiments

THE HOYA PROJECT

Participants: Michele Rhodda (Singapore Bot. Gdns.), Klaus Winter (STRI), Joe Holtum (JCU)

Project: To explore the expression of CAM photosynthesis in Hoya R.Br. (fam. Apocynaceae), a genus of 500+ species, mostly epiphytic climbers. A few species are associated with ants.

Distributed between the Himalayas and Australia, Hoya is most speciose in the humid tropics of south-east Asia, Philippines and the Indo-Australian Archipelago (see map below). They also inhabit areas with pronounced seasonality (e.g. southern and south-east Asia), including tropical, sub-tropical and warm-temperate montane areas.

Both Hoya and Dischidia, a closely related and generally morphologically similar genus of ca. 130 species, often have thick leaves. Water-conserving CAM photosynthesis has been documented in a handful of species from each genus.

Progress: We have measured the carbon isotope ratio (δ¹³C) and leaf thickness from over 100 species growing in two environments and have have measured nocturnal changes of leaf titratable acidity in many. Continuous gas-exchange has been measured for some Australian species over cycles of watering, withholding water and rewatering. In general, although the δ¹³C values range between -11 ‰, what one would expect for plants that obtain CO₂ solely during the night, and -33 ‰, what plants that obtain CO₂ solely during the day, we have yet to identify a species that does not express nocturnal leaf acidification, a defining characteristic of CAM photosynthesis.

Fewer Dischidia have been studied but, to date, the observations are similar to those for Hoya.