Terence P. McGonigle

Co-occurrence of Arum- and Paris-type morphologies of arbuscular mycorrhizae in cucumber and tomato

Colonization by arbuscular mycorrhizal (AM) fungi was investigated in cucumber (Cucumis sativus), tomato (Lycopersicon esculentum) and Clethra barbinervis (Ericales) grown in field-collected soil known from previous studies to generate Paris-type arbuscular mycorrhizae in C. barbinervis. Spores of Paraglomus, Acaulospora, Glomus, and Gigaspora were found in the soil. Formation of hyphal coils and arbusculate coils of Paris-type mycorrhizae and of arbuscules of Arum-type mycorrhizae in roots raised in this soil in the growth chamber were compared with the detection of DNA of AM fungi from the same root systems using Glomales-specific primers. Only Paris-type mycorrhizae with extensive arbusculate coils developed in C. barbinervis, but cucumber and tomato developed both Paris- and Arum-types in the same root systems. Glomaceae and Archaeosporaceae and/or Paraglomaceae were detected strongly in the DNA from both cucumber and tomato roots, in which Arum-type mycorrhizae were observed. In contrast, DNA of Glomaceae was detected more sparingly in C. barbinervis, in which Paris-type mycorrhizae dominated. Acaulosporaceae and Gigasporaceae were strongly detected in the DNA from both C. barbinervis and tomato, whereas they were more weakly detected in cucumber. These results indicate that the morphology of colonization is strongly influenced by the selection of fungi to colonize the host plant from among those in the soil environment.

Soil micro-habitat effects on fine roots of Chamaecyparis obtusa Endl. : a field experiment using root ingrowth cores

Ingrowth cores in the field were used to compare fine root characteristics of hinoki cypress (Chamaecyparis obtusa) among rooting substrate in the form of needle leaf litter, decomposing organic material, and mineral soil. Fine root growth, morphology, arbuscular mycorrhizal (AM) associations, and tissue C and N concentration were determined. The inorganic N leaching from each soil substrate was taken as a measure of N availability. Although there was no significant difference in total N leaching among substrates, more NH+4-N leached from the decomposing organic material than other substrates. Rapid fine root production was observed in the organic material, whereas root production in the litter substrate was suppressed. Annual net fine root productions in litter, organic material, and mineral soil were 51, 193, and 132 g m−2, respectively. In the leaf litter substrate, AM colonization was suppressed and specific root length was higher than in the other substrates, indicating severe nutrient limitation in the litter. These responses of hinoki cypress roots seemed to be a soil exploitation pattern whereby absorptive fine roots were arranged to maximize nutrient acquisition.

Titanium as an indicator of residual soil on arid-land plants.

Titanium (Ti) has been suggested as a soil contamination indicator for plant samples slated for trace element analysis because it is abundant in soil but not in plants. Based on results from our survey of regional soils and plants for cesium (Cs), we sought to confirm Ti as a valid soil contamination indicator reasoning that there are three sources of Ti associated with plant samples: (i) contamination during laboratory processing, (ii) vascular uptake via roots, and (iii) field soil residue on shoot surfaces. Our experiments showed that: (i) milling increased Ti by 4 mg.kg(-1), and Ti in reagents and on labware added another 5 to 6 mg.kg(-1); (ii) Ti in Crepis acuminata seedling shoots attributable to root uptake averaged 5 mg.kg(-1); (iii) soil-dusted seedlings showed elevenfold and eightfold increases in Ti and Cs, respectively. Further, investigation of shoot washing methods determined that (iv) none of seven washing agents removed all soil from any of two to seven plant species, and (v) Artemisia tridentata and Phlox hoodii specimens washed with water retained particles and displayed elemental signatures consistent with adhering soil. We conclude that Ti is a valid soil contamination indicator for arid-land plant samples, and that trace, soil-borne analytes measured in samples where Ti values are high and, hence, soil contaminated, should be described as plant associated. Furthermore, we give guidance on minimizing Ti contamination of samples during laboratory processing and on use of Ti together with washing to minimize yet gauge soil contamination during trace element analysis.