Effect of salt stress (NaCl) on whole plant CO₂-gas exchange in mango

Saline tolerance of mango rootstocks is, or will be, of major importance in many mango growing areas. In four experiments, different, Mangifera species (M. indica L. cv. '13-1' and 'Turpentine', M. zeylanica Hooker f.) were characterized in a controlled environment. During increasing levels of NaCl salinity (15 to 120 mM) physiological description data of whole plant CO ₂ gas exchange (CO₂ assimilation A, dark respiration R_D, root respiration R_R and carbon gain c_G) were simultaneously recorded over a period of up to one month. At the end of the experiments mineral composition was recorded in various plant parts. Three representative days with 24 measurements for every hour a day were selected in each experiment, snowing that low NaCl concentrations (15 and 30 mM) reduced A and R_D six days after the treatments started. After 3 more days and an increase to 60 mM NaCl a significant reduction of A was measured compared to control plants. In another experiment NaCl-salinity reduced A in both rootstock cultivars without clear difference, but M. indica '13-1' had lower R_R compared to M. indica 'Turpentine' and higher C_G. No significant differences in mineral contents were measured, but M. indica '13-1' had absolute higher Na⁺, CI^- and Ca²⁺ contents in leaves. M. indica '13-1' compared directly with M. zeylanica showed clear differences. At 60 mM NaCl A was 8 μmol CO₂ m^-2s^-1 in M. zeylanica and only 3 μmol CO₂ m^-2s^-1 in M. indica '13-1'. In M. zeylanica a higher R_R was measured. Significant lower Na⁺ contents were found in roots and young leaves of M. zeylanica as compared to M. indica '13-1'. The K⁺ contents and K/Na ratio were significantly higher in roots of M. zeylanica. Physiological characterization of whole plant CO₂ gas exchange under controlled environment conditions may be a useful tool for plant description. Data indicated only small differences in salt tolerance between M. indica '13-1' and M. indica 'Turpentine'. Promising higher tolerance may exist in M. zeylanica.