Photosynthetic Responses of C 3 and C 4 Wetland Species in a Tropical Swamp

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Abstract
(1) A tropical swamp near Nairobi, Kenya containing monotypic stands of the C4 sedges Cyperus papyrus, Cyperus latifolius and the C3 species Typha domingensis was investigated. (2) Aerial biomass of C. papyrs was 2050 g m-2, C. latifolius 2170 g m-2 and T. domingensis 1350 g m-2. The LAI of C. latifolius (15.3) was three times that of T. domingensis. The Cyperus canopies intercepted more than 95% and the Typha canopy more than 85% of incident photosynthetically active radiation. (3) Photosynthetic light response curves were constructued for the photosynthetic bracteoles of C. papyrus and leaves of C. latifolius and T. domingensis. Maximum rates of net photosynthesis were 26 .cntdot. 8 .mu.mol m-2 s-1 for C. papyrus, 21 .cntdot. 8 .mu.mol m-2 s-1 for C. latifolius and 17 .cntdot. 8 .mu.mol m-2 s-1 for T. domingensis. The apparent quantum yield of net photosynthesis from incident light was higher in C. papyrus than the other two species. (4) The relationship between net photosynthesis and stomatal conductance was almost linear for all species but the C4 plants achieved their highest rates of net photosynthesis at lower conductances than the C3 T. domingensis. (5) Both photosynthetic water-use efficiency and nitrogen-use efficiency in the C4 sedges were more than double the values for T. domingensis. (6) The adaptive advantage of C4 photosynthesis in wetlands is discussed and it is suggested that the higher nitrogen-use-efficiency of the C4 C. papyrus may explain why it is more tolerant of oligotrophic conditions than other (mainly) C3 species.

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