Varietal Differences of Yield, Morphological, and Biochemical Parameters of Allium cepa L. under Precipitation Excess in Different Phenological Phases

Flooding is an important factor, decreasing Allium cepa bulb yield and quality. A comparison, in terms of biometrical and biochemical parameters, of five Allium cepa cultivars, grown at two different locations, characterized by contrasting conditions of water availability, i.e., precipitation excess at the end (A) or at the beginning (B) of plant growth, revealed a significant decrease in bulb weight, height, and diameter, and an increase in oxidative stress parameters, such as total antioxidant activity and polyphenol content, monosaccharides, proline, malonic dialdehyde in the condition of excessive soil humidity at the end of the vegetation period (A). Among the five cultivars studied (Zolotnichok, Zolotie cupola, Black prince, Globus, and Myachkovsky), the lowest variations of the above parameters under precipitation excess at cycle end or beginning were recorded in Zolotnichok and Zolotie cupola, which was in accordance with their high adaptability. Cultivar Myachkovsky showed the highest differences of the parameters examined between A and B conditions. Outer scale biochemical parameters demonstrated the highest stability in both regions. The participation of proline, monosaccharides, total polyphenols, and total antioxidant activity in plant defense against hypoxia, caused by waterlogging, was proved by high correlation coefficients between inner scale parameters (‘r’ from +0.714 to +0.920) and the latter with bulb yield (‘r’ from −0.745 to −0.924). High adaptability cultivars (Zolotie cupola, Zolotnichok, Black prince) showed significantly lower MDA inner/outer scales ratio and lipids outer/inner scales ratio compared to cultivars with moderate adaptability in (B) conditions. The results provide important information regarding biochemical peculiarities of Allium cepa in diverse soil humidity, which should be considered in future breeding activities of onion genotypes, characterized by high adaptability to different water excess conditions.