Lutein from Deepoxidation of Lutein Epoxide Replaces Zeaxanthin to Sustain an Enhanced Capacity for Nonphotochemical Chlorophyll Fluorescence Quenching in Avocado Shade Leaves in the Dark

Leaves of avocado (Persea americana) that develop and persist in deep shade canopies have very low rates of photosynthesis but contain high concentrations of lutein epoxide (Lx) that are partially deepoxidized to lutein (L) after 1 h of exposure to 120 to 350 μmol photons m⁻² s⁻¹, increasing the total L pool by 5% to 10% (∆L). Deepoxidation of Lx to L was near stoichiometric and similar in kinetics to deepoxidation of violaxanthin (V) to antheraxanthin (A) and zeaxanthin (Z). Although the V pool was restored by epoxidation of A and z overnight, the Lx pool was not. Depending on leaf age and pretreatment, the pool of ∆L persisted for up to 72 h in the dark. Metabolism of ∆L did not involve epoxidation to Lx. These contrasting kinetics enabled us to differentiate three states of the capacity for nonphotochemical chlorophyll fluorescence quenching (NPQ) in attached and detached leaves: ∆pH dependent (NPQ ∆궜H ) before deepoxidation; after deepoxidation in the presence of ∆L, A, and Z (NPQ ∆LAZ ); and after epoxidation of A+ Z but with residual ∆L (NPQ ∆L ). The capacity of both NPQ ∆LAZ and NPQ ∆L was similar and 45% larger than NPQ ∆pH , but dark relaxation of NPQ ∆LAZ was slower. The enhanced capacity for NPQ was lost after metabolism of ∆L. The near equivalence of NPQ ∆LAZ and NPQ ∆L provides compelling evidence that the small dynamic pool ∆L replaces A+Z in avocado to "lock in" enhanced NPQ. The results are discussed in relation to data obtained with other Lx-rich species and in mutants of Arabidopsis (Arabidopsis thaliana) with increased L pools.