ASHS 2024 Conference

Plant growth regulators (PGRs) are a valuable tool in the ornamental industry. Whether it is for promoting or controlling growth, PGRs give growers the opportunity to produce high quality crops in the face of their production or shipping challenges. Often, substrate drench applications provide greater growth control, require less labor, and have shortened re-entry intervals than spray applications. Current PGR recommendations for ornamental millet (Pennisetum glaucum ‘Jester’) are for spray applications; therefore, the objectives of this study were to evaluate the efficacy of different PGRs applied as substrate drenches to improve marketability and control growth of ornamental millet ‘Jester.’ Seeds were received from a commercial supplier and sown into 288-cell plug trays (5-mL individual cell vol.) filled with a pre-moistened commercial soilless propagation substrate. After 24 d, young plants of similar heights, basal diameters and culm were transplanted with one plant per 11.4-cm-diameter container filled with a commercially available peat-based substrate. At 7 d after transplant, plants received 59-mL aliquots of solution containing 0 (deionized water control),1, 2, 4, 8, or 16 mg·L–1 ancymidol, flurprimidol, paclobutrazol, or uniconazole or 25, 50, 75, 100, or 200 mg·L–1 ethephon. Plants were grown at bench-level in a glass-glazed greenhouse with an air temperature set point of 23 °C, and supplemental day-extension lighting provided by light-emitting diode lamps from 0600 to 2200 hr (16-h photoperiod) to achieve a daily light integral of ~14 mol·m–2·d–1. Data were collected four weeks after drench. For each chemical, effects of PGR concentration were analyzed independently, and means were separated using Tukey’s honestly significant differences. For all analyses, a P ≤ 0.05 was used to determine significant effects. In general, ancymidol and ethephon provided the best growth control, while flurprimidol and uniconazole were found to be inadequate for drenches at the concentrations investigated due to unmarketable plants. Paclobutrazol resulted in unmarketable plants at concentrations ≥ 1 mg·L–1. Ancymidol substrate drenches containing 1 to 16 mg·L–1 resulted in plants that were 3% to 21% shorter and 3% to 14% smaller plants, respectively, than untreated plants. Similarly, increasing ethephon substrate drench concentrations from 1 to 16 mg·L–1 resulted in plants that were 7% to 36% shorter and 27% to 41% smaller plants compared to the untreated control. Results from this study establish substrate drench recommendations for ornamental millet ‘Jester’; however, further investigations are needed to assess growth control responses of other ornamental millet cultivars.

Heartleaf brunnera (Brunnera macrophylla L.) is a popular herbaceous perennial that is often used in the landscape. With large leaves and a mounding habit, growth control is often needed during greenhouse production. Plant growth regulators (PGR) applied as substrate drenches or foliar sprays can control growth necessary to produce compact, high-quality containerized plants. Our objectives were to evaluate the efficacy and growth control provided by uniconazole substrate drenches or sprays. Rooted liners of heartleaf brunnera ‘Jack Frost’ were transplanted into containers (16.6-cm; 2.8 L) filled with a commercially formulated peat-based substrate. After 10 d, eight single-plant replicates received either a substrate drench or foliar application of uniconazole. For substrate drenches, 296-mL aliquots of solution containing deionized water (0 mg·L–1; control) or 0.25, 0.875, 1.75, 2,5, 5.5, 7.5, or 10.0 mg·L–1 uniconazole were applied across the substrate surface. For foliar sprays, 0, 2.5, 5.0, 7.5, 10.0, or 20.0 mg·L–1 uniconazole were applied at a rate of 1.89 L/ 9.29 m2. Plants were grown in a glass-glazed greenhouse at 23 °C under ambient daylight supplemented with a photosynthetic photo flux density of ≈120 µmol·m–2·s–1 delivered from light-emitting diode lamps from 0600 to 2200 HR (16-h photoperiod) to achieve a daily light integral of 14 mol·m–2·d–1. At 7 weeks after treatment, data were collected and plants were destructively harvested. In general, uniconazole significantly controlled plant height, diameter, and dry mass for each uniconazole application method. Plant height and diameter were 15% to 51% (2.2 to 7.6 cm) shorter and 22% to 40% (7.4 to 13.5 cm) smaller, respectively, than untreated plants as uniconazole drench concentration increased from 0.875 to 5.0 mg·L–1, respectively. Plants treated with 0.875 to 5.0 mg·L–1 developed 3 to 7 fewer leaves. Shoot dry mass was 39.5% lower than untreated plants as uniconazole drench concentration increased from 0.875 to 5.0 mg·L–1, respectively. For foliar applications, plant height and diameter were both reduced but to varying degrees. Plant height was reduced by 6% (~1 cm) but the greatest amount of control observed was in plant diameter which was reduced by 15 to 21% (4.8 to 7.2 cm) as concentrations increased from 5.0 to 10.0 mg·L–1. Collectively, these results indicate that drenches of 0.875 to 5.0 mg·L–1 uniconazole or foliar sprays of 5.0 to 10.0 mg·L–1 uniconazole may be used to control growth of ‘Jack Frost’ heartleaf brunnera.

