Shoot and Root Growth of Castor Bean (Ricinus communis) in Response to Soil Salinity

Soil salinity is a serious threat to agriculture worldwide. Castor bean (Ricinus communis L.) is an oilseed crop containing 40–60% of highly valued oil. It is moderately sensitive to salinity. This study evaluated root and shoot responses of two castor bean genotypes, Zibo and Freo, to varying salinity levels (0, 100 and 200 mM NaCl) grown in soil in 1 m deep PVC tubes in a glasshouse. Salt stress was induced gradually, with 25 mM NaCl added every second day from 21 days after sowing (DAS) until the pre-determined salinity level was reached. Plants were harvested for assessments at 112 DAS. Salt-stressed Zibo flowered earlier than the control, while flowering time of Freo was not affected by salt stress. The 200 mM NaCl treatment reduced total root length and increased average root diameter of Zibo and Freo compared to the control. In addition, the 200 mM NaCl treatment significantly decreased total leaf area, chlorophyll content, shoot and root dry weight of both castor bean genotypes by 50%, 10.6%, 53.1%, and 59.4%, respectively, relative to the control. In contrast, the 100 mM NaCl treatment did not significantly affect these traits, indicating that both genotypes tolerated salt stress up to 100 mM NaCl. In general, Freo had greater salt tolerance than Zibo, due to its higher average root diameter, lower Na+ concentration, and higher K+/Na+ ratio in young leaves under salt conditions. In conclusion, Freo is recommended for cultivation in moderate saline soils and could be used as a parental genotypes to breed high-yielding and salt-tolerant castor bean genotypes.