COMPARATIVE EFFECTS OF VESICULAR-ARBUSCULAR MYCORRHIZAL INOCULATION AND PHOSPHORUS FERTILIZATION ON GROWTH AND PHOSPHORUS UPTAKE OF MAIZE (ZEA-MAYS L) AND SORGHUM (SORGHUM-BICOLOR L) PLANTS UNDER DROUGHT-STRESSED CONDITIONS

Maize (Zea mays L.) and sorghum (Sorghum bicolor L.) Moench (local var iety called ''Masakwat'') plants were grown in a sterilized low-P soil in the greenhouse for 12 weeks. Each plant species was either mycorrh izal with vesicular-arbuscular mycorrhizal (VAM) fungi, non-mycorrhiza l but minimally fertilized with soluble P, or non-mycorrhizal but high ly fertilized with soluble P. Drought stress was imposed after 4 weeks at weekly intervals. Under unstressed conditions, leaf area, shoot dr y weights, xylem pressure, and soil water potentials were similar for VAM and the two non-mycorrhizal P-fertilized treatments but each of th e VAM-infected species had a greater total root length. Total P uptake was similar for the maize treatments but higher for VAM than non-myco rrhizal P-fertilized sorghum treatments. Under drought-stressed condit ions, the growth parameters and soil water potential were similar for all maize treatments but they were reduced by mycorrhizal inoculation in sorghum. Greater water extraction occurred in drought-stressed myco rrhizal sorghum. In both plant species, total P uptake and P uptake pe r unit root length (including unstressed species) were significantly e nhanced in non-mycorrhizal P-fertilized treatments compared with the m ycorrhizal treatment. Except for the root dry weight of sorghum plants , there were no differences in the growth parameters and P uptake betw een minimally and highly P-fertilized non-mycorrhizal treatments for e ither maize or sorghum. The increased total root length in drought-str essed mycorrhizal sorghum plants and the similar infected root lengths in unstressed and drought-stressed sorghum plants may have caused hig h C partitioning to drought-stressed mycorrhizal roots and therefore c aused the reduced growth parameters in mycorrhizal plants compared to the non-mycorrhizal P-fertilized counterparts. The results indicate th at P fertilization in addition to mycorrhizal inoculation may improve the drought tolerance of maize and sorghum plants.