PHOTOSYNTHETIC AND GROWTH-RESPONSES OF VARIEGATED ORNAMENTAL SPECIES TO ELEVATED CO2

Variegated and completely green cultivars of oleander (Nerium oleander L.) and willow myrtle (Agonis flexuosa (Willd.) Sweet) were grown in controlled environment cabinets for 3 and 5 months, respectively, unde r either ambient levels of CO2 or with supplementary CO2 to a partial pressure of 800 mu bar. Photosynthesis of entirely green leaves and th e green portions of variegated leaves on both species was greatly stim ulated by high CO2 and there was no evidence of downward adjustment (a cclimation) of photosynthetic rates to high CO2 during the experiment. Dark respiration rates of these leaves were lowered by high CO2. The yellow portions of willow myrtle leaves showed a low level of photosyn thetic activity which was stimulated by high CO2; however, dark respir ation rates showed little response to elevated CO2. Green and yellow a reas on variegated leaves of willow myrtle had much lower dark respira tion rates than completely green leaves, but this difference was not e vident for oleander. Yellow portions of oleander leaves showed little evidence of photosynthetic capacity. This was also confirmed by a low photochemical efficiency as determined by chlorophyll fluorescence. A major effect of variegation was to slow overall plant growth compared with completely green plants. The respective 3-fold and 6-7-fold diffe rences in biomass between fully green and variegated cultivars of olea nder and willow myrtle was closely related to estimated net carbon gai n per day by the plant canopy. Variegation for both species averaged c lose to 50:50, green:yellow areas. Variegated plants developed about t wice the leaf area ratio and specific leaf area compared with their co mpletely green counterparts. The relative growth response to high CO2 was significantly greater for the variegated plants compared to the co mpletely green plants.