Manual pinching of apical meristems of horticultural crops removes apical dominance and promotes branching, increases flower numbers, and promotes the growth of a rounded, uniform plant canopy. However, manual pinching is a time-consuming and labor-intensive practice that can become costly for growers (Cheema, 2018; Starman, 1991). Meijón et al. (2009) noted that chemical pinching agents can reduce the cost associated with manual pinching, but the plant growth response is variable. Previous research classified sunflower (Helianthus spp.) as having strong apical dominance that can be broken only with the removal or manual pinching of the apical meristem (Bhattacharjee and Gupta, 1984; Cline 1978). Currently, some basal branching sunflower cultivars, referred to as multifloras, generally are not pinched; however, pinching single-stem sunflower cultivars promoted multiple axillary stems with smaller flowers (Armitage and Laushman, 2003).
Dikegulac sodium is a plant growth regulator (PGR) that chemically prevents apical dominance by inhibiting cell division in the apical meristem, thereby allowing lateral branching to occur (Arzee et al., 1977; Rezazadeh et al., 2015). Dikegulac sodium has been shown to effectively increase branching of several horticultural crops such as Sedum, Hydrangea, Phlox, and Nepeta (Banko and Stefani, 1995; Cline, 1978; Grossman et al., 2013; Sun et al., 2015). Within 1 d, dikegulac sodium was detected in the apical meristem after being applied to the basal foliage of Chrysanthemum morifolium. However, foliar application to the apical leaves provided the maximum effect for preventing apical dominance (Bocion and DeSilva, 1977).
In vitro studies involving Solanum nigrum showed that dikegulac sodium did not inhibit stationary or dormant cells, but that phytotoxicity did occur on actively dividing cells (Zilkah and Gressel, 1978). Further research by Zilkah and Gressel (1979) showed that cell leakage is quickly induced after application. Latimer and Whipker (2001) recommended trialing several concentrations of dikegulac sodium for new species because responses to the chemical were species-specific. Published works have examined the effects of dikegulac sodium on Helianthus physiology and enzyme activities (Arzee et al., 1977; Bhattacharjee and Gupta, 1981, 1984; Purohit, 1980). However, we are not aware of studies of the ability of dikegulac sodium to chemically pinch sunflower.
‘Sunfinity’ (Helianthus hybrida), a relatively new ornamental hybrid of sunflower, has a continuous branching habit. Manual pinching above the fourth node is recommended commercially to encourage continuous lateral branching and create a uniform and rounded plant canopy. A preliminary experiment was conducted in Spring 2021, with the objective of substituting manually pinching ‘Sunfinity’ above the fourth node with chemical pinching. When plants had developed five, six, or seven nodes, the foliar application of dikegulac sodium was sprayed across the plant canopy on actively growing axillary stems. This resulted in phytotoxicity of leaves and axillary stems and total plant senescence within 3 weeks after application.
The objective of this experiment was to treat ‘Sunfinity’ seedlings with increasing concentrations of dikegulac sodium (200, 300, 400, or 500 mg⋅L−1) at the time of growth when the first, second, or third node (N1, N2, or N3) was the apical node and when axillary buds were undeveloped to determine if the PGR effectively produced a well-branched plant comparable to that resulting from manually pinching. The overall goals were to determine which treatment resulted in growth similar to that of the manually pinched control, and to determine if chemical pinching is an effective alternative to manual pinching for the purpose of saving labor and costs. Foliar applications to the apical leaves while axillary buds were undeveloped were performed to prevent phytotoxic effects that were observed on actively growing axillary stems during our preliminary experiment.
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