Substrate type and salinity affect growth allocation, tissue ion concentrations, and physiological responses of carrizo citrange seedlings

Abstract

To gain insight into salinity tolerance of citrus, we studied growth, leaf, and root Cl- concentrations and physiological responses of 5-month-old seedlings of the citrus rootstock Carrizo citrange [Citrus sinensis (L.) Osb. × Poncirus trifoliate L.] grown in a greenhouse in 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 9 weeks. Without salinity, substrate type did not affect total plant growth although there were differences in shoot/root dry weight ratio and mineral nutrient relationships attributable to substrate. Predawn leaf water potential, midday CO2 assimilation, and leaf water use efficiency 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 as Cl- concentrations were lowest in Candler sand and highest in Floridana soil. Leaf Na+ was also highest in Floridana seedlings. In contrast to salt ions in leaves, roots of salinized seedlings in Candler sand had the highest Na+ and Cl- concentration. Salinity reduced net gas exchange of leaves similarly in all three substrates. Salinity reduced both leaf water potential and osmotic potential such that leaf turgor was increased. Thus, salinity-induced reductions in growth and net gas exchange were not the result of loss of turgor but more likely resulting from toxic ion accumulation in leaves. 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.