Water stress and light intensity effects on growth and nocturnal acid accumulation in a terrestrial CAM bromeliad (Bromelia humilis Jacq.) under natural conditions.

Seasonal variations in CAM performance of sunexposed and partially shaded populations of Bromelia humilis were measured under natural conditions in a semi-arid region in northern Venezuela. The sun population consisted of smaller plants, with lower chlorophyll and total nitrogen contents per unit leaf area compared with plants from the partial-shade population. During the dry season CAM activity, assessed as nocturnal acid accumulation, was higher in the partial-shade population. Acid accumulation was stimulated by irrigation in both populations within 24 h after treatment. Daily changes in concentration of soluble sugars were opposite to leaf acidity indicating their role as carbon source for acid synthesis during the night. The change in nocturnal sugar concentration was always more than the amount required for acid accumulation, suggesting other carbohydrate-consuming processes such as transportation of sugars out of the leaf. CAM activity was higher during the rainy season, and differences between populations were smaller. At the end of the rainy season reduction of CAM activity caused by drought was first detected in the sun population. Measured ratios of glucan/soluble sugar show a higher proportion of readily utilizable sugars during periods of active CAM and growth. Under conditions of continuous high light intensity and air temperature leading to all year round high potential evaporation in semiarid tropical regions, fully exposed populations of B. humilis show a pronounced reduction of metabolic activity. Partial shade favours growth and CAM activity in this constitutive CAM species. It is concluded that water stress, and not light intensity, is the predominant limiting factor for growth of this species under natural conditions.