Studies on growth depressions of mycorrhizal citrus under high phosphate concentrations

• Grant number: 93/03553-5
• Support Opportunities: Research Projects - Thematic Grants
• Start date: November 01, 1994
• End date: May 31, 1998
• Field of knowledge: Agronomical Sciences - Agronomy - Soil Science
• Principal Investigator: Elke Jurandy Bran Nogueira Cardoso
• Grantee: Elke Jurandy Bran Nogueira Cardoso
• Host Institution: Escola Superior de Agricultura Luiz de Queiroz (ESALQ). Universidade de São Paulo (USP). Piracicaba , SP, Brazil
• Associated research grant(s): 96/11504-2 - Raymond S. Pacovsky | United States Department of Agriculture - Estados Unidos, AV.EXT; 96/11903-4 - Michael a Lawton | the State University of New Jersey Rutgers - Estados Unidos, AV.EXT

Abstract

Citrus plants form a symbiosis with vesicular-arbuscular mycorrhizal (vam) fungi for the uptake of phos- phorus (p) in soils low in p since the morphology and architecture of the uncolonized root is inadequate for plant support. In p-rich soils, inoculation with vam fungi can result in a depression of growth, and the physiological mechanism behind this growth depression has still not been explained. The objective of this project is to determine the causes, at a physiological and molecular level, for this growth depression in mycorrhizal citrus plants when soil p is adequate. Four different hypotheses are considered: 1) toxin production, 2) alteration of photosynthesis and photoassimilate utilization, 3) alteration of p assimilation kinetics, and 4) alteration of the balance between external and internal fungal biomass. Toxin production is being determined by examining the intracellular fluid in mycorrhizal roots grown under high soil p and asaying if such fluid stimulates plant defense responses. An alteration of photosynthesis or photoassimilate partitioning is bering examined by determining rates of photosynthesis and transpiration by gas exchange, followed by measuring photoassimilatory enzymatic activities. The kinetic studies will determine p assim- ilatory parameters (km, Vmax) for vam roots utilizing various 32p-po4 solutions. Alterations of the external and internal mycelium of the vam fungus will be assayed, while the active extraradical mycelium will also be quantified using a vital stain. After determining the basis for this growth depression in citrus, it will be possible to make recommendations regarding the best management practices for superior productivity in mycorrhizal citrus, by specifying desirable p levels, fertilizer timing and mycorrhizal inoculum citrus rootstock compatibilities. In addition, this basic knowledge will permit new explanations for the balance between parasitism and mutualism in this symbiosis. (AU)