Manganese Toxicity in Watermelon Plants as Influenced by Nitrogen Form

Watermelon [ Citrullus lanatus (Thunb.)] ‘Sugar Baby’ seedlings were grown in sand culture leached with nutrient solution containing 2, 25, or 50 mg·liter −1 Mn in combination with NH 4 or NO 3 -N to evaluate the effects of N form on growth, composition, and development of Mn toxicity. Nitrogen form treatments consisted of continuous NO 3 , continuous NH 4 , and a treatment in which NH 4 was replaced by NO 3 after Mn toxicity symptoms were first observed on NO 3 -treated plants. Watermelon plants produced the most growth at all Mn levels with N supplied as NO 3 , least with NH 4 , and intermediate with the NH 4 -to-NO 3 shift treatment. Plants grown with NO 3 -N at 25 and 50 mg·liter −1 Mn had reduced growth and developed Mn toxicity symptoms with Mn concentrations >1700 μg·g −1 in shoot tissues. However, with NH 4 the Mn treatments had no effect on growth, no Mn toxicity symptoms developed, and Mn concentrations in shoot tissue were <800 μg·g −1 . Shifting plants from NH 4 -to-NO 3 nutrition at 50 mg·liter −1 Mn in solution stimulated rapid Mn absorption and produced Mn toxicity symptoms within 3 days.