The role of mycorrhiza in determining the response of Plantago lanceolata to CO 2 enrichment

Plantago lanceolata L. was grown for 104 d with (M) or without (NM) arbuscular mycorrhizal colonization under conditions of ambient ( C AMB =350 μl l −1 ) and elevated ( C ELEV =540 μl l −1 ) CO 2 . Sequential harvests (H) were taken at 41 (H 1 ), 76 (H 2 ) and 104 d (H 3 ) to determine the time‐course of mycorrhizal influence on the response of the plant to CO 2 enrichment. Total yields of M plants were greater than those of NM from H 2 onwards. Plants in the M ELEV treatment were significantly larger than those in the M AMB at 104 d. There were significant but much smaller differences in yield between NM ELEV and NM AMB . The differences in total yield arose through impact of C ELEV on both shoots and roots. Total root length was greater in M ELEV than in M AMB only at H 3 , but total length of mycorrhizal root was greater at H 2 and H 3 . The percentages of root length colonized and that occupied by arbuscules and vesicles were greater in M ELEV than in M AMB at the last two harvests, indicating increased sequestration of carbon in internal fungal structures. Though extraradical hyphal lengths were greater in M ELEV than in M AMB at H 2 and H 3 , the differences were not significant. Phosphorus inflow and P content of M plants were higher than those of NM plants at H 2 and H 3 , and were higher in M ELEV than in M AMB at H 3 . anova revealed no significant interactions between CO 2 and mycorrhizal treatment. The results are discussed in relation to carbon sequestration in mycorrhizal systems and likely impacts of CO 2 enrichment on P. lanceolata grown under field conditions. The importance of sequential harvesting for realistic determination of responses to CO 2 is stressed.