Influence of different Glomus species on the time-course of physiological plant responses of lettuce to progressive drought stress periods

Abstract

The effects of different arbuscular-mycorrhizal (AM) fungi of the genus Glomus on lettuce ( Lactuca sativa L. cv Romana) were compared in terms of foliar area and leaf-gas exchange parameters (photosynthetic activity, water use efficiency (WUE), transpiration and stomatal conductance) following progressive drought stress periods. Measurements were made during periods where AM lettuce plants were either well-watered or subjected to successive and progressive drought periods of 1 week duration (−0.06, −0.10 and −0.17 MPa of soil water potential) and rewatered for the last 2 weeks to test their capacity for stress recovery. Plant growth responses to progressive drought were greatly influenced by the Glomus species colonizing the roots. Leaf area was reduced in G. deserticola-plants by 9% due to drought, while in G. occultum-plants the decline was 70%. During the first period of plant growth (6 weeks), before the exposure to drought, the plant physiological parameters were shortly affected among plants colonized by the different endophytes. After the drought exposure, the activity of specific endophytes increased the differences in leaf gas exchange and related parameters among stressed mycorrhizal plants. Mycorrhizal symbiosis by specific Glomus spp. exhibited particular abilities (measured as photosynthetic activity, WUE, transpiration and conductance) according to the level of stress applied. Plants colonized by G. etunicatum, G. mosseae or G. occultum had a high sensitivity for decreasing CO 2 assimilation even to light stress (−0.06 MPa) while G. fasciculatum-infected plants decreased such parameters only under severe stress (−0.17 MPa). Plants colonized with G. etunicatum induced the highest transpiration and conductance values under all conditions and did not reduce water use efficiency until −0.17 MPa (severe stress). G. deserticola was the best endophyte in counteracting drought effects on physiological parameters under the drought regimes tested here. Following re-irrigation, G. deserticola showed the highest ability to recover CO 2 assimilation, WUE, conductance and transpiration from drought. These results suggest that the ability of different endophytes to protect the host plant against progressive drought stress cannot be ascribed either to a specific physiological mechanism or to the colonizing ability showed by the endophytes. These observations on the particular physiological responses to drought according to endophytes and stress intensity, clarify an important aspect in relation to the biodiversity of Glomus spp.