EFFECTS OF PHOTON IRRADIANCE ON THE GROWTH OF SHOOTS AND ROOTS, ON THE RATE OF INITIATION OF MYCORRHIZAL INFECTION AND ON THE GROWTH OF INFECTION UNITS IN TRIFOLIUM SUBTERRANEUM L.

Abstract

SUMMARYThe effects of photon irradiance on the growth of young plants (up to three weeks old) of Trifolium subterraneum L. and on the development of mycorrhizal root systems were studied with plants grown in a soil/sand mixture inoculated with Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe. Total plant growth was lower when phototi irradiance was 100 //mol m~'^ s~', compared with 450 /nnol m"^ s"'. Fresh weight of shoots was unaffected, but the fresh weight/dry weight ratio was increased at the lower irradiance in shoots, but not in roots. The main effect of decreased irradiance on the growth of the root system was a reduction in the numbers of first-and second-order lateral roots initiated. The average rate of extension of axial and lateral roots was only slightly reduced. The fraction of the root length infected was lower at the lower irradiance, particularly in the first two weeks. Development of infection was analyzed in terms of the role of formation of mycorrhizal entry-points (A) and the average rate of growth of infection units {B). Reduction in the fraction of the root length infected was found to be due to a reductioti in A. B was remarkably constant at the two irradiances and in the different root subsystems. Thus both roots atid infection units respond by makitig fewer units of unaltered rate of growth.

Effect of enhanced atmospheric CO2 on mycorrhizal colonization by Glomus mosseae in Plantago lanceolata and Trifolium repens

“…If this were not the case, we would expect a higher proportion of root length to be colonized by the fungus at elevated CO # , since more C is being sent below ground by the host plant. Indeed, mycorrhizal colonization rates are generally thought to be dependent on the carbohydrate status of the host plant ; for example, Tester et al (1986) have shown that, in Trifolium subterraneum, the mycorrhizal colonization rate was lower at lower irradiance (i.e. at lower C availability).…”

“…Obtaining time-course data, which allows the study of biomass partitioning between plant parts (i.e. allocation patterns), is therefore particularly important when studying mycorrhizas in the context of CO # concentrations, since the amount of fixed C going to the roots can play a role in mycorrhizal colonization (Tester et al, 1986). If data for single harvests are used, the only available measure of allocation pattern (between two plant parts) will be a ratio or fraction ; however, if there is a difference in total plant size between the two treatments (as it often is at different CO # levels), then such comparisons are meaningless.…”

Interactive effects of arbuscular mycorrhizal symbiosis, intraspecific competition and resource availability on Trifolium subterraneum cv. Mt. Barker

“…Our results do not seem to support this interpretation. If light was the only factor limiting the formation of mycorrhizas, a reduction in irradiance of 50%, which is enough to have a negative effect on mycorrhizal structures in T. subterraneum (Tester et al, 1985(Tester et al, , 1986, should result in a lower percentage infection. However, in the second experiment low light intensity did not modify infection.…”

“…plants at non-saturating irradiances of either 400 or 180 µmol m −# s −" (Hesketh & Moss (1963) cited in Tester et al (1985Tester et al ( , 1986), provided by incandescent and fluorescent lamps. The light intensity was reduced by placing a frame with a plastic mesh over the corresponding subplots.…”

“…We selected T. subterraneum as the host because the response of single plants of this species to mycorrhizal infection under different concentrations of P and light intensities has been studied in detail (Smith, 1982 ;Tester et al, 1985Tester et al, , 1986Nadian et al, 1996).…”