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Why are Nitrogen Concentrations in Plant Tissues Lower under Elevated CO 2 ? A Critical Examination of the Hypotheses
Author(s) -
Taub Daniel R.,
Wang Xianzhong
Publication year - 2008
Publication title -
journal of integrative plant biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.734
H-Index - 83
eISSN - 1744-7909
pISSN - 1672-9072
DOI - 10.1111/j.1744-7909.2008.00754.x
Subject(s) - transpiration , photosynthesis , stomatal conductance , shoot , transpiration stream , chemistry , nitrogen , root system , dilution , horticulture , biology , botany , organic chemistry , physics , thermodynamics
Plants grown under elevated atmospheric [CO 2 ] typically have decreased tissue concentrations of N compared with plants grown under current ambient [CO 2 ]. The physiological mechanisms responsible for this phenomenon have not been definitely established, although a considerable number of hypotheses have been advanced to account for it. In this review we discuss and critically evaluate these hypotheses. One contributing factor to the decreases in tissue N concentrations clearly is dilution of N by increased photosynthetic assimilation of C. In addition, studies on intact plants show strong evidence for a general decrease in the specific uptake rates (uptake per unit mass or length of root) of N by roots under elevated CO 2 . This decreased root uptake appears likely to be the result both of decreased N demand by shoots and of decreased ability of the soil‐root system to supply N. The best‐supported mechanism for decreased N supply is a decrease in transpiration‐driven mass flow of N in soils due to decreased stomatal conductance at elevated CO 2 , although some evidence suggests that altered root system architecture may also play a role. There is also limited evidence suggesting that under elevated CO 2 , plants may exhibit increased rates of N loss through volatilization and/or root exudation, further contributing to lowering tissue N concentrations.