Light-use properties in two sun-adapted shrubs with contrasting canopy structures

Abstract

We investigated the impact of high solar irradiance and elevated temperature on carbon gain by two, co-occurring, sun-adapted, dwarf shrub species, Planchonella obovata var. dubia (Koidz.) Hatusima and Hibiscus glaber Matsumura, growing on sun-exposed ridges in the Bonin Islands, in the subtropical Pacific Ocean. Planchonella had steeply inclined, longer lived, sclerophyllous leaves, whereas Hibiscus has thinner, more horizontally oriented, and shorter lived leaves. We tested the hypothesis that leaf physiological tolerance to high light is lower in Planchonella than in Hibiscus. Under relatively high irradiances (photosynthetic photon flux density, PPFD, > 500 μmol m-2 s-1), net photosynthetic rate (Pn) was about 8.0 and 0.4 μmol m-2 s-1 in mature and young leaves of Planchonella, and about 12.4 and 10.3 μmol m-2 s-1 in mature and young leaves of Hibiscus, respectively. Both Pn and photosystem II (PSII) quantum yield at a given PPFD were lower in Planchonella than in Hibiscus, whereas non-photochemical quenching (NPQ) at a given PPFD was higher in Planchonella. When leaf discs were exposed to high light (1900 μmol m-2 s-1 PPFD) at 37, 40 or 43 °C for 3 h, the recovery of PSII quantum yield (Fv/Fm) in the following 60-min dark period was slower in Planchonella than in Hibiscus, indicating that the ability of PSII to tolerate high light and high temperature was less in Planchonella than in Hibiscus. We postulate that there is a linkage between leaf display and leaf photochemical ability in sun-adapted shrub species.