Photosynthetic characteristics differ between emergent and floating-leaved macrophytes

Emergent and floating-leaved macrophytes,two different life forms of primary producers in shallow lakes and riparian zones,play important roles in nutrient cycling and energy flow of aquatic ecosystems. To investigate the differences in photosynthetic characteristics between the two forms of macrophytes,chlorophyll fluorescence,pigment composition,and special leaf weight( SLW) of four emergent macrophytes( Acorus calamus L.,Pontederia cordata L.,Thalia dealbata Fraser,Zizania latifolia( Griseb.) Stapf) and four floating-leaved macrophytes( Nymphaea mexicana Zucc.,Brasenia schreberi J. F. Gmel.,Nymphoides indica( L.) O. Kuntze,Trapa bispinosa Roxb.) growing in natural habitats were compared. In general, emergent macrophytes showed higher electron transport rate( ETR) and non-photochemical quenching( NPQ) under high irradiation,as well as relatively higher initial slope( α) of ETR curve under low light,than did floating-leaved plants. Broader ranges on maximum quantum yield of PSⅡ( F_v/F_m),ETR,NPQ,maximum potential ETR( Ps),maximum ETR( ETRm) and saturating irradiance( E_m) were also found among different species of emergentmacrophytes. Average chlorophyll( Chls) and carotenoid( Cars) contents were also higher in emergent macrophytes than in floating-leaved ones. However,due to the discrepancy of SLW,the trends of pigment amount among species on a fresh matter basis differed from those per leaf area unit. Among the four emergent macrophytes,T. dealbata and P. cordata exhibited relatively higher photosynthetic ability,and all fluorescence parameters together with pigment ratios( Chl a/b,Chls/Cars) indicated they belonged to the sun type. However,A. calamus and Z. latifolia exhibited relatively lower photosynthetic ability,and part of the parameters were closer to the shade type. All four floating-leaved species exhibited as sun type. This suggests that the heterogeneity of environments contributes to the discrepancy of leaf photosynthetic abilities.Emergent leaves grow in highly diverse environments,so their photosynthetic parameters have broad ranges,whereas the environments for floating leaves are relatively stable,so their parameters are limited to small ranges. It may not be necessary for floating-leaved macrophytes to develop NPQ and internal heat dissipation to prevent high irradiation damage. In conclusion,special attention should be paid to both environmental heterogeneity and discrepancy of adaptation abilities when selecting aquatic macrophytes for