Nutrient Status and Mycorrhizal Enhancement of Water Transport in Soybean

Abstract

MATERIALS AND METHODSMycorrhizal soybean (Glycine max L. Merr. var. plants have lower resistances to water transport than nonmycorrhizal plants after 4.5 weeks of growth. Although resistances of whole plants differ by 40 %, there were no differences in the resistances of stems plus leaves, indicating that the major effect of the mycorrhizae was to reduce the resistance of the roots. Since the fungitoxicant, p-chloronitrobenzene, had no effect on resistances to water transport, reduced resistances were probably not caused by a direct modification of the transport pathway by the fungus. Differences in resistance between mycorrhizal and nonmycorrhizal soybean were essentially eliminated by the application of nutrients to the soil. Thus, lowered resistances of mycorrhizal roots growing in soil with low levels of nutrients probably resulted from the enhanced nutrient status of the plant brought about by the fungus. Mycorrhizal infection increased growth at both low and high nutrient levels.The vesicular-arbuscular mycorrhizal relationship, one of the most common forms of symbiosis, increases the growth of many host plant species (1-3, 10, 15, 16, 20, 25, 29) including soybean (30, 31). The increased growth has been attributed to an enhanced nutrient uptake by the host plant (2,12,15,(18)(19)(20)26). Mycorrhizal plants take up more phosphorus than nonmycorrhizal controls when relatively unavailable sources of phosphorus are supplied to the roots (12,26). In most cases the addition of more readily available phosphorus eliminates differences in growth and phosphorus uptake (3,12,13,20,26).We have recently shown that VA3 mycorrhizae also decrease the resistance to water transport in whole soybean plants (31). The present study was undertaken to determine where the fungus acts and whether the lowered resistance to water transport can be attributed to increases in the nutrient status of the host brought about by the mycorrhizae.'This work was supported in part by The method used for determining resistance to water transport was that of Boyer (5, 6), which has previously given results that compare well with independent methods (6, 31) and provides consistent determinations of whole plant resistances in mycorrhizal and nonmycorrhizal soybean (31). The method involves measurement of the recovery in water potential of a moderately water-deficient leaf on an intact plant. The time required for recovery, which is determined when no transpiration is occurring, is dependent on the resistance to water transport.For measurement of the recovery of a whole plant, the blade of an intact soybean leaf which was moderately water deficient was sealed in a thermocouple psychrometer chamber that measures the water potential of intact leaves (4). The soil and root system were then submerged in degassed water, excess water was drained away, and the recovery in water potential of the leaf was recorded. After determining that the data conformed to the transfer equation for a plane sheet (5, 9, 1 1), the resistance of the whole plant was calc...

The Effect of Removal of Extraradical Hyphae on Water Uptake by Vesicular‐arbuscular Mycorrhizal Plants

“…Comparisons of root hydraulic conductivities both in P-deficient and P-fertilized non-mycorrhizal and VAM plants by Safir, Boyer & Gerdemann (1972) and Graham & Syvertsen (1984) led to the conclusion that it was enhanced P status of VAM plants which accounted for greater root conductivities. However, the present study emphasizes the importance of extraradical hyphae in water uptake and it is suggested that the reported eliminations of differences in VAM and non-mycorrhizal root water uptake with increased P fertilization may have been due in part to reductions in the quantity of extraradical hyphae which occur under conditions of high P availability (Bethlenfalvay, Bayne & Pacovsky, 1983).…”

Hongo endomicorrízico y bacteria fijadora de nitrógeno inoculadas a Coffea arabica en vivero.

“…en el caso de Leucaena sembrada en la costa de chiapas, también se obtuvieron resultados semejantes (aguirre-medina y Velazco 1994). Safir et al (1972) señalan que los hongos micorrízicos incrementan las concentraciones de n y ca, y ames (1987) demostró el transporte del nitrógeno por las hifas de las micorrizas.…”

COMPARATIVE WATER RELATIONS AND PHOTOSYNTHESIS OF MYCORRHIZAL AND NON‐MYCORRHIZAL BOUTELOUA GRACILIS H.B.K. LAG EX STEUD.

“…Daft and Okusamya (1973) reported an increased number of vascular bundles in maize following mycorrhizal infection which could reduce resistance to water transport. Safir et al (1972) estimated that root resistance to water uptake was reduced 40% by mycorrhizal infection in soybean. They also noted that phosphate fertilizers reduced root resistance and attributed the improved water transport to improved nutritional status, especially phosphorus.…”

“…Christensen, 1977; Allen, personal observations). Infection with VA mycorrhizae has been shown to enhance water status in soybean (Safir, Boyer and Gerdemann, 1972), onion (Mosse and Hayman, 1971), citrus (Levy and Krikun, 1980) and to improve survival of Atriplex canescens in arid mine spoils (Aldon, 1975). However, only a few investigators have considered possible physiological mechanisms which could contribute to these effects.…”

“…Thus, whole-plant resistance (i?^) was reduced by mycorrhizal infection. Assuming that Rj, approaches the inverse ofthe hydraulic conductivity at high transpiration rates (Fiscus and Markhardt, 1979), B. gracilis had fairly high whole-plant resistances when compared with crop plants (Boyer, 1971;Safir et al, 1972;Fiscus and Markhardt, 1979). However, as reported previously by Safir et al (1972), whole-plant resistances dropped sharply with mycorrhizal infection.…”