A reassessment of the prevalent organic solutes constitutively accumulated and potentially involved in osmotic adjustment in pear leaves

In higher plants, osmotic adjustment at the various levels of plant organization is partly achieved through accumulation of a range of osmolytes especially LMW organic solutes often termed as osmotic solutes. A metabolite profiling of crude extracts of mature pear leaves of a range of 8 Pyrus genotypes was performed using current HPLC, UPLC and ¹H NMR spectroscopy techniques in order to identify such putative compounds. Using as variables the concentrations of 45 identified substances and those of a restricted number of unknowns, all belonging to LMW carbohydrates, polyols, organic acids, amino acids and phenolics on the one hand, and the varieties investigated as individuals on the other, we generated a set of data analyzed further by PCA. Those varieties were discriminated into three clusters respectively comprised of the four Asian varieties, the European variety Williams grafted onto 4 different rootstocks, and the two other European varieties Conference and Angelys. These metabolic phenotypes were shown to rely more on scion genotypes than on rootstocks. High to very high amounts of sorbitol (average content of 363 μmol g^?1 DW) associated with low amounts of mannitol and myo-inositol were found in all genotypes as well as in a local ecotype of P. communis where the hexitol accounted for 7.3% DW. Sorbitol actually represented up to 30-40% of the total osmotically active organic solutes accumulated in the set of pear leaves investigated, and it was shown to be significantly more abundant in the variety Williams than in Asian ones (p < 0.01). In contrast, the other well-known compatible solute glycine betaine, barely detectable using ¹H NMR spectroscopy or HPLC, occurred in leaves of all pear varieties at weak levels lower than 2 μmol g^?1 DW which suggested a minor role in osmotic adjustment. Its amount does not seem to be altered in response to an osmotic upshift applied to detached leaves or to depend on exogenously supplied ABA. For non-sustantiated reasons, these results are in contrast with those showing elsewhere very high accumulation of GB in the Asian genotype Su li. In this study mature leaves of this genotype collected from the same tree in July 2007 and July 2006 were shown to contain respectively 0.59 ± 0.04 and 0.85 ± 0.04 μmol g^?1 DW. Other abundant organic substances like arbutine, quinic acid, malic acid, sucrose as well as chlorogenic acid and other quinic acid adducts, might also behave as osmotically active substances. In addition to arbutine, its derivative hydroquinone, chlorogenic acid and structurally related substances might be involved in protective functions against secondary oxidative stresses induced by abiotic and biotic stresses encountered during the growing season.