The contribution of leaf angle to photoprotection in the mangroves Avicennia marina (Forssk.) Vierh. and Bruguiera gymnorrhiza (L.) Lam. under field conditions in South Africa

The putative contribution of leaf angle to photoprotection in the mangrove species Avicennia marina and Bruguiera gymnorrhiza was evaluated under field conditions using chlorophyll fluorometry. At light intensities < 300 mu mol m(-2) s(-1) there were no differences in PSII quantum yield (Delta F/F-m') between species, while at higher irradiance, yield was significantly higher in A. marina leaves. North facing (sun) leaves exhibited significantly higher electron transport rate through PSII (ETR) than south-facing (shade) leaves in A. marina, but not in B. gymnorrhiza. In north-facing leaves of both species, there was significant mid-day depression in the intrinsic PSII quantum yield in dark-adapted state (F-v/F-m), which was greater in B. gymnorrhiza. This decrease in F-v/F-m was due to decreases of both F-m and F-o in B. gymnorrhiza but only of F-m in A. marina. In both species, there was complete recovery of F-v/F-m in the afternoon. Differences in F-v/F-m between leaves orientated in different directions were most pronounced between north- and south-facing leaves and were greater in B. gymnorrhiza than in A. marina. Generally, angled leaves reduced the amount of solar radiation impinging on the leaf in both species, as well as midday depression in F-v/F-m. In B, gymnorrhiza, the maximum fraction of direct solar radiation received by leaves was strongly correlated with the degree of mid-day depression in F-v/F-m. Angled leaves of B. gymnorrhiza benefited more from reduced radiation loads than those of A. marina. Although both species possessed photoprotective mechanisms, avoidance of radiation by steep leaf angles was more important in B. gymnorrhiza than in A. marina, especially during months with high solar altitude and high radiation loads.