Comparisons between P-fertilized and mycorrhizal plants

In experimentation with vesicularoarbuscular mycorrhizal (VAM) fungi, the availability of non-VAM control plants of equal size to VAM plants is a fundamental requirement. The purpose of this work was to determine nutrient regimes needed to achieve growth equivalence between VAM and non-VAM plants. Soybean [Glycine max (L.) Merr. ] cv. Amsoy 71 and sorghum [Sorghum bicolor (L.) Moench] cv. Bok 8 plants were grown under controlled conditions in a soil (Josephine silty clay loam, mesic Typic Haploxerult) low in plant-available P. Soybeans were inoculated with one of four species and sorghum with one of two species of VAM fungi. Non-inoculated control plants received nutrient solutions that contained 0.0, 0.2, 0.4, or 1.0 mM P. While the growth of P-supplemented controls may be equivalent to VAM plants, an important question remains: Are these plants also equivalent in terms of such functional parameters as leaf development, dry matter partitioning, and nutrient assimilation? The objective of this experiment was to answer these questions. The response to VAM colonization was similar in both hosts, although less extensive colonization was observed ~n sorghum. Dry weight, leaf area, and P content increased exponentially with nutrient solution P level. Plants colonized wiCh VAM fungi grew 3 to 6 times larger than the P-free controls but attained only 35 to 65% of maximum growth possible with high fertilizer P input. Host response to VAM colonization was equivalent to that of plants receiving between 0.12 and 0.22 mM P for phytomass, leaf area, and N content. Mycorrhizal plants contained less P, Mn, and root Fe but more Zn and Cu than comparable plants fertilized with P. It was concluded that P-treated, non-VAM plants differed physiologically and anatomically from VAM plants of equivalent size grown under P stress. It may therefore be necessary to establish the comparability of VAM plants and of "VAM-equivalent controls" separately for each plant parameter of interest. Even then, differential growth responses in VAM-host associations may prevent complete comparability between VAM and P-fertilized plants. Additional index words: Gloraus, Micronutrient, Phosphorus nutrition, Sorghum, Soybean. p exhibit a wide range of host responses to vesicular-arbuscular mycorrhizal (VAM) colonization under different environmental conditions (32). External conditions affecting P (3, 28) and bohydrate (7) availability to the symbiotic partners influence the magnitude of hostoplant~growth. The nutritional status the non-VAM plants used as controls affects the interpretation of mycorrhizal effects (31). In all comparisons of VAM and non-VAM plants, a fundamental problem is to obtain plants of similar size and development. Non-VAM plants supplied with additional P to compensate for enhanced P uptake by VAM fungi have been used experimentally in an attempt to obtain comparable controls (25, 26~ 31). While apparent growth of such controls may be equivalent to VAM plants, an important question remains: Are these plants also equivalent in terms of such functional parameters as leaf development, dry matter partitioning, and nutrient assimilation? The objectives of this study were: i) to measure the growth response of soybean [Glycine max (L.) Merr. ] and sorghum [Sorghum bicolor (L.) Moench] to P fertilization or VAM colonization, ii) to determine the input needed by non-VAM plants to produce growth equivalent to that of VAM plants, and iii) to compare some morphological characteristics and nutrient content of VAM plants with those of non-VAM plants. MATERIALS AND METHODS Biological Materials Soybean (cv. Amsoy 71) plants were inoculated with one of four species of VAM fungi or remained non-inoculated and received one of four P-amended nutrient solutions. Three species of VAM fungi, Glomus epigaeum Daniels and Trappe (12), Glo~nus ~nosseae (Nicol. and Gerd.) Gerd. and Trappe (14), and an undescribed Glomus pecies, used in the study were collected from the Anzo-Borrego Desert State Park, California (6). The fourth species, the Gerdemann isolate of Glomusfasciculatum (Thaxter sensu Gerd.) Gerd. and Trappe was obtained from Abbott Laboratories (Long Grove, IL 60047) s. All fungi were cultured for 6 months on sorghum (cv. G766) grown in a sand/perlite (2/ 1,v/v) medium. Spores of each species were collected by wet sieving. Approximately 100 + 7 spores of each VAM isolate were used as inoculum for plants grown in this experiment. Soybean seeds (0.5 to 0.6 g) were surface sterilized successively in 0.7 L L-~ ethanol (1 min), 1.0 g -~ HgCI~ (10 rain), and 0.01 N HCI (10 min); rinsed with sterile, distilled water; and germinated for 2 days at 27°C. Three seedlings were planted in 1.5 L pots and were initially treated with a leachate of the original VAM cultures free of VAM propagules to establish similar biota in VAM and control treatments. The two smallest plants per pot were removed after 1 week. Each treatment was replicated six times in a randomized-block design. Plants were harvested after 9 weeks. Sorghum (cv. Bok 8) was tested for response to G. fasciculatum and G. ~nosseae only. For each pot, six seeds (0.15 to 0.25 g) were surface sterilized using the procedure described for soybean, planted, and thinned to three plants per pot after 1 week. Each treatment was replicated five times in a randomized-block design. Plants were grown for 9 weeks.