Survey of thermal energy dissipation and pigment composition in sun and shade leaves

A survey was conducted of the magnitude of energy dissipation in photosystem II (expressed as nonphotochemical quenching of chlorophyll fluorescence, NPQ) as well as leaf carotenoid composition of a wide range of different plant species growing in deep shade and/or full sun. Consistently higher levels of the reversible component of NPQ as well as higher degrees of rapidly attainable de-epoxidation of the xanthophyll cycle (VAZ) pool were observed in sun leaves compared to deep shade leaves. It is concluded that these altered features of the xanthophyll cycle allowed sun leaves to dissipate excess energy more effectively over the short term. In addition to the rapid increase in reversible NPQ, shade leaves exhibited a slow further, and sustained, increase in NPQ, In contrast to these deep shade leaves experimentally exposed to high PFDs, understory leaves experiencing highly variable PFD in their natural environment appeared to Be able to dissipate excess excitation energy adequately via xanthophyll cycle-dependent thermal dissipation. Furthermore, very consistent trends across plant species were observed for changes in carotenoid composition (pools of carotenes, VAZ, and other xanthophylls) in response to light environment, as long as it is assumed that in some species beta-carotene can be replaced by alpha-carotene and in a few plant species lutein can be replaced by lactucaxanthin. Sun leaves consistently exhibited much greater levels of VAZ than shade leaves but very similar levels of lutein and neoxanthin (all on a Chi basis) as well as an only slightly higher ratio of total carotenes to the pool of all xanthophylls.