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Tolerance to high solar irradiation is an important aspect of stress tolerance for landscape plants, particularly for species native to understory conditions. The objective of this study was to evaluate differential tolerance to high solar irradiation and underlying photosynthetic characteristics of diverse taxa of Illicium L. grown under full sun or 50% shade. Eleven commercially available taxa of Illicium were evaluated for light tolerance by measuring light-saturated photosynthetic capacity (Amax), dark-adapted quantum effi ciency of photosystem II (Fv/Fm), and relative chlorophyll content using a SPAD chlorophyll meter. Comparisons of Amax indicated that three of the 11 taxa (I. anisatum L., I. parvifl orum Michx. ex Vent., and I. parvifl orum 'Forest Greenʼ) maintained similar rates of light-saturated carbon assimilation when grown in either shade or full sun. All other taxa experienced a signifi cant reduction in Amax when grown in full sun. Chlorophyll fl uorescence analysis demonstrated that Fv/Fm was similar between sun and shade plants for the same three taxa that were able to maintain Amax. These taxa appeared to experience less photoinhibition than the others and maintained greater maximum photochemical effi ciency of absorbed light. SPAD readings were not signifi cantly reduced in these three taxa either, whereas most other taxa experienced a signifi cant reduction. In fact, SPAD readings were signifi cantly higher in I. parvifl orum 'Forest Greenʼ when grown under full sun, which also maintained the highest Amax of all the taxa. These results suggest that there is considerable variation in light tolerance among these taxa, with I. parvifl orum 'Forest Greenʼ demonstrating superior tolerance to high light among the plants compared. A more rigorous examination of I. parvifl orum 'Forest Greenʼ (high light tolerance) and I. fl oridanum Ellis (low-light tolerance) demonstrated that I. parvifl orum 'Forest Greenʼ had a considerably higher Amax, a higher light saturation point, greater potential photosynthetic capacity, reduced susceptibility to photoinhibition as indicated by superior PSII effi ciency following light exposure, greater capacity for thermal de-excitation as indicated by a higher rate of nonphotochemical quenching (NPQ) under full sun, greater apparent electron transport rate (ETR) at mid-day, and higher concentrations of the free-radical scavenger myo-inositol. All of these factors contribute potentially to a greater capacity to use light energy for carbon fi xation while minimizing photodamage.

Photosynthesis, Chlorophyll Fluorescence, and Carbohydrate Content of Illicium Taxa Grown under Varied Irradiance