NONHYDRAULIC ROOT-TO-SHOOT SIGNALING IN MYCORRHIZAL AND NONMYCORRHIZAL SORGHUM EXPOSED TO PARTIAL SOIL DRYING OR ROOT SEVERING

Our objectives were (1) to determine if arbuscular mycorrhizal symbios is could modify leaf response to nonhydraulic root-to-shoot communicat ion of soil drying in Sorghum bicolor (L.) Moench, and (2) to compare the sensitivity of leaf growth and stomatal conductance (C-s) to the n on-hydraulic signal. Seedlings were grown in a greenhouse with root sy stems split among four pots. Treatments were applied in a 2 (colonized or not colonized by Glomus intraradices Schenck and Smith) x 2 (roots severed or dried) x 4 (roots in 0, 1, 2 or 3 pots dried or severed) e xperimental design. Plants with roots in three pots dried or severed s howed reduced leaf elongation, C-s and leaf water potential (psi) comp ared with fully watered (control) plants and thus were probably hydrau lically affected by root treatment. Drying or severing roots in one po t did not affect leaf elongation, C-s or psi in either mycorrhizal or non-mycorrhizal plants. In non-mycorrhizal plants having two pots drie d, final leaf area and total leaf length were reduced by 18 and 10%, r espectively, relative to controls. Stomatal conductance of these half- dried nonmycorrhizal plants remained unchanged, suggesting that the de crease in leaf growth was not hydraulically induced. Non-mycorrhizal p lants having roots severed in two pots continued to have leaf growth s imilar to that of the controls, suggesting that growth reductions in h alf-dried non-mycorrhizal plants did not result from a reduction in ro ot water gathering capacity. Mycorrhizal symbiosis appeared to elimina te inhibition of leaf growth that was not hydraulically induced, becau se mean leaf area and total leaf length were not reduced in half-dried mycorrhizal plants, relative to controls. However, final leaf area of mycorrhizal plants having one or two pots dried was negatively correl ated with the product of drying root mass and the time for which roots were exposed to mild drought, suggesting that mycorrhizal plants were also susceptible to growth inhibition that was not hydraulically indu ced. Reductions in leaf extension rate that were not hydraulically ind uced, when viewed as a function of actual soil matric potential, were similar in mycorrhizal and non-mycorrhizal plants, suggesting that the differences in overall growth inhibition between half-dried mycorrhiz al and non-mycorrhizal plants may have been related to differences in soil drying rate.