Mycorrhizal colonization and drought stress affect delta C-13 in (CO2)-C-13-labeled lettuce plants

We studied the role of different arbuscular-mycorrhizal (AM) fungi on lettu ce (Lactuca sativa L.) plant carbon metabolism under drought stress. Plants were grown in pots maintained at two levels of soil moisture and labeled d uring photosynthesis with (CO2)-C-13. P-fertilized plants were used as a no n-mycorrhizal control. Well-watered mycorrhizal plants showed similar growt h to that of P-fertilized plants. The level of mycorrhizal root infection w as not significantly affected by fungal species or by water treatment. In c ontrast, important differences in delta(13)C between P-fertilized and AM pl ants were found in shoot and root tissues as a consequence of both water li mitation and fungal presence. delta(13)C in shoots and roots increased in n on-mycorrhizal treatment as compared with the well-watered plants, whereas this parameter decreased significantly in mycorrhizal plants. Photosyntheti c 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 rath er than being stored in roots.