Effects of cotyledon removal on the root growth and physiology properties during early seedling stage of Ricinus communis under salt stress.

Castor (Ricinus communis L.) is an important oilseed crop species that demonstrates strong adaptability to saline-alkaline conditions. Early seedling development is the most vulnerable period to salinity stress during plant establishment. Cotyledons, as the earliest emerging photosynthetic organs, play a crucial role in supporting seedling establishment and contributing to salt stress tolerance. However, the effects of cotyledon removal (CR) on root adaptive strategies and physiological regulation under salt stress remain unclear. Here, the growth, root ultrastructure, ion homeostasis, antioxidant defences, and carbon-nitrogen metabolism of castor roots with the degree of CR (no cotyledon removed, NR; one cotyledon removed, OR; both cotyledons removed, TR) were measured under both salt and non-salt treatments. The results showed that CR inhibited biomass accumulation and damaged root architecture, and the scanning electron microscopy (SEM) analysis revealed that TR induced loss of root hairs and severe epidermal damage. Under saline conditions, CR had a more significant effect, leading to elevated Na+ concentrations and reduced K+ content in castor roots. In addition, TR treatment decreased MDA content and antioxidant enzyme activity under 150 mM NaCl, compared with NR seedlings. Moreover, CR adversely affected root carbon and nitrogen metabolism. Principal component analysis (PCA) further highlighted distinct physiological profiles, with TR150 samples clustering separately due to severe ionic and oxidative imbalances. Together, these findings provide new insight into the physiological functions of cotyledons and the above and below ground interactions of castor plants under salt conditions.