Leaf metabolic response to water deficit in Pinus pinaster Ait. relies upon ontogeny and genotype

Pinus pimaster Aiton displays marked heteroblasty during its vegetative phase, showing complex interactions between developmental processes and environmental adaptation, with a patent differentiation in functional performance according to leaf type resulting from heteroblasty. In this work, untargeted quantitative metabolic profiling in adult and juvenile needles of four water-deficit stressed P. pinaster genotypes was investigated to know if metabolic responses to drought are genotype-dependent and vary according to the stage of needle ontogeny. Among the changes in metabolome, it highlights the increase of osmotically active substances, as well as the overproduction of antioxidant compounds by up-regulating ascorbate, shikimate/phenylalanine and phenylpropanoid metabolic pathways. Moreover, significant inter-clonal quantitative variations were also found at different levels, in both primary and secondary metabolism pathways, although terpenoids and specially flavonoids showed the highest significance. A surprising result was the ontogenetic changes in metabolic profiles. Juvenile needles contain significant higher levels of UV screener and antioxidant flavonoids. The level of drought tolerance of a genotype and the ontogenetic stage were important factors in determining the drought-induced level of flavonoid increase in needles of a given genotype. Regarding terpenoids, the significant drought-induced accumulation of neutral diterpenes in juvenile needles, and mono- and sesquiterpenes in adults, was genotype dependent regardless its degree of drought tolerance. These results point to flavonoids as very versatile metabolites in maritime pine needle, acting like important compounds modulating not only drought acclimation but also changes in needle metabolism associated with developmental processes such as heteroblasty.