The increasing diversity of butterfly bush (Buddleia × hybrida) cultivars presents new challenges for growers, particularly in adjusting plant growth retardants (PGRs) to manage plant size effectively. Therefore, the objective of this study was to evaluate uniconazole substrate drench concentrations for growth control of two popular cultivars of butterfly bush (Buddleia × hybrida). Liners of ‘Grand Cascade’ and ‘Prince Charming’ butterfly bush were individually transplanted into containers (16.5-cm; 1.7 L) filled with a commercially formulated bark-based substrate. After 10 d, eight single-plant replicates received a substrate drench of 266-mL aliquots of solution containing deionized water (0 mg·L–1; untreated) or 0.25, 0.5, 1, 2, or 4 mg·L–1 uniconazole. Plants were grown in a glass-glazed greenhouse at 20 °C under ambient daylight supplemented with a photosynthetic photo flux density of ≈125 µmol·m–2·s–1 delivered from light-emitting diode arrays from 0600 to 2200 HR (16-h photoperiod) to achieve a daily light integral of 14 mol·m–2·d–1. At 5 weeks after drench, data were collected. Plant height, plant diameter, growth index (GI), and shoot dry weights were unaffected by cultivar or cultivar × uniconazole concentration interaction but varied by uniconazole concentration (P < 0.0001); therefore, all data were pooled and analyzed by uniconazole concentration. Increasing uniconazole substrate drench concentrations effectively controlled ‘Grand Cascade’ and ‘Prince Charming’ butterfly bush plant height, plant diameter, GI, and shoot dry weights. Plant height and diameter were 16% to 32% (6.9 to 13.8 cm) shorter and 10% to 24% (6.9 to 16.9 cm) smaller than untreated plants as uniconazole drench concentration increased from 1 to 4 mg·L–1, respectively. Shoot dry mass was 24% to 34% (5 to 7 g) lower than untreated plants as uniconazole drench concentration increased from 1 to 4 mg·L–1, respectively. Overall, these results indicate that 1 to 4 1 to 4 mg·L–1 uniconazole applied as a substrate drench may be used to control growth of ‘Grand Cascade’ and ‘Prince Charming’ butterfly bush. Time to visible bud and flower was not negatively influenced by increasing uniconazole concentrations; however, growers should trial drench concentration and adjust as needed for desired market dates. Additionally, further investigations with uniconazole are warranted for other butterfly bush cultivar introductions because it is the is the preferred PGR for perennial growth control.

1-aminocyclopropane-1-carboxylic acid (ACC) is the immediate precursor of ethylene in plants. Accede SG containing ACC as active ingredient has been registered as a chemical thinner in the US for stone fruit and apple. The objective of this study was to investigate the effects of weather parameters on Accede SG efficacy. In a series of experiments in Oregon and California in the field and growth chambers, we evaluated the relationship between flower/fruitlet abscission caused by ACC and weather parameters in peaches. In field trials, ACC at 300 mgL-1 and 600 mgL-1 was sprayed on a daily basis to different set of peach trees throughout the bloom period and correlated fruit set data with daily weather parameters. From these field trials, it became apparent that flower/fruitlet abscission caused by ACC is in negative relationship with daily minimum and maximum temperatures. These findings were confirmed by greenhouse studies where increase in nighttime temperature reduced thinning efficacy of ACC. No close relationship between thinning efficacy and relative humidity was found. We created three models to predict ACC thinning efficacy using weather parameters and ACC spray concentration.

1-aminocyclopropane-1-carboxylic acid (ACC) is assumed to cause flower abscission via the ethylene pathway when used as a chemical thinner in peaches. The objectives of this study were to investigate the uptake of ACC via flower parts, and determine if ethylene is the main cause of flower abscission by ACC. In a series of field trials in Oregon, we determined that ACC is a non-mobile compound when applied as a foliar spray to peach trees. ACC does not translocate between branches and movement of externally applied ACC is very limited even between flower parts. When the ACC solution was applied via paintbrush to various parts of the flowers, it became evident that for sufficient flower thinning activity, the presence/uptake of ACC is needed through the pedicel and/or Abscission Zone tissues of the pedicel (AZ1-2). Application of ACC to the petals only, resulted in petal drop only but not in flower abscission. When evaluating the mode of action of ACC, we established a close relationship between ethylene production of the flowers and flower abscission. However, when ethylene production of the flowers was reduced with the addition of ACC oxidase inhibitors (2-picolinic acid, pyrazinecarboxylic acid) in the ACC spray solution, the flower abscission rate remained the same as in the ACC application alone. These results indicate that ethylene might not be the only factor in flower abscission caused by ACC.

