Foliar drought tolerance of mycorrhizal and nonmycorrhizal cowpea, soybean, bush bean, and basil

The objective of this research was to compare the ability of mycorrhizal (M) plants (Glomus intraradices) and nonmycorrhizal plants to withstand dehydration. The legume experiment with cowpea (Vigna unguiculata), soybean (Glycine max L) and bush bean (Phaseolus vulgaris) and the basil (Ocimum basilicum L) experiment were conducted using three treatments; mycorrhizal (M), nonmycorrhizal with low phosphorus (NL), and nonmycorrhizal with high phosphorus (NH). Soybean, bush bean, cowpea, and basil plants were exposed to a slow, continuous drying episode and drought tolerance of intact foliage was studied. The drying episode continued until plants reached the lethal point, described in terms of leaf water potential (Ψ), or the water potential of the last surviving leaves. Stomatal conductance, osmotic adjustment, lethal soil matric potential (Ψ_m) and lethal leaf Ψ were measured to determine soil and leaf water status of M vs. NL or NH plants. Lethal leaf Ψ and soil matric (Ψ_m values did not differ among treatments in both experiments, except for soybean plants where the NL treatment was -3.60 MPa compared to -3.19 for the M treatment and -3.33 for the NH treatment. Cowpea NL plants developed more osmotic adjustment than NH cowpea plants. Basil M plants had an osmotic adjustment of 0.16 MPa compared to 0.06 and 0.08 MPa in NL and NH plants, respectively. Differences in lethal leaf Ψ among treatments suggest that mycorrhizal symbiosis has changed the physiology of the host plant. In these experiments there were no differences in foliar dehydration tolerance or avoidance, however, in the basil study, M plants had greater osmotic adjustment during drought.