Substrate Type Affects Growth, Mineral Nutrition and Physiological Responses of Salinized Carrizo Citrange Seedlings

We studied growth, mineral nutrition and physiological responses to salinity of five-month-old seedlings of Carrizo citrange rootstock grown in a greenhouse in containers of three different substrates: Candler sand soil, Floridana sandy clay soil or a commercial soilless peat/perlite/vermiculite potting media. Plants were kept well watered with a complete nutrient solution plus either no salt (control) or 50 mM NaCl for nine weeks. Without salinity, substrate type did not affect plant growth even though there were differences in mineral nutrient relationships attributable to substrate. Pre-dawn leaf water potential, midday CO₂ assimilation and leaf water use efficiency, however, were highest in seedlings grown in the soilless peat. The salt treatment decreased leaf and root growth, reduced leaf Ca²⁺ and increased leaf K⁺ concentration in all the three substrates. Overall, plant growth was negatively related to leaf Cl^-. Leaf growth reductions were least in Candler-grown seedlings and greatest in Floridana soil. This was related to leaf Cl^- concentrations which were lowest in Candler sand and highest in Floridana soil. Leaf Na⁺ was also highest in Floridana seedlings. In contrast to leaf ions, roots of salinized seedlings in Candler sand had the highest Na⁺ and Cl^- concentration. Salinity reduced net gas exchange of leaves and plant transpiration similarly in all three substrates. Salinity reduced both leaf water potential and osmotic potential such that leaf turgor was increased. Based on the relative rankings of leaf growth and leaf Cl^- concentrations, Carrizo seedlings from Candler sand had the highest salt tolerance and those grown in Floridana soil had the lowest salt tolerance. Substrate type should be considered when characterizing plant growth and physiological responses to salinity.