It has been known that body size has a profound influence on almost all characteristics of plants. Scaling, which is the study of the influence of body size on form and function, provides a useful tool to understand morphological and physiological phenomena, such as respiration, growth and reproduction. Based on the fractal branching of vascular systems of typical plants, the West, Brown and Enquist theory (denoted henceforth as the WBE theory) predicts that plant as well as animal respiration should scale as the 3/4 power of body size. Because plant respiration plays a crucial role in a wide range of ecological phenomena, ranging from the biomass accumulation of individual trees to global atmospheric CO₂ concentrations, the WBE theory has stimulated a vigorous debate concerning its validity and predictive value over the past decade, and it has receive both supportive and opposing evidence. In this review, we first considerate the effects of different regression protocols on scaling exponents, focusing on ordinary least squares (OLS) and reduced major axis (RMA) regression analysis. We then review the recent advances that have improved our understanding of the scaling of plant respiration with respect to biomass, and explicitly point out that the scaling exponent varies markedly depending on plant developmental stage and nitrogen content. Briefly, the initial confirmation of the WBE theory with respect to plants was mostly derived from indirect (surrogate) measurements of metabolic rates (e.g. diameter growth rates, biomass production rates, and leaf biomass). However, substantial deviations from the predictions of the WBE theory have been observed for particular taxonomic groups or ecosystems. Using the direct measurement of plant respiration, recent studies have suggested that the scaling exponent for plant respiration is close to unity for saplings but numerically decreases as trees grow. This numerical shift in the plant respiration exponent is thought to reflect physical and physiological constraints on the allocation of plant biomass between photosynthetic and nonphotosynthetic organs over the course of plant growth. Furthermore, plant metabolism is more closely related to nitrogen content rather than total biomass because nitrogen is largely confined to living tissues and is fundamental to physiological process such as respiration. Lastly, we discuss issues to be resolved in future research that promise to further deepen our understanding of plant metabolic scaling relationship.