Morpho-anatomical, physiological and biochemical adaptive responses to saline water of Bougainvillea spectabilis Willd. trained to different canopy shapes

Bougainvillea is widely used as potential ornamental shrub in revegetation and landscaping projects as well as in urban landscape design in Mediterranean areas characterized by saline and sodic soils. The current research aimed at elucidating the morphological, physiological and biochemical changes occurring in Bougainvillea spectabilis 'Alexandra' grown under salt stress conditions (1 mM [S1, electrical conductivity EC = 2.0 dS m−1] non-salt control, 40 mM [S40, EC = 6.5 dS m−1] and 80 mM [S80, EC = 11.0 dS m−1] NaCl). The response of plants trained to two canopy shapes (globe [GSh] and pyramid [PSh]) has been compared in order to unravel possible adaptive mechanisms, combining morpho-functional traits at different levels, to cope with saline water. PSh plants under moderate saline water (40 mM NaCl) showed higher leaf dry weight, leaf number and area as well as higher number of flowers per plant than those observed in GSh Bougainvillea plants. Saline water-triggered leaf anatomical changes were also harmonized to physiological adjustments in both canopy shapes in such a way to counteract the reduction in transpiration due to stomatal limitations by adopting traits improving CO2 diffusion in the mesophyll and investing more resources in the assimilatory tissue (e.g. increase in intercellular spaces and incidence of palisade parenchyma over the total mesophyll thickness). Bougainvillea plants enact many-sided strategies to acclimate to S80 salinity by harmonizing several mechanisms including sodium and chloride inclusion in leaves and their compartmentalization in vacuoles, retention of high levels of potassium in the cytosol to osmotically balance the cytoplasm and other organelles within the vacuole, modulation of leaf functional anatomical traits, in addition to a C2 Kranz mechanism of photosynthesis which allow plants to successfully cope with stress.