Mycorrhizal Stimulation of Leaf Gas Exchange in Relation to Root Colonization, Shoot Size, Leaf Phosphorus and Nitrogen: A Meta-analysis Using Meta-regression

Arbuscular mycorrhizal (AM) symbiosis often stimulates gas exchange rates of the host plant. This may relate to mycorrhizal effects on host nutrition and growth rate, or the influence may occur independently of these. Using ISI Web of Science, we identified 220 articles containing 1019 studies from which effects size and moderator information were extracted. Then, in a meta-analysis using meta-regression, we tested the strength of the relationship between AM-induced increases in gas exchange, and AM size and leaf mineral effects across the literature. With few exceptions, AM stimulation of carbon exchange rate (CER), stomatal conductance (g_s) and transpiration rate were significantly associated with mycorrhizal stimulation of shoot dry weight, leaf phosphorus, leaf nitrogen: phosphorus ratio and percent root colonization. Increasing leaf P was positively correlated with increases in host CER and g_s. Conversely, increases in the leaf N:P ratio in AM plants were associated with decreases in CER and g_s. Leaf N:P response ratio was a better statistical predictor of gas exchange response to AM symbiosis than leaf phosphorus by itself. Mycorrhizal stimulation of photosynthesis, while overall about twice as large as stimulation of stomatal conductance and transpiration, has declined markedly over the 35 years of published investigations. Carbon exchange rate has been more sensitive than stomatal conductance or transpiration rate to increased percent colonization and shoot sizes. The positive correlation of percent root colonization with AM influence on CER may be related to the other regression moderators studied here; more highly infected roots may be better able to scavenge soil nutrients and plants may grow more quickly as a consequence. The results of this meta-analysis indicate that mycorrhizal influences on leaf phosphorus and plant size are positively correlated with their elevated gas exchange rates. However, the strongest correlation, leaf N:P ratio of AM plants, was a negative one.