WATER RELATIONS OF MYCORRHIZAL AND PHOSPHORUS-FERTILIZED NON-MYCORRHIZAL CITRUS UNDER DROUGHT STRESS

Rootstock seedlings of Carrizo citrange (Poncirus trifoliata (L.) Raf. x C. Sinensis (L.) Osbeck) and sour orange (Citrus aurontium L.) were grown in a sandy soil low in phosphorus (P) and either inoculated with Glomus intraradices Schenck & Smith (vesicular-arbuscular mycorrhizal; VAM) or fertilized with soluble P (non-mycorrhizal; NM). Five-month-old VAM and NM seedlings of each rootstock were comparable in size, P sufficiency, and relative growth rate whether they were kept well-watered or subjected to two drought-stress cycles of short duration. Under well-watered conditions, whole plant transpiration, leaf water status, and root hydraulic conductivity were similar for VAM and NM plants of each rootstock. During drought-stress and recovery periods, VAM plants also had very comparable whole-plant transpiration rates and leaf water potentials to NM plants, but mycorrhiza reduced root hydraulic conductivity of Carrizo citrange and sour orange 66 and 49%, respectively. These data do not support the hypothesis that mycorrhiza significantly enhance water relations of citrus under the drought-stress conditions studied.