Morpho-anatomical adaptations of Callistemon citrinus to CaCl2 and NaCl solutions

Abiotic stress such as salinity is the major abiotic stress that reduce plant growth worldwide, especially in semi-arid regions. Ornamental plants are classified as tolerant species due to their ability to adapt under salt stress conditions. However, most studies concerning the salt tolerance of ornamental species have been based on studies in which NaCl is the predominant salt, whereas information is lacking concerning the CaCl2 effects on morphological, physiological and anatomical adaptations of ornamentals. The aim of this study was to assess the adaptation/tolerance of green house grown Callistemon citrinus an important potted flowering shrub in terms of plant growth, leaf water potential, leaf gas exchange, leaf anatomy and mineral composition, to two chloride salts NaCl and CaCl2. The experiment was performed in terms of equimolar concentrations in order to assess the ion effects of the two salinity sources. Plant growth parameters such as leaf area, leaf number, shoot and root dry weight were negatively affected by salinity, with more pronounced effects recorded under CaCl2 irrigation. The growth reduction was related to a significant decrease of the photosynthetic rate and stomatal conductance. A reduction of stomata size and palisade tissue size was also observed under NaCl irrigation. The differential responses of Callistemon citrinus plants to water salinity by CaCl2 or NaCl elicited several considerations. Other than the ion imbalance, in the CaCl2 treatment ion toxicity might have played a major role on the growth and Pn rate, since the amount of Cl ions accumulated in the leaves of these plants was doubled compared to NaCl treated ones. The adaptation mechanisms developed by the plant to cope with the salinity stress conditions are more effective under NaCl, such as the increased density of glands and trichomes.