Transition from juvenility to maturity strengthens photosynthesis in sclerophyllous and deciduous but not in semi-deciduous Mediterranean shrubs

Abstract

Reproductive maturity was tested as a factor of photosynthetic differentiation in adults and juveniles of native Mediterranean species. An evergreen sclerophyllous shrub (Nerium oleander), a semi-deciduous shrub with leaf dimorphism (Phlomis fruticosa) and a winter deciduous tree with shrub-like habitat (Cercis siliquastrum) were used. In previous work, the adults of the above species appeared to create more effective leaves than juveniles in terms of photosynthesis during leaf development. Our purpose was to examine if those differences sustain after leaf maturation or change through seasons in response to the main environmental stresses of the Mediterranean climate. For this reason, the photosynthetic activity was recorded for two consecutive years in fully developed leaves. The selection of individuals was based on the minimum possible size difference between adults and juveniles growing in the field and on full exposure to solar irradiance. Differentiation of photosynthetic characteristics between adults and juveniles did not exhibit a common pattern but was dependent on the growth form and the survival strategy of the plant. Photosynthesis in adults of N. oleander was superior to juveniles, with higher CO2 assimilation and electron transport rate as well as carboxylation efficiency, while differences increased during the dry period when adults take advantage of their deeper root system. In the deciduous C. siliquastrum, photosynthetic activity was generally lower in juveniles, similarly to N. oleander, but differentiation was of lower magnitude. Increased need for photoprotection in juveniles of both species in summer was recorded. However, juveniles exhibited lower photosynthetic activity also in spring, most likely due to the co-existent reproductive process. Suppression of photosynthesis due to low temperature stress in winter, was more pronounced in juveniles of N. oleander affecting mainly the electron transport rate, carboxylation efficiency and mesophyll conductance. In P. fruticosa no notable difference between adults and juveniles was observed at any time of the year. It appears that the rich repertoire of mechanisms against the adverse environmental conditions in adults of P. fruticosa, is already present in juveniles in which the need for survival overcomes any differentiation associated with allocation and usage of photosynthates due to their immaturity.