A General Model For Respiratory Metabolism During Seed Germination And Early Seedling Development

The structural characteristics and physiological properties of organisms and their allometric scaling relationships have been a great concern for biologists.Especially the development of Metabolic Scaling Theory has inspired many interests in allometric scaling relationships.Based on MTE and the principles of energy metabolism,ecologists have developed ontogenetic growth models for animals(from an oosperm to a mature individual)and plants(from small seedlings/saplings to large trees),respectively.All the models above assume that this ontogenetic stage could be considered as a continuous process.However,little attention is paid on the energy metabolism and allocation during seed germination and seedling development.At this stage,energy is provided by catabolizing storage reserves to resume metabolic activities and induce germination while individual biomass increase is null.Thus it is a relatively independent energy-consuming process.To investigate how respiratory metabolism is regulated during this early ontogenetic stage,we assume that:(1)Enzyme kinetics of any single chemical reaction in respiratory metabolism can be modelled by the Michaelis-Menten equation.(2)Water is considered as a key limiting resource in regulating respiratory activities.Then we derived a general mathematical model to quantify the relationship between metabolic rate,body size and water content based on the assumptions above.We conducted a tracking experiment which measured the metabolic rate,body size,water content,nitrogen and carbon content for 40 plant species including annual herbs,perennial herbs and woody plants to test our model.Also we determined the correlation between maximal mass-specific metabolic rate and initial seed body size,initial seed carbon and nitrogen content,time needed to germinate and plant water content when seeds germinate using multiple linear regression.Our study showed that:1)The intraspecific scaling slope between whole plant dry mass and total nitrogen,carbon content and respiration rate for 40 species is close to 0 respectively,while the intraspecific scaling slope is close to 1 respectively.2)The model(2-13)fits well with our data with an overall r~2 of 0.84.Also our model is general regardless of plant life forms including annual herbs,perennial herbs and woody plants.The scaling slope between corrected mass-specific respiration and corrected water content is 1.013,which is very close to the estimated value 1.This suggests that during seed germination and early seedling development,water plays a crucial role in metabolism due to its role as reactants of respiration activity and transporter of nutrients.3)Seed germination occurs at the point when the curvature of our model curve reaches maximal,indicating that seeds would not germinate before reaching critical water content.4)There are no significant differences of modelling curve among plant life forms,while desert plants have relatively higher specific respiration rate at a certain water content,which can be explained by the evolution strategies of desert plants.Water deficiency in desert ecosystems forces plants to evolve to increase their WUE and thus become more competitive in long-term survival and reproductivity.5)Because of the effect of water content,body fresh mass and dry mass are different variables to represent body size.6)The maximal mass-specific metabolic rate is only correlated with seed initial body size and plant water content when seeds germinates rather than other variables.Based on cell energetics and enzyme kinetics,we have derived a mathematical model which quantifies the effect of body size and water content on respiratory metabolism during seed germination and early seedling development and fits well with our measured data of 40 species.Our model would have many important implications for the further development of MTE.Furthermore,our model also predicts that in water-stressed environment such as dryland ecosystems,plant individual metabolic rate is also determined by body size and water content,which would provide theoretical basis for ecological patterns at ecosystem level including species distribution and biodiversity.