Increased Fertilizer Levels Do Not Prevent ABA-Induced Chlorosis in Pansy

Abscisic acid (ABA), the hormone responsible for stomatal closing, has been studied for years for its potential to enhance the shelf life of ornamental plants. By inducing stomatal closure, sprays with ABA solutions can reduce plant water use and delay the onset of drought stress when plants are not watered in the retail setting. ABA has been shown to do this effectively, but can also induce unwanted side effects, including chlorosis. Pansy (Viola × wittrockiana) has been shown to be particularly susceptible to ABA-induced chlorosis. The objective of this study was to determine whether the fertilization rate affects the severity of chlorosis. Pansy ‘Delta Premium Pure Yellow’ seedlings were transplanted in 10 cm pots filled with a peat-perlite substrate. A controlled-release fertilizer (Harrell 19-4-8 with micros) was incorporated at seven rates from 0 – 8 g/L of substrate. When plants had reached a salable size, half the plants were sprayed with a solution containing 1,000 ppm ABA, while the other plants were sprayed with water. Plants were subirrigated daily with plain water throughout the entire study. Leaf chlorophyll was monitored for two weeks following ABA application. Plant dry weight was lowest without the addition of controlled release fertilizer to the substrate (1.47 g/plant), but similar in any of the treatments with fertilizer (2.15 g/plant). Leaf chlorophyll content increased greatly as fertilizer rate increased from 0 to 2 g/L, with little or no further increase in leaf chlorophyll at even higher fertilizer rates. ABA induced chlorosis, irrespective of the fertilizer rate. ABA also reduced the final dry weight of the shoots by 24%, regardless of fertilizer rate. This reduced dry weight as the result of ABA applications is likely due to reduced photosynthetic rates, which may be due to both reduced leaf chlorophyll content and stomatal closure limiting CO₂ diffusion into the leaves. We conclude that ABA-induced chlorosis of pansy cannot be prevented by the application of additional fertilizer.