Low NaCl Concentrations Increase Cotyledon Growth in Chinese White Radish (Raphanus sativus L. var. longipinnatus Bailey) Seedlings via Aquaporin-Mediated Water Transport

This study aimed to elucidate the specific role of low NaCl concentrations, particularly 10 and 20 mM, in stimulating cotyledon growth in Chinese white radish (<i>Raphanus sativus</i> L. var. <i>longipinnatus</i> Bailey) seedlings. Chinese white radish seeds were cultivated in sand culture and subjected to daily watering with solutions containing 0, 10, 20, 50, or 100 mM NaCl. Growth, water status, aquaporin gene expression, ion contents, and physiology-related parameters were assessed 4 days after sowing. Applying 10 and 20 mM NaCl significantly promoted the growth of 4-day-old seedlings. Notably, the cotyledons exhibited the most significant growth, achieving a rate of 177% compared with the 125-138% growth observed in the hypocotyl and root parts. This substantial enhancement in cotyledon growth, including biomass, cotyledon area, cotyledon thickness, and mesophyll cell size, was induced by an optimal concentration of 10 mM NaCl. This induction correlated with the increased water content, degree of succulence, and expression of aquaporin genes, specifically within <i>PIP1-1</i>, <i>PIP1-2</i>, <i>PIP2-1</i>, <i>PIP2-2</i>, and <i>TIP1-1</i>, in addition to the maintenance of the Hill reaction, heightened free radical scavenging, and the elevated accumulation of Na⁺, Cl^-, K⁺, proline, total N, and C. These findings suggest a beneficial role of low NaCl levels in optimising early-stage seedling growth.