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

We investigated the impact of high solar irradiance and elevated temperatur e on carbon gain by two, co-occurring, sun-adapted, dwarf shrub species, Pl anchonella obovata var. dubia (Koidz.) Hatusima and Hibiscus glaber Matsumu ra, growing on sun-exposed ridges in the Bonin Islands, in the subtropical Pacific Ocean. Planchonella had steeply inclined, longer lived, sclerophyll ous leaves, whereas Hibiscus has thinner, more horizontally oriented, and s horter lived leaves. We tested the hypothesis that leaf physiological toler ance to high light is lower in Planchonella than in Hibiscus. Under relativ ely high irradiances (photosynthetic photon flux density, PPFD, > 500 mu mo l m(-2) s(-1)), net photosynthetic rate (P-n) was about 8.0 and 0.4 mu mol m-2 s(-1) in mature and young leaves of Planchonella, and about 12.4 and 10 .3 mu mol m(-2) s(-1) in mature and young leaves of Hibiscus, respectively. Both P-n and photosystem II (PSII) quantum yield at a given PPFD were lowe r in Planchonella than in Hibiscus, whereas non-photochemical quenching (NP Q) at a given PPFD was higher in Planchonella. When leaf discs were exposed to high light (1900 mu mol m(-2) s(-1) PPFD) at 37, 40 or 43 degreesC for 3 h, the recovery of PSII quantum yield (F-nu/F-m) in the following 60-min dark period was slower in Planchonella than in Hibiscus, indicating that th e ability of PSII to tolerate high light and high temperature was less in P lanchonella than in Hibiscus. We postulate that there is a linkage between leaf display and leaf photochemical ability in sun-adapted shrub species.