Mycorrhizal colonization and drought stress affect δ13C in 13CO2-labeled lettuce plants

We studied the role of different arbuscular-mycorrhizal (AM) fungi on lettuce (Lactuca sativa L.) plant carbon metabolism under drought stress. Plants were grown in pots maintained at two levels of soil moisture and labeled during photosynthesis with (CO2)-C-13. P-fertilized plants were used as a non-mycorrhizal control. Well-watered mycorrhizal plants showed similar growth to that of P-fertilized plants. The level of mycorrhizal root infection was not significantly affected by fungal species or by water treatment. In contrast, important differences in delta(13)C between P-fertilized and AM plants were found in shoot and root tissues as a consequence of both water limitation and fungal presence. delta(13)C in shoots and roots increased in non-mycorrhizal treatment as compared with the well-watered plants, whereas this parameter decreased significantly in mycorrhizal plants. Photosynthetic activity was increased in AM plants in well-watered and droughted plants. G. deserticola was the most beneficial endophyte for water use efficiency in both water treatments. Transpiration rate was not affected by any of the treatments. On the basis of total C-13 in plant tissues, in AM plants the newly fixed C seemed to be preferentially utilized for fungal activity rather than being stored in roots.