Abstract
The present work evaluated the indicators of photosynthetic efficiency and antioxidative protection in cashew tree seedlings subjected to salinity stress. The study was conducted with seedlings of two advanced dwarf cashew clones (CCP09 and CCP76) subjected to salt stress with increasing doses of NaCl (0, control; 25; 50; 75; 100 mM) in the nutrient solution for 30 days under greenhouse conditions. The variables of gas exchange, CO2 assimilation (PN), stomatal conductance (gS), transpiration (E), intercellular CO2 concentration (CI), photochemical activity, potential quantum efficiency (Fv/Fm), effective quantum efficiency (ΔF/Fm’) of photosystem II (PSII), photochemical quenching (qP), non-photochemical quenching (NPQ) electron transport rate (ETR) as well as the indicators of damage and oxidative protection were measured. Under these conditions, there was an intense accumulation Na+ associated with a reduction in the K+/Na+ ratio in the leaves of both clones in response to salt, with higher values for this ratio in clone CCP09 than in CCP76 the highest concentration of NaCl (100 mM). Salinity reduced PN, gS and E in the two clones evaluated, with lower reductions in CCP09 than in CCP76 at the highest salt dose. Instantaneous carboxylation (PN/CI) and water use (PN/E) efficiencies were strongly restricted by salinity but were less affected in CCP09 than in CCP76. Salinity stress also increased hydrogen peroxide (H2O2) levels in CCP09, whereas lipid peroxidation decreased in both progenies. The clones presented specific antioxidant responses due to greater enzymatic and non-enzymatic activity in CCP76, in addition to the activity of phenol peroxidase (POX) in CCP09.
Highlights
Excess of salt in the soil solution causes metabolic disturbances in plants due to the osmotic and ionic effects of salinity, leading to reduced crop growth and productivity (Khan & Panda, 2008; Lima, Nobre, Gheyi, Soares, & Silva, 2014)
Ionic toxicity caused by salinity stress results from increased Na+/K+, Na+/Ca+2, Na+/Mg+2 and Cl-/NO3- ratios in plant tissue, causing cellular disorders related to the physiological function of these essential nutrients (Abbaspour, Kaiser, & Tyeman, 2014; Bessa, Lacerda, Amorim, Bezerra, & Lima 2016)
These authors affirmed that salinity stress decreases plant growth due to the energy consumption required for synthesis of osmotically active organic compounds, which are associated with osmotic adjustment processes and maintenance of water relations in plants
Summary
Excess of salt in the soil solution causes metabolic disturbances in plants due to the osmotic and ionic effects of salinity, leading to reduced crop growth and productivity (Khan & Panda, 2008; Lima, Nobre, Gheyi, Soares, & Silva, 2014). Damage to PSII can occur due to the photooxidation of structures such as pigments (chlorophylls) and proteins, the D1 protein, caused by the generation of excess reactive oxygen species (ROS) at PSII (Goh, Ko, Koh, Kim, & Bae, 2012) This damage affects PSII repair systems and can lead to chronic photoinhibition, causing non-stomatal photosynthetic limitation (Hussain et al, 2012). Genetic variability in relation to characters involved in salt resistance, such as ionic partitioning (Ponte et al, 2011) and oxidative protection (Ferreira-Silva et al, 2011), exists Despite these few reports in the literature, metabolic disturbances related to photosynthetic limitation in cashew plants cultivated under salinity conditions are not characterized. The effects of salinity on stomatal modulation and photochemical activity associated with the K+/Na+ ratio in leaf tissue are discussed
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