Light absorption efficiency and the package effect in the leaves of the seagrass Thalassia testudinum

Abstract

In this study, I evaluate the variability in light absorption efficiency of Thalassia tes- tudinum leaves and the magnitude of the package effect affecting seagrass leaves. The large vari- ability observed in pigment density and leaf absorptance of T. testudinum leaf segments, was com- pared to the variation in light absorption efficiency reported for leaves of Mentha aquatica, a species that maintains a specialized bifacial mesophyll and has 3 times higher leaf pigment content. Pigment light absorption efficiency of T. testudinum leaves was also compared to the interspecific variability reported for leaf sections of 12 seagrass species collected in 10 tropical (Mexican Caribbean and Philippine Indo-Pacific) and 31 temperate (Spanish Mediterranean, Portuguese Atlantic and Danish fjords) coastal areas. The results of this comparison confirmed that T. testudinum leaves are affected by the package effect, because pigment light absorption efficiency decreases non-linearly as pigment content per unit area increases. The finding that M. aquatica leaves have a 1.5 times higher pigment light absorption efficiency than T. testudinum leaves for similar pigment content, does not necessar- ily indicate that the specialized leaf anatomy developed by seagrasses (i.e. a pigmented epidermis and an unpigmented leaf mesophyll) has led to a reduction in light absorption efficiency. This is because temperate seagrass leaves, which have a 2.5 times higher chlorophyll content than flat- shaped leaves of tropical species, exhibit a 1.4 times higher efficiency than M. aquatica leaves of sim- ilar pigment density. The possible effect of leaf morphology (i.e. leaf thickness and the specific leaf area) on the variation of leaf absorptance of T. testudinum was finally addressed to clarify the ca- pacity of seagrass leaf morphology in counterbalancing pigment self-shading within the thin epider- mis.