Onion Plant Growth, Bulb Quality, and Water Uptake following Ammonium and Nitrate Nutrition

Abstract

Nitrogen applied as NH 4 -N or NO 3 -N (75 mg·liter -1 ) affected onion (AIlium cepa L.) plant growth when grown in solution culture. Nitrate alone or in combination with NH 4 -N increased leaf fresh and dry weight, leaf area, root fresh and dry weight, and bulb dry weight when compared to growth with NH 4 -N as the sole N source. Bulb fresh weight was highest with an NH 4 -N: NO 3 -N ratio between 1:3 and 3:1. Maximum leaf fresh weight was not necessary to produce maximum bulb fresh weight when onions were subjected to different N-form ratios. Precocious bulbing resulted when NH 4 -N was the sole N source; however, high bulbing ratios early in plant development were not correlated with final bulb fresh weight. Nitrogen form also influenced water uptake and pungency, as measured by enzymatically developed pyruvate concentration, but did not affect bulb sugar concentration.