Salinity Tolerance of ‘Hamlin’ Orange Trees on the Hybrid Rootstocks US-897 and x639 Is Greater than of Trees on Cleopatra Mandarin

Abstract

Tree growth, leaf gas exchange, root and leaf Cl and Na concentrations of salinized 2-year-old ‘Hamlin’ orange trees were evaluated in a greenhouse. We compared trees on the relatively salt tolerant Cleopatra mandarin (Cleo) rootstock with two of its commercial hybrid rootstocks, Cleo × Flying Dragon trifoliate (TF; US-897) and Cleo × Rubidoux TF (x639). Trees on these rootstock hybrids have some horticultural advantages over the parent type Cleo but their relative tolerance to salinity has not been tested. Well-fertilized and well-watered trees were grown in a high peat, soilless potting mix and treated with 0, 30 or 60 mM NaCl (maximum EC= 7.36 dS·m–1 or TDS = 5,152 ppm) for 4 months. Trees on Cleo were the smallest, had the highest root/shoot (Rt/Sh) dry weight ratio, and used the least water regardless of salinity level. Trees on US-897 grew the most and had the lowest RT/SH ratio. The lower Rt/Sh ratios of trees on both hybrid rootstocks than of trees on Cleo, implied that the hybrids had more efficient root systems that support greater shoot growth than Cleo root systems. Total tree dry weight (TDW) of trees on all three rootstocks was reduced similarly by salinity as Rt/Sh ratio was affected little by salt stress. Rootstock had little effect on leaf Na but leaf Cl concentration and Cl accumulation were lowest in the smallest trees on Cleo. Roots of x639 had some ability to sequester Cl at the intermediate salt level (30 mM) but this ability was overcome at 60 mM as all leaves accumulated more Cl than roots at the high salinity level regardless of rootstock. Net assimilation of CO2(ACO2) was lowest in leaves on Cleo at 0 and 30 mM NaCl but high salinity reduced ACO2similarly across rootstocks. Reductions in leaf gas exchange were more strongly related to high concentrations of leaf Cl than to high leaf Na. The greater shoot growth and higher leaf Cl levels of trees on both US-897 and x639, support the idea that both hybrid rootstocks were similarly more salt tolerant than Cleo.