Abstract
CO 2 exchange rate (CER), internal CO 2 concentration (Ci), photosynthetical phosphorus use efficiency (PPUE), nodule activity (ARA) and growth parameters were compared in mycorrhizal nodulated (VAM) and P-compensated nodulated alfalfa plants, under both well watered and drought acclimated conditions. Leaf area ratio significantly decreased in P-fertilized plants in response to drought treatment whereas mycorrhizal plants maintained rather constant values. As stress progressed, VAM plants developed lower root-shoot and higher leaf area ratios than non-VAM. Total dry weight decreased for both treatments in response to drought, but reduction was more pronounced in P-fertilized plants. Under watered conditions, CER and PPUE were approximately 30% and 55% higher respectively in VAM than in non-VAM plants while specific nodule activity was similar in both groups of plants. Throughout drought, the three parameters (CER, PPUE and ARA) maintained significantly higher values in myocorrhizal plants. Internal CO 2 concentration was always higher in P-compensated than in VAM plants. Results show that under water stress the tripartite symbiosis maintains higher values of photosynthetic and nodule activities than non-myocorrhizal plants suggesting enhanced drought tolerance by mycorrhizal condition.
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