Accumulation and localization of aluminium in root tips of loblolly pine seedlings and the associated ectomycorrhiza Pisolithus tinctorius

ABSTRACT Evidence from past studies suggests that loblolly pine may be tolerant of Al. The experiments described in this manuscript were initiated to examine Al tolerance and Al accumulation in the pine root and the degree of Al accumulation in fungal hyphae when pine roots were colonized with the ectomycorrhiza Pisolithus tinctorius. The experiments used lumogallion staining and confocal microscopy to localize Al in root and fungal structures. The results clearly showed that loblolly pine seedlings were highly resistant to Al. A decrease in primary root extension could not be detected until Al+3 activities approached 40 µmol L−1, and extension was suppressed only 30% at an Al+3 activity of 580 µmol L−1. This contrasted with the response of the Al‐sensitive ‘check’ species soybean, where primary root extension was severely restricted at Al+3 activities lower than 5 µmol L−1. Tissue Al measurements and lumogallion fluorescence of longitudinal sections of the pine root tip indicated that tolerance was associated with both Al exclusion from the tip region and compartmentalization of absorbed Al in peripheral cell areas outside of the meristem. In lateral roots colonized with ectomycorrhizae, lumogallion fluorescence showed that large amounts of Al accumulated at the fungal mantle and in areas with the Hartig net. At higher magnification, lumogallion indicated substantial Al accumulation inside hyphae. Little Al could be detected in lateral root cells. The results show that pine possesses multiple mechanisms that can contribute to Al tolerance in acid field soils.