Hypersensitivity of Ranunculus asiaticus to Salinity and Alkaline pH in Irrigation Water in Sand Cultures

Ranunculus, grown as a field crop in southern and central coastal California, is highly valued in the cut flower and tuberous root markets. However, concerns regarding the sustainability of ranunculus cultivation have arisen when the plantations are irrigated with waters of marginal quality because the viability of the tuberous roots may be compromised. A study was initiated to evaluate the effect of saline irrigation waters, with and without pH control, on the growth of plants and tuberous roots of ranunculus. Treatments consisted of four irrigation water solutions with increasing concentration of Ca2+, Mg2+, Na+, SO42−, and Cl− to meet an electrical conductivity (EC) of 2, 3, 4, and 6 dS·m−1 and pH 6.4. The 3, 4, and 6 dS·m−1 solutions were replicated with uncontrolled pH, which averaged 7.8 over the trial. Ranunculus ‘Yellow ASD’ and ‘Pink CTD’ seedlings were transplanted into greenhouse sand tanks and irrigated twice daily with treatment solutions. Shoot dry weight of plants irrigated with 2 dS·m−1 solutions was 7.20 g and 6.66 g in ‘Yellow ASD’ and ‘Pink CTD’, respectively; however, increasing EC from 2 to 3 dS·m−1 induced an 83% and 78% decrease, respectively. Tuberous root fresh weight of control plants, 7.45 g and 8.42 g for ‘Yellow ASD’ and ‘Pink CDT’, respectively, was decreased by 82% and 89% when EC was 6 dS·m−1. High pH of irrigation water caused an additional decrease in shoot dry weight and tuberous root weight. In control plants, 83% and 76% of tuberous roots of ‘Yellow ASD’ and ‘Pink CTD’, respectively, that were transplanted in the following season produced new shoots; however, tuberous roots sprouting percentage from plants irrigated with EC 4 dS·m−1 water decreased to 42.9% and 58.3% and to 11.1% and 45.0% with EC 6 dS·m−1. The hypersensitivity of ranunculus to salinity was associated with a significant decrease in Ca2+ and K+ tissue concentration. In ‘Yellow ASD’, Ca2+ decreased from 202 mmol·kg−1 in control plants to 130 mmol·kg−1 in plants irrigated with 3 dS·m−1 solutions and pH 6.4. In ‘Pink CTD’, the decrease was from 198 mmol·kg−1 to 166 mmol·kg−1. Potassium was similarly affected. Compared with control plants (405 mmol·kg−1), shoot Na+ concentration was increased by 101% in ‘Yellow ASD’ and by 125% in ‘Pink CTD’ when irrigated with 6 dS·m−1 water. Salt sensitivity of ranunculus, as determined by growth of the flowering stems and viability of the tuberous roots, was increased by irrigation with alkaline waters, which was associated with additional increases in Na+ and Cl– tissue concentration and decreased iron accumulation. Hypersensitivity to salinity makes ranunculus crop a poor candidate for water reuse systems; however, further research is warranted to elucidate the possibility of enhancing its tolerance to salinity by supplemental Ca2+ and K+ and acidification of irrigation water.