Why are higher plants green? Evolution of the higher plant photosynthetic pigment complement

The physiological reason that higher plants are green is unknown. Other pho tosynthetic organisms utilize pigments that strongly absorb green light; th erefore, there must have been natural forces that 'selected' the photosynth etic pigments found in higher plants. Based on previously published data an d our recent findings about green light and photosynthesis within leaves (S un et al,), a specific functional role is described for the primary photosy nthetic pigments of higher plants, that were derived from green algal proge nitors. The particular absorptive characteristics of chlorophylls a and b a ppear to perform two contradictory, but necessary functions in higher plant s. Firstly, chlorophylls a and b absorb light for maximum utilization under non-saturating conditions, a function that is well understood. Secondly, t hey can act as protective pigments under over-saturating light conditions, when absorbed light is dissipated as heat. Under such conditions, a signifi cant portion of light can also be efficiently utilized, especially in the b ottom portion of the leaf, that is mainly illuminated by green light and no t down-regulated, The second function may have been the selective force tha t gave rise to the extremely successful terrestrial plants, that evolved fr om green algae.