The Regulatory Mechanism Of Exogenous Phosphorous In Cadmium Resistance Of Mangroves

Mangrove ecosystem is one of the ecosystems which enjoy the richest biodiversity and plays an important role in the stability of coastal zone and its adjacent area.In recent years,with the increasing population of the coastal zone and the rapid development of urbanization,how to protect threatened mangroves reasonably and effectively has become one of the most urgent problems in the new situation.Cadmium(Cd)is a common heavy metal pollutant in mangrove ecosystem,which can affect the normal physiological and biochemical processes of mangrove plants directly or indirectly.At the same time,mangroves have high productivity,turnover rate,and exchange rate between land and sea ecosystems.Mangrove plants play a vital role in the biogeochemical cycle of biogenic elements.Phosphorus is a basic and important element of estuary,and becomes an important component of nucleic acid,membrane lipid and energy metabolism.Phosphorus(P)is very important for the growth and development of mangrove plants growing in intertidal zone.At present,the existing studies on the interactions between Cd and P mostly focus on the effect of P on Cd mobility,solubility and bioavailability in plants and the objects are usually crops.The study of mangrove plants is extremely rare.While P and Cd often coexist in mangrove ecosystem,and there is a certain interaction between them.Therefore,it is very important to clarify the effect of phosphorus on mangrove plants in the context of Cd.The present study focuses on the role of exogenous phosphorous in cadmium tolerance of mangroves,with Avicennia marina(Forsk.)Vierh.(salt secretion species)and Kandelia obovata(S.,L.)Yong(salt exclusion species)as testing materials.This paper mainly discusses the following aspects:Firstly,the study investigated the differences in Cd accumulation and physio-ecological response of two mangrove seedlings under P and Cd exposure;Secondly,it explained the effect of P on the rate of radial oxygen loss,root porosity and iron plaque in mangrove seedlings under Cd stress;Thirdly,it revealed that the role of root cell wall polysaccharide in Cd resistance of mangrove seedlings under P and Cd treatments.The main conclusions are as follows:(1)The study illustrated the difference in the eco-physiological mechanism influenced by different P supplement under Cd stress.Results showed that the distribution order of Cd accumulation in two mangrove seedlings was root>stem>leaf.Salt excretion could not significantly change the distribution pattern of heavy metals.P application decreased the content of malondialdehyde,but increased proline and photosynthetic pigments contents as well as induced the synthesis of non-protein thiols,glutathione and phytochelatins in the leaves of two mangrove seedlings under the stress of Cd.These results illustrate that P addition promoted the detoxification of two tested mangrove seedlings on Cd,while the response strategies of mangrove plants to P and Cd interaction were species-specific.A.marina seedling showed stronger adaptability and tolerance to Cd.(2)We measure the rate of radial oxygen loss and root porosity using Ti3+-citrate oxidation method and pycnometer method,respectively.The results indicated that Cd stress inhibited the radial oxygen loss and root porosity of the test mangroves,while P significantly improved the inhibition effects of Cd on mangrove plants.There was a significant positive correlation between the root porosity and the rate of radial oxygen loss in mangrove plants.A.marina seedlings had higher rate of radial oxygen loss and root porosity than that of K.obovata,suggesting that A.marina had a stronger ability to adapt to anaerobic environment.Further research shows that P supplement reduced the Fe content but increased the Cd content in iron plaque of mangrove seedlings.Furthermore,the formation of root iron plaque could act as a barrier and buffer against Cd absorption by mangrove plants,but capacity-constrained.The root system is still the main barrier of Cd entering into plants.(3)The application of P significantly increased the content of Cd in the root cell wall of two tested mangrove plants under the stress of Cd.Moreover,the content of pectin and hemicellulose 1 in root cell wall were significantly enhanced under P and Cd co-application.This effect was even more significant in the seedling of A.marina.Besides,P significantly increased PME activity of root cell wall in mangrove seedlings in the context of Cd.A.marina seedling had a remarkably greater PME activity than K.obovata.This means high levels of pectin and PME activity could induce more free carboxyl groups.This will thus lead more Cd binding to the cell wall of root and increase the resistance of mangrove seedlings to Cd.In conclusion,these results illustrated that P could medicate Cd stress in mangrove plants and provided a novel method for controlling Cd pollution.