Plantago lanceolata L. was grown for 104 d with (M) or without (NM) arbuscular mycorrhizal colonization under conditions of ambient (CAMB=350 μl l−1) and elevated (CELEV=540 μl l−1) CO2. Sequential harvests (H) were taken at 41 (H1), 76 (H2) and 104 d (H3) to determine the time-course of mycorrhizal influence on the response of the plant to CO2 enrichment. Total yields of M plants were greater than those of NM from H2 onwards. Plants in the MELEV treatment were significantly larger than those in the MAMB at 104 d. There were significant but much smaller differences in yield between NMELEV and NMAMB. The differences in total yield arose through impact of CELEV on both shoots and roots. Total root length was greater in MELEV than in MAMB only at H3, but total length of mycorrhizal root was greater at H2 and H3. The percentages of root length colonized and that occupied by arbuscules and vesicles were greater in MELEV than in MAMB at the last two harvests, indicating increased sequestration of carbon in internal fungal structures. Though extraradical hyphal lengths were greater in MELEV than in MAMB at H2 and H3, the differences were not significant. Phosphorus inflow and P content of M plants were higher than those of NM plants at H2 and H3, and were higher in MELEV than in MAMB at H3. anova revealed no significant interactions between CO2 and mycorrhizal treatment.
The results are discussed in relation to carbon sequestration in mycorrhizal systems and likely impacts of CO2 enrichment on P. lanceolata grown under field conditions. The importance of sequential harvesting for realistic determination of responses to CO2 is stressed.