INCIDENCE OF BORON DEFICIENCY IN BEDDING PLANTS CAUSED BY DROUGHT STRESS OR ABSCICIC ACID APPLICATION

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Growers have reported boron (B) deficiency in pansy (Viola ×wittrockiana) plug production, specifically in plugs grown in the high heat and humidity conditions of summer. Past studies have reported that soil moisture has an impact on boron availability. To simulate drought conditions, ‘Dynamite Yellow’ pansy plants were grown in a peat-based substrate which was allowed to dry down to 40, 30 or 20% container capacity (CC), 10 or 20 days after sowing (DAS) or on a continual basis. Boron tissue concentration was not affected by any induced drought stress, with the exception of those allowed to dry to 40% CC 20 DAS, which had higher levels of boron than the control. To simulate a physiological response to drought stress, exogenous abscisic acid (ABA) was applied as a substrate drench or foliar spray at concentrations of 150 or 300 mg L-1 to ‘Dynamite Yellow’ pansy plants grown in a peat-based substrate 10 or 20 DAS. All treatments resulted in lower tissue concentrations of B compared to an untreated control. Plants treated 10 DAS with a substrate drench (300 mg L-1) or a foliar spray (150 mg L-1) and plants treated 20 DAS with a foliar spray (150 or 300 mg L-1) of ABA showed a reduction in transpiration. Plants had lower ratios of transpiration/leaf area when ABA was applied as a 300 mg L-1 substrate drench 10 DAS and as foliar spray at both concentrations at either application time. The lower B tissue concentrations coupled with lower transpiration rates were similar to circumstances in greenhouse production of fall pansy crops. Boron deficiency is most common in August when high temperatures and relative humidity cause the plants to transpire less.