Creeping bentgrass is an important cool-season turfgrass species widely used for golf course putting greens, however it experiences summer stress and quality decline in the U.S. transition zone and other regions with similar climates. Silicon (Si) may improve abiotic stress of creeping bentgrass, but mechanism of its impact on plant drought and heat tolerance has not well understood. The objectives of this study were to investigate physiological mechanism of Si on tolerance to drought and heat stress in creeping bentgrass under growth chamber and field conditions. The five treatments from two Si products (Potassium silicate at 0.95 and 1.91 mL m-2, and Ortho-Si at 0.16 and 0.32 mL m-2) were applied biweekly to creeping bentgrass, and treated grass was subjected to heat and drought stress for 56 days and also the treatments were applied to creeping bentgrass putting green in the field conditions. Turfgrass quality, physiological parameters and root growth characteristics were evaluated biweekly. Deficit irrigation was applied to induce drought stress in June and July in the field plots. Foliar application of the Si products improved turf quality, photochemical efficiency, leaf chlorophyll and carotenoid content, antioxidant enzyme activity and endogenous Si content. The Si treatments at the high rates also improved root biomass, length, surface area, volume, and root viability when compared to the control. The results from the field study confirmed the findings in the growth chamber study. The exogenous Si may improve drought and heat tolerance by enhancing root growth and viability, Si uptake by roots, and up-regulation of antioxidant activity, protecting photosynthetic function. The results of this study suggest that foliar application of Si products may be considered as an effective approach to improve turf quality and physiological fitness of creeping bentgrass during the summer months in the U.S. transition zone and other regions with similar climate.

Among numerous abiotic environmental factors, varying light quality and intensity elicit photosynthetic responses that can play vital roles in the optimization of crop production in controlled environment agriculture. Earlier and preliminary studies on photosynthetic activity reported that amber light (595 nm) induces higher photosynthetic rates and quantum yield of plants is a wavelength-dependent response. To resolve the most accurate ePAR curves in cannabis (Cannabis sativa), this study investigated the spectral response of photosynthesis by examining the effects of the leaf versus the whole plant on the impact of photosynthetic activity. A customized CO2 chamber equipped with relative humidity, temperature, and CO2 was used to collect leaf and whole plant photosynthetic data from 5 week old clones with different monochromatic wavelengths (380–750 nm) using the LI-6800 Portable Photosynthesis System (LI-COR) equipped with the Large Leaf and Needle Chamber (LI-COR 6800-13). Differences and correlation between photosynthetic activity at the leaf level and the whole plant were determined. Subsequent studies will involve the combinations of different wavelengths at different ratios. Findings will expand the current understanding of the photosynthetic response of plants to light and provide highly resolved spectral quantum yield curves.

The adoption of supplemental lighting in horticulture has allowed greenhouse growers to increase the yield of multiple crops by at least 10%, with a 300% yield increase documented for cucumbers when transitioning from unlit to lit production. Since then, horticultural lighting has advanced significantly to now include dynamic LED lighting, which provides unparalleled control over the light intensity, spectrum and zoning within the greenhouse. This has allowed greenhouse growers to further maximize crop productivity, but also to target improvements in crop quality and to diversify their production. For example, the greenhouse industry has seen the introduction of multiple berry crops, leafy crops, Asian cucumbers, etc. in recent years, all of which have varying lighting requirements ranging from 12 to 25 mol of light per square meter per day. The ideal spectrum also differs between crops, as do the light saturation points and target intensities. Beyond enhancing photosynthesis and increasing crop productivity, dynamic LED lighting can also be used to improve the nutritional quality and taste of various crops through spectral adjustments during the growth cycle. In a trial comparing the impact of broad and narrow spectra on basil, a broad spectrum produced a flavor profile stronger in eucalyptol whereas a narrow spectrum produced an estragole-dominant crop. This resulted in a milder flavor under a broad spectrum and a stronger flavor under a narrow spectrum. With dynamic lighting, growers can adjust the spectrum to target different flavors and thus different markets. Further, the application of high levels of blue light during the last week of production has been repeatedly shown to enhance the antioxidant capacity of red leafy greens through the bioaccumulation of anthocyanin. The Brix, or sweetness, of fruiting crops has also been improved under dynamic lighting, with commercial trials showing a minimum of 7% increase in Brix in speciality cherry tomatoes compared to fixed spectrum lighting. This increase could potentially be enhanced with end-of-day (EOD) light treatments, which early trials have suggested to enhance the translocation of sugars from leaves to fruit. As such, dynamic LED lighting can improve both the productivity and nutritional quality of greenhouse crops, allowing forward-thinking growers to meet the growing population’s needs in terms of both quality and quantity.