Plant age-related changes in cytokinins, leaf growth and pigment accumulation in juvenile mastic trees

Abstract

Despite plant age is a key factor to understand the performance and survival of any given species in the field, little is known about its influence on the endogenous levels of cytokinins in juvenile individuals of perennial plants. In this study, we hypothesized that plant age in juvenile mastic trees may affect cytokinin levels and lead to significant changes in cytokinin-related processes, such as leaf growth and pigment accumulation. Two groups of juvenile plants aged 2 and 5 years old were grown under Mediterranean field conditions to evaluate the age-related differences in the endogenous levels of cytokinins and auxins, leaf biomass and area, pigments levels and the efficiency of PSII photochemistry (Fv/Fm ratio). Furthermore, regression analyses between cytokinin levels and several indicators of stress performance were carried out in a population of juvenile mastic trees. Results showed that endogenous levels of cytokinins, particularly 2-isopentenyladenine (2-iP), increased during periods of intense leaf growth. However, plant age did not affect 2-iP levels, but those of trans-zeatin (Z); 2-year-old plants showing 2-fold higher levels of Z compared to 5-year-old plants, the latter having 11-fold higher total plant biomass. Plant age-related reductions in cytokinins were not associated with reductions in leaf growth in the largest individuals, but with an increased chlorophyll (Chl) a/b ratio. Further regression analyses revealed that although enhanced Chl a/b ratios predispose leaves to suffer photooxidative stress, as indicated by increased anthocyanins levels and lowered Fv/Fm ratios in juvenile mastic trees, individuals of different ages showed a similar degree of stress tolerance. It is concluded that plant age may alter the endogenous composition of cytokinins and the pattern of pigment accumulation in leaves, but this does not reduce leaf growth or stress tolerance in this species.