| Soils are generally characterized by lack of phosphorus(P),magnesium(Mg)and calcium(Ca),and abundance of iron(Fe)and aluminum(Al)in subtropical China.While in phosphate mining areas of subtropics(e.g.,the Central Platuea of Yunnan),P-rich soils formed based on these P-rich ores,accompanied by the enrichment of Mg and nitrogen(N)in soils.It provides a natural and ideal experiment site for the comparative study of plant development and the trophic cascade effect between plants and herbivores at the P-rich and P-deficient sites.In previous studies,based on stoichiometric traits and metabolic components,the coping strategies of plant vegetative organs(e.g.,leaves)to the different soil nutrients of P-rich and P-deficient sites have been revealed,but little has been reported on the adaption and regulation mechanism of plant reproductive organs(e.g.,seeds)to the different soil nutrients at the contrasting-P sites.Quercus variabilis Bl.is a kind of broad-leaved deciduous tree species,which is widely distributed in East Asia.It is also an important kind of forest constructive species in Central Yunnan,and has typical large seeds,which form in June,and mature and fall down in September to October.Weevil(Curculio davidi Fairmaire),which is parasitic in Q.variabilis seed,is a holometabolous phytophagous insect.It generally completes the initial larvae development and the energy storage in seeds.According to the physiological and ecological traits of plants,soil nutrients not only can affect element concentrations of Q.variabilis seeds,which have certain correlations with the element concentrations in leaves,but also can affect the chemical composition of parasitic insects via the trophic cascade effect.In the present study,to explore the traits of element and metabolic components in developing seeds,the relationships of metabolites as well as elements between leaves and seeds,and the bottom-up cascade effect of seeds on parasitic weevils,we took Q.variabilis populations at P-rich and P-deficient sites as research objects.The main results are as follows:1.Changes of morphological traits presented similar patterns in Q.variabilis seeds at P-rich and P-deficient sites during developing process,and availability of soil elements mainly affected the morphology of mature seedsDuring developing process(from July to October),the height,width,shell width and weight(fresh weight per seed and dry weight per seed)of seeds all presented obviously increasing trends;Seeds got to the physiological maturity in October,when the seed weight showed no difference from that in September;Q.variabilis seeds presented typical characteristics of recalcitrant seeds,namely,there was a sharp decline in the water content of seeds in the late stage,but the water content was still high(>40%)in September.There existed a certain consistency in changes of seed height,width,shell width,weight and water content at the P-rich and P-deficient sites and they showed no significant differences between the two sites.Besides,under soil nutrient gradient across the P-rich and P-deficient sites,dry weight of ripe seeds(October)declined with concentrations of C,N,P,K,Ca,Mg,Fe,Mn,Cu and Al increasing,respectively(p <0.05).It illustrated the trade-off ecological strategy between reproduction and growth of plants in adaptation to "poor soils" in subtropical China.2.Element composition and stoichiometric traits of Q.variabilis seeds varied significantly with the developing process,and different availability of soil elements affected the characteristics of seed elements at different stages of developmentThere were significant differences both in element composition and stoichiometric traits of Q.variabilis seeds at different developing stages(based on principal component analysis).Compared to other elements,energy related elements(C,H and O),protein synthesis related elements(N and S),and osmotic regulation related element(K)accounted for a relatively higher proportion at both two sites at every developmental stages.Concentrations of C,N,S and K in seeds significantly increased at the late stage.Meanwhile,both element composition and stoichiometric traits showed significant differences in seeds between P-rich and P-deficient sites at each stage.Discriminating elements(O,N,P,S,Mn,and Cu)and element stoichiometries(C/P,N/P,Mg/P,Mn/P,Cu/P,and Al/P)between the two sites varied with different developmental stages.At the late stage,concentrations of O and Mn were higher in seeds at P-deficient sites,but concentrations of N,P,S,and Cu were lower than those at P-rich sites.3.Metabolite concentrations of Q.variabilis seeds varied significantly with the developing process,and in terms of different soil nutrients,seeds developed specific metabolite phenotypes at the two sitesDuring developing process,variations of seed metabolite concentrations showed consistency at P-rich and P-deficient sites.Concentrations of seed metabolites presented sequential dynamics with the development,among which,concentrations of orthophosphate,d-fructose,sucrose and myo-inositol presented more significant changes,and concentrations of most metabolites showed maximum in August;Sugar metabolism,TCA cycle and amino acid metabolism were important metabolic processes,involved in the accumulation of seed substances.Meanwhile,at each developmental stage,metabolite composition at the two sites differed significantly,with sugars and organic acids as the major metabolites in distinguishing Q.variabilis seeds at the two sites;Concentrations of orthophosphate and most sugars(especially d-fructose and sucrose)in seeds at P-rich sites were higher,but concentration of erythrose was lower than those at P-deficient sites.Besides,differing from Q.variabilis seeds at P-rich sites,seeds at P-deficient sites might tend to use pentose phosphate pathway in energy metabolism,which consumed less P than glycolysis pathway,to improve P use-efficiency and ensure the substance accumulation of seeds.4.Elements and metabolites showed close relationships between Q.variabilis leaves and seeds,and these relationships were affected by the P-rich and P-deficient site typesThere were certain correlations both in some enzyme and protein/nucleic acid-related elements(e.g.,N,P,Mn and Mg)and metabolites(mainly sugars)between leaves and seeds of Q.variabilis.Concentrations of enzyme and protein/nucleic acid-related elements(N,P,Ca,Mg,Mn and Cu)were lower,but concentrations of most sugars and organic acids were higher in seeds than in leaves.The correlations of elements and metabolites between leaves and seeds were both different at P-rich and P-deficient sites.Besides,Q.variabilis leaves storied more Mn than seeds to adapt to high soil Mn content in Central Yunnan of subtropical area.Seed Mn was negatively related to leaf N and Ca at P-deficient sites,but at P-rich sites,it was positively related to leaf Ca,and not related to leaf N.These revealed,based on metabolite and element components,there were both significant differences and close connections between leaves and seeds of Q.variabilis,and different soil nutrients between the two sites changed the correlation patterns of elements and metabolites between leaves and seeds.5.Weevil larvae hold strong post-ingestion regulation to maintain homeostasis,and availability of soil elements could affect the cascade effect between seeds and weevil larvaeIn the transport of elements from seeds to weevils,concentrations of protein/nucleic acids(N,P and S),enzymes(Zn and Mg),lipids(C and H)related elements were higher,and concentrations of other elements(O,K,Ca and Mn)were lower in weevils than in seeds.The body size of weevils was positively related to seed N concentration,and negatively related to seed Mn concentration.Meanwhile,element stoichiometric traits of weevils were significantly different between P-rich and P-deficient sites(based on principal component analysis),and N,Cu,Na,C/Cu,C/Na and C/Al contributed a lot in reflecting the differences.Besides,weevils at P-deficient sites absorbed more N and released more Mn than those at P-rich sites.These revealed that parasitic insects adapted to the environment of different soil nutrient supply in Central Yunnan,and formed post-ingestion regulation-based element homeostasis mechanism and stoichiometric properties.Based on all the results,in theory,the study deepened the understanding of the ecological strategies of the adaption of plant reproductive organs(seeds)to soils at P-rich and P-deficient sites,revealed the relationships of elements between seeds and leaves,proclaimed the effect of seeds on the element composition and growth of parasitic insects at the two sites,and clarified the characteristics of homeostasis regulation in parasitic insects.In practical applications,it could provide theoretical basis for the management of natural secondary oak forest in the subtropics. |
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