Quantification of water uptake by arbuscular mycorrhizal hyphae and its significance for leaf growth, water relations, and gas exchange of barley subjected to drought stress.

Arbuscular mycorrhizal fungi alleviate drought stress in their host plants via the direct uptake and transfer of water and nutrients through the fungal hyphae to the host plants. To quantify the contribution of the hyphae to plant water uptake, a new split-root hyphae system was designed and employed on barley grown in loamy soil inoculated with Glomus intraradices under well-watered and drought conditions in a growth chamber with a 14-h light period and a constant temperature (15 degrees C; day/night). Drought conditions were initiated 21 days after sowing, with a total of eight 7-day drying cycles applied. Leaf water relations, net photosynthesis rates, and stomatal conductance were measured at the end of each drying cycle. Plants were harvested 90 days after sowing. Compared to the control treatment, the leaf elongation rate and the dry weight of the shoots and roots were reduced in all plants under drought conditions. However, drought resistance was comparatively increased in the mycorrhizal host plants, which suffered smaller decreases in leaf elongation, net photosynthetic rate, stomatal conductance, and turgor pressure compared to the non-mycorrhizal plants. Quantification of the contribution of the arbuscular mycorrhizal hyphae to root water uptake showed that, compared to the non-mycorrhizal treatment, 4 % of water in the hyphal compartment was transferred to the root compartment through the arbuscular mycorrhizal hyphae under drought conditions. This indicates that there is indeed transport of water by the arbuscular mycorrhizal hyphae under drought conditions. Although only a small amount of water transport from the hyphal compartment was detected, the much higher hyphal density found in the root compartment than in the hyphal compartment suggests that a larger amount of water uptake by the arbuscular mycorrhizal hyphae may occur in the root compartment.

Effect of heavy metal contaminated shooting range soils on mycorrhizal colonization of roots and metal uptake by leek.

We grew leek (Allium porrum) in soils of two shooting ranges heavily contaminated with heavy metals in the towns of Zuchwil and Oberuzwil in Switzerland as a bioassay to test the activity of arbuscular mycorrhizal (AM) fungi in these soils. Soil samples were taken from (1) front of the shooting house (HOUSE), (2) the area between house and target (FIELD) and (3) the berm (BACKSTOP). Samples of Ribwort plantain (Plantago lanceolata) growing naturally within the shooting ranges were also collected and the colonization of its roots by mycorrhizal fungi was measured. The number of AM spores in the soils was significantly reduced concomitant with the increase in the degree of soil contamination with metals. In Zuchwil, mycorrhizal fungi equally colonized roots of Ribwort plantain sampled from BACKSTOP and HOUSE. In Oberuzwil, however, plants from BACKSTOP had lower colonization when compared with those sampled from HOUSE. Colonization of leek was strongly reduced in the BACKSTOP soil of Zuchwil and slightly reduced in the BACKSTOP soil of Oberuzwil when compared with plants grown in respective HOUSE soil. Concentrations of Cd, Cr, Cu, Ni, Pb and Zn in the leaves of leek grown in the BACKSTOP soil was within the range considered toxic for human consumption. This points to the high degree of bioavailability of these metal in these soils. Significant decrease in the number of mycorrhizal spores in the BACKSTOP soils in Zuchwil and the low colonization of leek roots grown in these soils point to possible changes in the species diversity of mycorrhizal fungi in these soils.

Diversity and structure of AMF communities as affected by tillage in a temperate soil.

Arbuscular mycorrhizal fungi (AMF) were studied in differently tilled soils from a long-term field experiment in Switzerland. Diversity and structure of AMF communities were surveyed either directly on spores isolated from the field soil or on spores isolated from trap cultures, planted with different host plants. Single-spore cultures were established from the AMF spores obtained from trap cultures. Identification of the AMF was made by observation of spore morphology and confirmed by sequencing of ITS rDNA. At least 17 recognised AMF species were identified in samples from field and/or trap cultures, belonging to five genera of AMF--Glomus, Gigaspora, Scutellospora, Acaulospora, and Entrophospora. Tillage had a significant influence on the sporulation of some species and non- Glomus AMF tended to be more abundant in the no-tilled soil. The community structure of AMF in the field soil was significantly affected by tillage treatment. However, no significant differences in AMF diversity were detected among different soil tillage treatments. AMF community composition in trap cultures was affected much more by the species of the trap plant than by the original tillage treatment of the field soil. The use of trap cultures for fungal diversity estimation in comparison with direct observation of field samples is discussed.

Arbuscular mycorrhiza infection enhances the growth response of Lolium perenne to elevated atmospheric pCO(2).

Elevated atmospheric pCO(2) increases the C-availability for plants and thus leads to a comparable increase in plant biomass production and nutrient demand. Arbuscular mycorrhizal fungi (AMF) are considered to play an important role in the nutrient uptake of plants as well as to be a significant C-sink. Therefore, an increased colonization of plant roots by AMF is expected under elevated atmospheric pCO(2). To test these hypotheses, Lolium perenne L. plants were grown from seeds in a growth chamber in pots containing a silica sand/soil mixture for 9 weeks with and without inoculation with Glomus intraradices (Schenck and Smith). The growth response of plants at two different levels of N fertilization (1.5 or 4.5 mM) combined with ambient (35 Pa) and elevated atmospheric pCO(2) (60 Pa) was compared. The inoculation with G. intraradices, the elevated atmospheric pCO(2) and the high N fertilization treatment all led to an increased plant biomass production of 16%, 20% and 49%, respectively. AMF colonization and high N fertilization increased the plant growth response to elevated atmospheric pCO(2); the plant growth response to high N fertilization was also increased by AMF colonization. The root/shoot ratio was reduced by high N fertilization or elevated atmospheric pCO(2), but was not affected by AMF colonization. The unchanged specific leaf area indicated that if AMF colonization represented an increased C-sink, this was fully covered by the plant. Elevated atmospheric pCO(2) strongly increased AMF colonization (60%) while the high N fertilization had a slightly negative effect. AMF colonization neither improved the N nor P nutrition status, but led to an improved total P uptake. The results underline the importance of AMF for the response of grassland ecosystems to elevated atmospheric pCO(2).

Decreasing the NO3 and increasing the vitamin C contents in spinach by a nitrogen deprivation method.

Excessive use of nitrogen fertilizers is known to increase the NO3 and reduce the vitamin C contents in fruits and vegetables. We investigated the concentration of these compounds in spinach leaves when plants were transferred to nitrogen-free media prior to their harvest. It was noted that a pre-harvest transfer of spinach to N-free media reduces the NO3 and increases the vitamin C content of the leaves by a substantial amount in a 2-3 day period. It is suggested that this technique may be suited to produce spinach or other leafy vegetables with low NO3 and high vitamin C contents under commercial hydroponic conditions.

Vesiculated Hairs: A Mechanism for Salt Tolerance in Atriplex halimus L.

The concentration of salts in the vesiculated hairs of Atriplex halimus L. was measured and was remarkably higher than that of the leaf sap and xylem exudate. In spite of their unusually high salt content, these hairs when immersed seemed unable to absorb water, in apparent contradiction to the previously held hypothesis that vesiculated hairs make it possible for such plants to absorb water from the atmosphere. Although growing the plants under saline conditions increased the salt content of the hairs from 2.3 m Na+K to 11.6 m, salt content of the expressed leaf sap from young leaves did not change significantly. This observation indicates that in A. halimus the vesiculated hairs play a significant role in removing salt from the remainder of the leaf and preventing the accumulation of toxic salts in the parenchyma and vascular tissues. Thus, a nearly constant salt content is maintained in leaf cells other than the hairs.