SURVEY OF THERMAL-ENERGY DISSIPATION AND PIGMENT COMPOSITION IN SUN AND SHADE LEAVES

A survey was conducted of the magnitude of energy dissipation in photo system II (expressed as nonphotochemical quenching of chlorophyll fluo rescence, NPQ) as well as leaf carotenoid composition of a wide range of different plant species growing in deep shade and/or full sun. Cons istently higher levels of the reversible component of NPQ as well as h igher degrees of rapidly attainable de-epoxidation of the xanthophyll cycle (VAZ) pool were observed in sun leaves compared to deep shade le aves. It is concluded that these altered features of the xanthophyll c ycle allowed sun leaves to dissipate excess energy more effectively ov er the short term. In addition to the rapid increase in reversible NPQ , shade leaves exhibited a slow further, and sustained, increase in NP Q, In contrast to these deep shade leaves experimentally exposed to hi gh PFDs, understory leaves experiencing highly variable PFD in their n atural 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 observe d 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-c arotene and in a few plant species lutein can be replaced by lactucaxa nthin. Sun leaves consistently exhibited much greater levels of VAZ th an 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 caro tenes to the pool of all xanthophylls.