GAS-EXCHANGE AND METABOLITE FLUCTUATIONS IN GREEN AND YELLOW BANDS OFVARIEGATED LEAVES OF THE MONOCOTYLEDONOUS CAM SPECIES AGAVE AMERICANA

The variegated leaves of the Crassulacean acid metabolism (CAM) specie s Agave americana have a large central longitudinal green band with na rrow yellow bands on either side. The yellow bands had 97% less pigmen t content, 84% lower ribulose-1,5-bisphosphate carboxylase/oxygenase a ctivity, but only 20% lower phosphoenolpyruvate carboxylase activity t han the green band. The green bands exhibited gas exchange typical of CAM plants, with most CO2 uptake occurring at night, leading to a dail y net CO2 uptake of 127 mmol m(-2) day(-1). The yellow bands had some nighttime net CO2 uptake but a larger loss during the daytime, indicat ing that they were sink tissues. Nocturnal citrate and malate accumula tions for the yellow bands were 65 and 75%, respectively, of those of the green bands; sucrose supported 64-83% of their nocturnal acid accu mulation. This is the first evidence that agaves, which are malic-enzy me-type CAM plants, use sucrose as the carbon source for nocturnal aci d accumulation. About 44% of the carbon demand of the yellow bands can be supplied by sucrose diffusing via the symplast From the adjacent g reen band, about 25% from fructose and glucose diffusion, and some via the apoplast.