| Brasenia schreberi J.F.Gmel is a perennial freshwater macrophyte characterized by a thick mucilage on young buds and leaves in the Nymphaeaceae family.The species has been widely cultivated as an aquatic vegetable in China for many years but is now listed as an endangered species,as wild populations of B.schreberi have decreased sharply due to habitat fragmentation and loss,anthropogenic impacts,etc.Small cultivating B.schreberi populations have replaced wild populations in recent decades,and the establishment of conservation zones for the germplasm resources of B.schreberi in Lichuan(LC)and Hangzhou(HZ)could promote the protection of endangered B.schreberi populations.Recent studies showed that different B.schreberi populations in China have low levels of genetic diversity,and most of genetic diversity was within populations.Main B.schreberi cultivating areas such as Suzhou(SZ),HZ,LC and Shizhu(ShZ)have faced the problems of degradation in quality and decline in production,etc.Although environmental issues like global warming and water eutrophication have been widely recognized nowadays,the mechnisms of environmental factors affecting the growth and mucilage accumulation of B.schreberi are not clear.Researchers have demonstrated differences in phenotypic properties between different B.schreberi populations,whether the differences are results of local adaptation or genetic differentiation still need further study.Furthermore,colors of the underside of leaves(leaf colors)of B.schreberi are varied,but few reports have been focused on the potential function of such leaf color polymorphism.To investigate the questions raised above and provide theoretical basis for the conservation of B.schreberi populations,we conducted the following experiments:(1)comparing environmental factors(water quality,sediment nutrients and air temperature)and plants properties(buds and leaves)of different B.schreberi populations in SZ,HZ and LC across growing seasons;(2)comparing phenotypic properties of B.schreberi populations from SZ,HZ,LC and ShZ(SZ-P,HZ-P,LC-P and ShZ-P,respectively)in a common garden experiment;(3)four strains with different leaf colors,a full dark-red type(DR-type),a light-red type(LR-type),an edge-red type(ER-type)and a green type(G-type)were verified,and chlorophyll fluorescence parameters,composition of photosynthetic pigments and stomatal traits of different strains were comparatively measured.The main findings are as follows:(1)For all the environmental parameters measured,water permanganate index(CODMn),total nitrogen(TNW),electrical conductivity(ECw)and dissolved oxygen(DOw),and sediment organic carbon(SOC)and total nitrogen(TNs)were the main factors influencing the growth and mucilage accumulation of B.schreberi based on Redundancy Analysis,which accounted for82.2%of the variation of mucilage accumulation.Mucilage content(MucC),thickness(MucT)and single bud weight(SBW)were negatively correlated with CODMn,TNw and ECw,but positively correlated with SOC and TNs,and correlation between rolled-leaf length(RLL)and DOw was positive and highly significant.Besides,high air temperature in summer may have a negative impact on mucilage accumulation of the species.The results indicate that clean water and sediment enriched with nutrients can promote the growth and mucilage accumulation of B.schreberi.(2)In the common garden experiment of different B.schreberi populations,phenotypic properties of buds(including MucC,MucT and SBW)and leaves(including leaf colors,area(LA),length(LL),width(LW)and LL/LW)between SZ-P and HZ-P were similar,and those between LC-P and ShZ-P were similar,too.Mucilage accumulation of SZ-P and HZ-P was higher than LC-P and ShZ-P.Although most of variation in phenotypic properties was within populations based on Variance of Analysis(about70%),significant differences were existed between different populations.The results suggest that the variation of phenotypic properties may be the results of both local adaptation and genetic differentiation,and the environmental adaptability of SZ-P and HZ-P is higher than LC-P and ShZ-P.(3)Among the four strains with different leaf colors,ER-type showed highest values for all measured chlorophyll fluorescence parameters and derived parameters of ETR(electron transport rate)curves except photoinhibition,and those of LR-type were lowest except maximum quantum yield of PSII and non-photochemical quenching.Photoinhibition occurred with photosynthetically active radiation(PAR)more than 2000μmol m-2s-1 for all strains.DR-type had the highest photosynthetic pigment content,and followed by ER-type.With the ratio of red decreased,the dry weight/fresh weight ratio went down from DR-to G-type.The ER-type was characterized with highest stomatal density(Ds)and smallest stomatal size(agc)and pore size(ap),which was opposite with LR-type,but differences in fractional stomatal cover(fgc)and stomatal conductance(gs)between ER-and LR-type were not significant.We conclude that higher photosynthetic capacity of ER-type may be related with its smaller stomata,as faster opening and closing of smaller than of larger stomata could allow closer tracking of environmental(mainly light)variations based on previous studies.Our results suggest that clean water and nutrients enriched sediment favor the growth and mucilage accumulation of B.schreberi,and habitat conservation particularly water quality is important for maintaining B.schreberi populations.In areas of high temperature like Nanjing,SZ-P and HZ-P with higher environmental adaptability are high quality germplasm resources compared with LC-P and ShZ-P.Strains with different leaf colors showed differences in photosynthetic and physiological characteristics,and ER-type with higher photosynthetic efficiency is the high quality germplasm resource.Effective approaches should be taken to protect the ER-type from extinction as which is diminishing rapidly in recent decades and few remained. |
