Phosphate Uptake and Arbuscular Activity in Mycorrhizal Allium cepa L.: Effects of Photon Irradiance and Phosphate Nutrition

Abstract

The effect of photon irradiance on phosphate uptake and growth of onion plants was investigated in an experiment aimed at determining whether development and activity of mycorrhizal infection and arbuscules are linked to physiological effects. Mycorrhizal (inoculated with Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe) and non- mycorrhizal plants of Allium cepa. L. were grown at three soil phosphate levels and two photon irradiances (410 and 190 �mol m-2 s-1). There was one harvest at 6 weeks. Data were obtained for phosphate uptake (specific uptake rate), plant growth and mycorrhizal infection (total and active infection by trypan blue staining and succinate dehydrogenase (SDH) activity respectively). There was a positive growth response to mycorrhizal infection at the lower P levels (P0 and P1) but not at P2 with high irradiance. This response was smaller at low irradiance. Furthermore, data for root/shoot and fresh-weight/dry-weight ratios indicated that allocation of dry matter was altered in mycorrhizal plants in accord with their increased demand for photosynthate. Reduced irradiance was associated with lower rates of P uptake in all treatments, with the greatest effect in mycorrhizal plants at P0 and P1. These reductions were not associated with reductions in incidence or intensity of mycorrhizal infection nor in the development of arbuscules. At P2 the incidence of infection was reduced under low irradiance, but development of arbuscules within infected regions of the roots remained the same as in other treatments. The use of nitroblue tetrazolium chloride as a vital stain for SDH activity showed that during the 6-week period of the experiment, all the infection revealed by trypan blue staining was physiologically active. Branched arbuscules (counted in freehand sections of fresh roots) always had SDH and alkaline phosphatase activity. Thus, in young plants, observations of arbuscules gives a good indication of the amount of active infection. This may not be so in older roots.