Photosynthetic and Growth Responses of Variegated Ornamental Species to Elevated CO₂

Abstract

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, under either ambient levels of CO₂ or with supplementary CO₂ to a partial pressure of 800 μbar. Photosynthesis of entirely green leaves and the green portions of variegated leaves on both species was greatly stimulated by high CO₂ and there was no evidence of downward adjustment (acclimation) 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 photosynthetic activity which was stimulated by high CO₂; however, dark respiration rates showed little response to elevated CO₂. Green and yellow areas on variegated leaves of willow myrtle had much lower dark respiration rates than completely green leaves, but this difference was not evident 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 differences in biomass between fully green and variegated cultivars of oleander and willow myrtle was closely related to estimated net carbon gain per day by the plant canopy. Variegation for both species averaged close to 50:50, green:yellow areas. Variegated plants developed about twice the leaf area ratio and specific leaf area compared with their completely green counterparts. The relative growth response to high CO₂ was significantly greater for the variegated plants compared to the completely green plants.