LEAF ELONGATION AND WATER RELATIONS OF MYCORRHIZAL SORGHUM IN RESPONSE TO PARTIAL SOIL DRYING - 2 GLOMUS SPECIES AT VARYING PHOSPHORUS FERTILIZATION

Arbuscular mycorrhizal symbiosis has previously been shown to alter th e response of sorghum leaves to probable non-hydraulic signals of soil drying. Our objectives here were to determine: (1) how changes in pho sphorus nutrition affect this root-to-shoot signalling in sorghum, (2) if mycorrhizal symbiosis can affect the signalling process independen tly of effects on host P nutrition, and (3) how two Glomus species com pare in their influence on signalling. Sorghum bicolor (L.) Moench 'G1 990A' plants were grown with root systems split between two pots. The 3x3x2 experimental design included three levels of mycorrhizae (Glomus infraradices Schenck and Smith, Glomus etunicatum Becker and Gerd., n on-mycorrhizal), three levels of phosphorus fertilization and two leve ls of water (fully watered, half-dried). Declines in leaf elongation w ith soil drying were more consistent in non-mycorrhizal than mycorrhiz al plants. Relative growth rate (RGR) of both mycorrhizal and non-myco rrhizal plants initially declined when water was withheld from about h alf of the root system. With further soil drying, RGR of mycorrhizal p lants eventually returned to control levels, while RGR of non-mycorrhi zal plants remained depressed throughout the drying episode. By the en d of the drying episode, mycorrhizal symbiosis had eliminated drying-i nduced declines in total plant leaf length. Shoot and root dry weight declines of half-dried plants were not affected by mycorrhizae. Declin es in stomatal conductance with soil drying were larger and more frequ ent in non-mycorrhizal than mycorrhizal plants. Leaf osmotic potential and relative water content remained similar in control and half-dried plants during drying, suggesting that altered leaf behaviour of half- dried plants was due to some non-hydraulic factor. The two fungi did n ot differ substantially in their influence on leaf behaviour. The appl ied phosphorus treatments did not affect either growth or stomatal res ponse of half-dried plants to the root-to-shoot signal, but length dec lines were related to actual leaf phosphorus concentrations. Rate of s oil drying did not appear to influence ultimate growth reductions. We conclude that mycorrhizal fungi can modify leaf growth response to the root-to-shoot signal of soil drying, and that this mycorrhizal effect can occur independently of mycorrhizal effects on plant size or phosp horus nutrition. However, plant size and nutrition, which are commonly affected by mycorrhizal symbiosis, can also modify the signalling pro cess.