The Water And Nutrition Relationships Between Mistletoe And Its Host Mangrove Species

Mistletoes are xylem-tapping hemi-parasites.They obtain heterotrophic carbon coupled with water and mineral nutrients via haustoria which connect to the internal tissues of their hosts,however,they are capable of photosynthesis because they contain chlorophyll.Xylem-tapping mistletoes generally have higher transpiration rate(E), lower CO₂ assimilation rate(A) and lower water-use efficiency(WUE) than their hosts. The higher E has been attributed to N gathering mechanisms.Here,Viscum ovalifolium infecting on Sonneratia caseolaris(a mangrove tree growing on saline environment) were investigated.The diurnal courses of gas exchange and chlorophyll fluorescence were followed,and light response and ACi curves were measured under control conditions.Especially,influence of mesophyll conductance (gm),an important but sometimes neglected factor,on gas exchange was researched. Chlorophyll contents,leaf water potential,carbon isotope ratio(δ¹3C),and mineral element in leaves was compared.In addition,in order to analyze the influence of haustoria on mineral elements,elements in xylem were also compared.Significantly lower E and gs were found from the mistletoe,which didn't within the general pattern.However the mistletoe had high foliar N concentrations although it transpired less than its host.Therefore,the result was still consistent with the N-parasitism hypothesis.Because the mangrove host was N sufficient,mistletoe can obtain N easily from the xylem sap without having to maintain high transpiration.It was proved that if sufficient N supplied,mistletoes growing on mangroves,may keep lower E in order to avoid excessive salt accumulation in leaves.The mistletoes accumulated high salt in leaves and have more negative leaf potential(ψ) than those of their mangrove hosts.Mistletoe A was significatively decreased,and they have low light saturation(Lsp).However,mistletoe didn't accumulate significative excess energy which primarily determines the rate of PSâ…¡photoinactivation and destroyed the photosynthetic centers.High thermally dissipated (D) and nonphotochemical quenching(NPQ) in mistletoes indicated that the excess energy is dissipated by heat,which is the photoprotection mechanism for mistletoe leaves.On the other hand,mangroves have mechanisms to cope with the low water potentials and high levels of salt in leaves.In this research,both of gs and gm are decreased in response to salinity stress in mistletoes,as a consequence,mistletoes maintain a low A although they have the same high leaf N contents to their host.The decreased A in mistletoes is affected by diffusion limitations(i.e.gs and gm) more than biochemical limitation under salinity stress.It was proved that mistletoes have lower maximum carboxylation rate(V_cmax) and maximum electron transport rate(J_max) values,which indicated a fundamental limitation limitation on photosynthetic function at the biochemical level.However,it was suggested that gm is an important factor causing the photosynthetic decline of mistletoe leaves with most stress conditions.That why mistletoes always maintain a low A although they have high leaf N contents.In most stress case,mistletoes have high gs and Tr,while have low gm.The high gs result in more water lost,while the low gm result in low A,this may be the reason that why mistletoes have lower WUE than their hosts.The xylem elements indicated that the xylem continuity between host and mistletoe didn't exist at all.