Leaf elongation and water relations of mycorrhizal sorghum in response to partial soil drying: two<i>Glomus</i>species at varying phosphorus fertilization

Arbuscular mycorrhizal symbiosis has previously been shown to alter the response of sorghum leaves to probable non-hydraulic signals of soil drying. Our objectives here were to determine: (1) how changes in phosphorus nutrition affect this root-to-shoot signalling in sorghum, (2) if mycorrhizal symbiosis can affect the signalling process independently of effects on host P nutrition, and (3) how two Glomus species compare in their influence on signalling. Sorghum bicolor (L.) Moench ‘G1990A’ plants were grown with root systems split between two pots. The 3×3×2 experimental design included three levels of mycorrhizae (Glomus intraradices Schenck & Smith, Glomus etunicatum Becker & Gerd., non-mycorrhizal), three levels of phosphorus fertilization and two levels of water (fully watered, half-dried). Declines in leaf elongation with soil drying were more consistent in non-mycorrhizal than mycorrhizal plants. Relative growth rate (RGR) of both mycorrhizal and non-mycorrhizal plants initially declined when water was withheld from about half of the root system. With further soil drying, RGR of mycorrhizal plants eventually returned to control levels, while RGR of non-mycorrhizal plants remained depressed throughout the drying episode. By the end of the drying episode, mycorrhizal symbiosis had eliminated drying-induced declines in total plant leaf length. Shoot and root dry weight declines of half-dried plants were not affected by mycorrhizae. Declines in stomatal conductance with soil drying were larger and more frequent 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 not differ substantially in their influence on leaf behaviour. The applied phosphorus treatments did not affect either growth or stomatal response of halfdried plants to the root-to-shoot signal, but length declines were related to actual leaf phosphorus concentrations. Rate of soil 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 phosphorus nutrition. However, plant size and nutrition, which are commonly affected by mycorrhizal symbiosis, can also modify the signalling process.