BLACK, KEVIN, DEREK MITCHELL & BRUCE OSBORNE.
Department of Botany, University College Dublin, Dublin 4, IRELAND.
Previous studies on arbuscular mycorrhizal (AM) plants have shown that increased photosynthetic rates may be related to an enhanced `sink' demand. This sink-effect could be due to the drain of carbon (C) from the host to the AM fungus. Most leaf models of non-mycorrhizal plants that implicate a sink-related control of photosynthesis, propose a central regulatory role for phosphorus (P). In order to investigate this possibility in mycorrhizal plants, the influence of additional externally-applied P and AM colonization on the host photosynthetic performance and C allocation of cucumber was investigated. Experiments on plants supplied with 130 µM P showed a significant increase in growth, leaf area, net photosynthesis, electron transport capacity and foliar P concentration in +AM, compared to non-mycorrhizal (-AM) plants. Measurements of C allocation were carried out using C-14 labelling studies on +AM and -AM plants supplied with 130 µM P. Additional controls (-AM+P) were grown in extra P (190 µM P) to produce plants with foliar P concentrations similar to those of +AM plants at 130 µM P. Data obtained from HPLC and inorganic phosphate (Pi) analyses on leaves indicate an inverse relationship between starch and Pi concentration. There was also a shift in C allocation from starch to sucrose in +AM and -AM+P plants in comparison to -AM controls, indicating that this is specifically a consequence of the improved P status of cucumber.