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 symbiosis could modify leaf response to nonhydraulic root-to-shoot communication of soil drying in Sorghum bicolor (L.) Moench, and (2) to compare the sensitivity of leaf growth and stomatal conductance (C-s) to the non-hydraulic signal. Seedlings were grown in a greenhouse with root systems 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) experimental design. Plants with roots in three pots dried or severed showed reduced leaf elongation, C-s and leaf water potential (psi) compared with fully watered (control) plants and thus were probably hydraulically affected by root treatment. Drying or severing roots in one pot did not affect leaf elongation, C-s or psi in either mycorrhizal or non-mycorrhizal plants. In non-mycorrhizal plants having two pots dried, final leaf area and total leaf length were reduced by 18 and 10%, respectively, relative to controls. Stomatal conductance of these half-dried nonmycorrhizal plants remained unchanged, suggesting that the decrease in leaf growth was not hydraulically induced. Non-mycorrhizal plants having roots severed in two pots continued to have leaf growth similar to that of the controls, suggesting that growth reductions in half-dried non-mycorrhizal plants did not result from a reduction in root water gathering capacity. Mycorrhizal symbiosis appeared to eliminate inhibition of leaf growth that was not hydraulically induced, because 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 correlated 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 induced. Reductions in leaf extension rate that were not hydraulically induced, 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 mycorrhizal and non-mycorrhizal plants may have been related to differences in soil drying rate.