NONHYDRAULIC SIGNALING OF SOIL DRYING IN MYCORRHIZAL MAIZE

Our objectives were to (1) verify that nonhydraulic signalling of soil drying can reduce leaf growth of maize, (2) determine if a mycorrhiza l influence on such signalling can occur independently of a mycorrhiza l effect on leaf phosphorus concentration, plant size or soil drying r ate, and (3) determine if leaf phosphorus concentration can affect res ponse to the signalling process. Maize (Zea mays L. 'Pioneer 3147') se edlings were grown in a glasshouse with root systems split between two pots. The 2 x 3 x 2 experimental design included two levels of mycorr hizal colonization (presence or absence of Glomus intraradices Schenck & Smith), three levels of phosphorus fertilization within each mycorr hizal treatment and two levels of water (both pots watered br one pot watered, one pot allowed to dry). Fully watered mycorrhizal and nonmyc orrhizal control plants had similar total leaf lengths throughout the experiment, and similar final shoot dry weights, root dry weights and leaf length/root dry weight ratios. Leaf growth of mycorrhizal plants was not affected by partial soil drying, but final plant leaf length a nd shoot dry weight were reduced in half-dried nonmycorrhizal plants. At low P fertilization, effects of nonhydraulic signalling were not ev ident. At medium and high P fertilization, final total plant leaf leng th of nonmycorrhizal plants was reduced by 9% and 10%, respectively. T hese growth reductions preceded restriction of stomatal conductance by 7 d. This and the fact that leaf water potentials were unaffected by partial soil drying suggested that leaf growth reductions were non-hyd raulically induced. Stomatal conductance of plants given low phosphoru s was less influenced by nonhydraulic signalling of soil drying than p lants given higher phosphorus. Soil drying was not affected by mycorrh izal colonization, and reductions in leaf growth were not related to s oil drying rate (characterized by time required for soil matric potent ial to drop below control levels and by time roots were exposed to soi l matric potential below typical leaf water potential). We conclude th at mycorrhizal symbiosis acted independently of phosphorus nutrition, plant size or soil drying rate in eliminating leaf growth response to nonhydraulic root-to-shoot communication of soil drying.