Effects of Plant Growth Regulators on Bolting and Several Traits of Onion (Allium cepa) cv. Texas Early Grano

Bolting in onion reduces yield and quality of bulbs. To control bolting chemically, a glasshouse experiment was conducted using Texas Early Grano which is widely cultivated in subtropical areas of Karman Province. In this research, Paclobutrazol (0, 1000, 2000 mg/lit), ethephon (500, 1000, 2000, 3000, 4000 mg/lit), cycocel (500, .1000 mg/lit) and mixtures of ethephon and cycocel were sprayed on onion plants prior to vernalization. At harvest time, morphological characters (bolting percentage, shoot length, shoot dry weight, bulb maturity index) and biochemical characters (chlorophyll a, b and total in shoots, reducing sugars and soluble proteins in roots, bulbs and shoots) were recorded. Paclobutrazol reduced bolting, reducing sugars, soluble proteins and shoot length but increased leaf chlorophylls, proteins, sugars and bulb weight with no effect on shoot dry weight. Although ethephon reduced bolting, shoot growth, sugars, proteins and leaf chlorophyll and increased maturity index, sugars and proteins in bulbs, it had no effect on bulb yield. Cycocel increased bolting, sugars, proteins, chlorophylls and dry weight in shoots but had no effect on shoot length, leaf chlorophyll, sugars and protein in roots and bulb weight. Cycocel also reduced sugars and protein in bulbs. Mixtures of cycocel and ethephon reduced bolting, leaf chlorophyll, shoot length and dry weight but had no effect on bulb yield and other characters measured.