FIELD EVIDENCE FOR THE CARBON COST OF CITRUS MYCORRHIZAS

Mycorrhizas can produce negative crop responses when phosphorus availa bility is sufficient in agricultural soils because the fungi are of no benefit in nutrient acquisition yet continue to colonize roots and in voke parasitic costs. Benomyl fungicide was used to test this predicti on in the field by limiting mycorrhizal colonization of 2-yr-old Valen cia orange trees (Citrus sinensis (L.) Osbeck) on four rootstocks of v arying mycorrhizal dependency in P-deficient soil fertilized with and without phosphate. No known fungal pathogens of citrus roots controlle d by benomyl were present on the trees or in the field soil. Young tre es with or without P fertilization and benomyl treatment remained suff icient in P (greater than or equal to 0.10% leaf P) throughout the 27- month study. Root zone drenches of benomyl reduced mycorrhizal coloniz ation and leaf P status of Valencia orange trees on the three slower-g rowing rootstocks, trifoliate orange (Poncirus trifoliata (L.) Raf.), Swingle citrumelo (Citrus paradisi Macf. x P. trifoliata) and sour ora nge (Citrus aurantium L.), for the duration of three growing seasons. Benomyl affected root colonization and P status of trees on the faster -growing rootstock, Volkamer lemon (Citrus volkameriana Tan. and Pasq. ), less than for trees on the slower-growing rootstocks and the effect s were sustained for only two seasons. The shorter duration of benomyl effect for trees on Volkamer lemon rootstock compared with the slower -growing rootstocks was explained by the loss of inhibition of mycorrh izal activity when roots grew out of the drench zone and mycorrhizas w ere no longer in direct contact with the fungicide. Benomyl treatment increased growth rate of Valencia orange on the slow-growing rootstock s from 5 to 17 % after three seasons, and from 2 to 9 % on Volkamer le mon rootstock after two seasons compared with the non-benomyl treated trees. The benomyl effect was attributed to reduction of costs of root colonization over time, and consequently, a greater availability of c arbon assimilate for shoot growth of trees. Since mycorrhizal fungi ar e ubiquitous in fertilized agricultural soils and obligate biotrophs o n the roots of most crop species, these results indicate a need to fur ther investigate whether negative growth responses of P-sufficient pla nts in the field occur because mycorrhizal fungi are no longer behavin g as mutualists.