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 fiel d conditions using chlorophyll fluorometry. At light intensities < 300 mu m ol 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 hig her 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 depre ssion 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 de creases 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 we re 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, g ymnorrhiza, the maximum fraction of direct solar radiation received by leav es 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 loa ds 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 so lar altitude and high radiation loads.