Arbuscular mycorrhizal symbiosis and nonhydraulic: Signaling of soil drying in Vigna unguiculata (L) Walp

We examined the influence of Glomus intraradices on nonhydraulic signaling of soil drying, in a drought-avoiding plant having stomates that are extremely sensitive to changes in soil moisture. Cowpea [Vigna unguiculata (L.) Walp. 'White Acre'] seedlings were grown in a greenhouse with root systems split between two pots. The 2x3x2 experimental design included two levels of mycorrhizal colonization (presence or absence of Glomus intraradices Schenck & Smith UT143), three levels of phosphorus fertilization within each mycorrhizal treatment and two levels of water (both pots watered or one pot watered, one pot allowed to dry). Stomatal conductance was mostly similar in fully watered mycorrhizal and nonmycorrhizal controls. However, g(s) of half-dried, nonmycorrhizal plants was reduced on fewer days and to a lesser extent than g(s) of half-dried, mycorrhizal plants, perhaps related to quicker soil drying in mycorrhizal pots. The partial soil drying treatment had little effect on leaf relative water content or osmotic potential, indicating that declines in g(s) and leaf growth were induced by some nonhydraulic factor. Leaf growth was inhibited only in nonmycorrhizal plants, evidently due to a difference in phosphorus nutrition between mycorrhizal and nonmycorrhizal plants. The mycorrhizal effect on g(s) was not associated with phosphorus nutrition. Inhibition of g(s) was directly related to extent of soil drying, while inhibition of leaf growth was inversely related to extent of soil drying.