The Rare-earth Metallome of Pecan and Other Carya

Abstract

We report the composition of the rare-earth (REE) metallome component of the foliar ionomes of pecan (Carya illinoinensis) and other North American Carya and how accumulation of specific REEs relate to ploidy level and to accumulation of essential divalent nutrient elements. REE accumulation within the foliar ionomes of 12 Carya species, growing on a common site and soil, indicates that REEs accumulate according to the Oddo-Harkins rule with Ce, La, Nd, and Y (Ce > La > Nd > Y) being the dominant REEs with accumulated concentration typically being La > Ce > Nd > Y > Gd > Pr > Sm > Dy > Er > Yb > Ho > Tb >Tm > Sc >Lu. Carya species quantitatively differ in accumulation of REEs with all but C. aquatica accumulating at much greater concentrations than non-Carya tree species and with tetraploid Carya accumulating to approximately twice the concentration as diploid Carya. Carya tomentosa was an especially heavy accumulator of REEs at 859 μg·g−1 dry weight, whereas C. aquatica was especially light at 84 μg·g−1. Accumulation of REEs was such that any one element within this elemental class was tightly linked (generally r ≥ 0.94, but 0.81 for Ce) to all others. Accumulation of REEs is negatively correlated with Ca accumulation and positively correlated with Mn and Cu accumulation in diploid Carya. In tetraploid Carya, accumulated Mg, Ca, and Fe is positively associated with foliar concentration of REEs. Total concentration of REEs in pecan's foliar ionome was 190 μg·g−1, about equivalent to that of Mn. Circumstantial evidence suggests that one or more of the physiochemically similar REEs increases physiological plasticity and subsequent adaptive fitness to certain Carya species, especially tetraploids. Because all tetraploid Carya are high REE accumulators and are native to more xeric habitats than diploids, which typically occupy mesic habitats, it appears that REEs might play a role in Carya speciation and adaptation to certain site-limiting environmental stresses. REEs appear to play an unknown metabolic/physiological role in pecan and most Carya species, especially tetraploids; thus, their nutritional physiology merits further investigation.