SUMMARYSix citrus cultivars were grown with and without the mycorrhizal fungus Glomus fasciculatus under three fertilizer regimes all without phosphorus. The average mycorrhizal dependency (Gerdemann 1975) of Rough lemon and Brazilian sour orange were greater than the mycorrhizal dependecies of Alemow, Troyer citrange, Bessie sweet orange and Trifoliate orange. The mycorrhizal dependency of each cultivar, except Bessie sweet and Trifoliate orange, was substantially altered by at least one of the fertilizer regimes, and therefore the order of mycorrhizal dependency was different at all three fertilizer regimes. On the average, citrus rootstocks exhibited the greatest mycorrhizal dependency with the least fertilization. The average percentage phosphorus in non-mycorrhizal leaf tissues was inversely correlated with the mycorrhizal dependency of citrus cultivars at the medium fertilizer regime. An inverse correlation was observed between the dry weights of non-mycorrhizal roots of the citrus cultivars and the mycorrhizal dependency of the citrus cultivars.
The interaction between P fertilization of citrus and the mycorrhizal fungus Glomus fasciculatus was examined in a greenhouse study. A low fertility loamy sand (4.6 ppm P) was provided with all necessary soil nutrients except P. This soil received superphosphate [Ca(H2PO4)2 · H2O] at rates of 0, 6, 28, 56, 278, and 556 ppm of P. Seven replicate mycorrhizal and nonmycorrhizal Brazilian sour orange and Troyer citrange seedlings were grown at each P fertility level. After 5 months, mycorrhizal sour orange which received 0 fertilizer P were similar in size to non‐mycorrhizal sour orange which were fertilized with 278 ppm P. Mycorrhizal citrange which received 0 fertilizer P were similar in size to nonmycorrhizal citrange which were fertilized with 56 ppm P. Dry weights of mycorrhizal sour orange seedlings were significantly greater than nonmycorrhizal sour orange seedlings when fertilized with 0, 6, 28, and 56 ppm P (947, 1089, 347, and 253% greater, respectively). Dry weights of mycorrhizal citrange seedlings were significantly greater than nonmycorrhizal citrange seedlings only when fertilized with 0 and 6 ppm P (746 and 300% greater, respectively). The %P in leaves of both citrus cultivars was enhanced by the mycorrhizal association at all P fertility levels. Variation in absorption of K, Mg, and Na by the citrus cultivars appeared to be primarily influenced by P concentrations of the seedlings. Variation in absorption of Zn, Cu, and Mn appeared to be influenced by both P concentrations of the seedlings and the presence of the mycorrhizal fungus. The number of G. fasciculatus spores/cm3 of soil which were associated with inoculated plants decreased with increasing P fertility levels. With mycorrhizal sour orange, numbers of spores decreased from 10.1 spores/cm3 soil at 56 ppm P soil to 0 spores/cm3 soil with plants at 556 ppm P. With mycorrhizal citrange, numbers of spores decreased from 5.7 spores/cm3 soil at 28 ppm P to 0 spores/cm3 soil at 56 ppm P. No correlation was found between soil P or % P in leaves, roots or stems of seedlings and the elimination of spore production.
The effects of plant nutrients on citrus fruit quality cannot be considered independently of their effects on yield. In some cases quality can be improved by sacrificing some yield;however, from the overall economic point of view, it is usually advantageous to sustain maximum fruit yield even though there may be some sacrifice in fruit quality. This report emphasizes the nutrient effects on quality in the ranges in which we expect maximum yield to be sustained. If the deficient ranges for yield are included, the degree of effects on quality is greater. The nutrient ranges and effects discussed are primarily those encountered by the authors under California conditions.
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