Extension of the phosphorus depletion zone in VA-mycorrhizal white clover in a calcareous soil

Abstract

To examine the influence of vesicular-arbuscular (VA) mycorrhizal fungi on phosphorus (P) depletion in the rhizosphere, mycorrhizal and non-mycorrhizal white clover (Trifolium repens L.) were grown for seven weeks in a sterilized calcareous soil in pots with three compartments, a central one for root growth and two outer ones for hyphae growth. Compartmentation was accomplished by a 30-/zm nylon net. The root compartment received a uniform level of P (50 mg kg -1 soil) in combination with low or high levels of P (50 or 150 mg kg -1 soil) in the hyphal compartments. Plants were inoculated with Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or remained uninfected.Mycorrhizal inoculation doubled P concentration in shoot and root, and increased dry weight, especially of the shoot, irrespective of P levels. Mycorrhizal contribution accounted for 76% of total P uptake at the low P level and 79% at the high P level, and almost all of this P was delivered by the hyphae from the outer compartment. In the non-mycorrhizal plants, the depletion of NaHCO 3-extractable P (Olsen-P) extended about 1 cm into the outer compartment, but in the mycorrhizal plants a uniform P depletion zone extended up to 11.7 cm (the length of the hyphal compartment) from the -3 root surface. In the outer compartment, the mycorrhizal hyphae length density was high (2.5-7 m cm soil) at the various distances (0-11.7 cm) from the root surface. Uptake rate of P by mycorrhizal hyphae was in the range of 3.3 -4.3 • 10 -15 mol s -1 cm -1

Plant growth and phosphorus uptake in mycorrhizal rangpur lime seedlings under different levels of phosphorus

“…The total EM increased with time and reached about 19 m g -1 in the substrate with +AMF plants at 90 DAT, in all P levels. These data are in agreement with previous works (Abbott et al, 1984;Schubert et al, 1987;Sylvia, 1988;Li et al, 1991), in which the total EM ranged from less than 1 to about 26 m g -1 .…”

“…It is notorious that mycorrhizal citrus have higher P concentrations than control plants (Peng et al, 1993;Melloni et al, 2000) but, in this case, only at 25 and 200 mg kg -1 P. In these cases, shoot P concentrations in +AMF plants were above the sufficiency level of 1.9 g kg -1 for citrus, whereas the -AMF plants remained below the deficiency level of 1 g kg -1 (Graham et al, 1997). The EM may be responsible for more than 75% of the plant P uptake (Li et al, 1991). Based on the specific P uptake rate it could be seen that similar values were achieved by roots of +AMF plants, at the two lowest P levels.…”

“…Total EM and active EM always had higher figures in the substrate with +AMF plants, at all P levels, along the experiment. Although some authors (Abbott et al, 1984;Sylvia, 1988;Li et al, 1991) suggested the subtraction of the mycelium estimated in -AMF treatments from their respective +AMF counterparts, both data were presented, because it is quite difficult to distinguish mycorrhizal mycelium from that from other filamentous fungi (Sylvia, 1988;Kabir et al, 1996;. In addition, interactions between AMF and saprophytic fungi are unknown, whether stimulating or inhibiting each other.…”

“…There are evidences that plant response to mycorrhiza cannot be explained solely by the internal colonization. The beneficial effect of mycorrhiza is mainly attributed to the fungal hyphae spreading up to 11 cm through the soil beyond the rhizosphere, which enables more efficient soil exploitation for nutrients (Li et al, 1991), being responsible for up to 80% of the P and 60% of the Cu uptaken by plant (Marschner & Dell, 1994).…”

Phosphorus efficiencies and responses of barley (Hordeum vulgare L.) to arbuscular mycorrhizal fungi grown in highly calcareous soil

“…The role of AMF in plant nutrition is likely to be important even when plants are genetically modified with other traits, such as root exudation of citrate (Lopez-Bucio et al 2000;Zhu et al 2001). The beneficial effects of AMF are due mainly to the ability of mycorrhizal fungal hyphae to acquire P well beyond the limits of the rhizosphere depletion zone (Li et al 1991). It has been widely reported that responsiveness to (or dependency on) mycorrhizal colonisation, in terms of improved growth and/or P uptake, varies between crop cultivars (Krishna et al 1985;Koide et al 1988;Bryla and Koide 1990;Rao et al 1990;Mercy et al 1990;Baon et al 1993;Hetrick et al 1993;Khalil et al 1994;Hetrick et al 1995).…”