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Amir Rezazadeh and Richard L. Harkess

Purple firespike (Odontonema callistachyum), native to Central America, has potential for use as a new flowering potted plant. The effects of number of pinches (zero, one, or two) and number of cuttings (one, two, or three) per 6-inch pot were evaluated on the control of plant height. Plant height was suppressed as the pinch number increased. The greatest reduction was recorded with one cutting per pot and two pinches. The maximum number of branches per pot was recorded with two pinches and three cuttings per pot. In a second experiment, plant growth regulators (PGR) were also tested for efficacy of height control; 2 weeks after pinching, foliar sprays of paclobutrazol, flurprimidol, daminozide, chlormequat, and a tank-mix of daminozide + chlormequat or media drenches of paclobutrazol, uniconazole, or flurprimidol were applied. Plant height, leaf area, and leaf dry weight were recorded at 3, 6, and 9 weeks after PGR application. Maximum height control was obtained with uniconazole drench at 8 ppm, resulting in plants 22 cm tall, 61% shorter than the untreated control (56 cm); however, it resulted in severe leaf distortion. Plant height was 56% and 46% shorter than the control using drenches of paclobutrazol at 30 ppm and flurprimidol at 15 ppm, respectively. Daminozide spray at 2000 ppm and tank-mix of daminozide + chlormequat at 4500/1500 ppm suppressed stem elongation by 20.3% and 19%, respectively. Plants treated with paclobutrazol drench at 30 ppm reduced leaf area and leaf dry weight compared with other PGRs. Chlormequat spray at tested concentrations was ineffective for controlling firespike plant growth. The most attractive potted plants were produced using a drench application of paclobutrazol at 10 or 15 ppm.

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Amir Rezazadeh and Eric T. Stafne

The present study assessed the effect of photoperiod on budbreak of cuttings of three interspecific hybrid grape (Vitis) cultivars that had received different chilling hours. Stem cuttings were collected at 100-hour intervals of chilling (200, 300, 400, 500, 600, 700, and 800 hours) from the vineyard and kept in three growth chambers with daylengths of 8, 16, and 24 hours. Another group of cuttings were maintained in a greenhouse with a natural daylength range of 10.5–13 hours [8 Dec. 2017 to 4 May 2018 (average = 12 hours)]. Chilling requirements, days to budbreak, and budbreak rate were determined after plants were exposed to different chilling hours and daylengths. Results of our study demonstrated that the chilling requirements of all three cultivars were adequately reached at 600 hours or more. Increasing chilling exposure from 600 to 800 hours shortened the time to budbreak in all cultivars. Overall, ‘MidSouth’ had an average budbreak rate of 90% when receiving at least 600 hours chilling. ‘Blanc du bois’ and ‘Lake Emerald’ had 62% and 65% average budbreak, respectively. Longer days (24 hours) reduced time to budbreak by 14, 6, and 8 days, respectively, in ‘Blanc du bois’, ‘Lake Emerald’, and ‘MidSouth’ at 600 hours chilling. A combination of 24-hour photoperiod and chilling of 600 hours resulted in a maximum budbreak rate of 70%, 70%, and 100% in ‘Blanc du bois’, ‘Lake Emerald’, and ‘MidSouth’, respectively. Our results indicate that breaking dormancy may be controlled by both temperature and photoperiod in these three cultivars.

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Amir Rezazadeh, Richard L. Harkess and Guihong Bi

Red firespike (Odontonema strictum) is an ornamental shrub with potential for use as a flowering potted plant due to its dark green foliage and attractive red flower spikes. To stimulate branching and improve quality of red firespike, foliar spray applications of dikegulac sodium (DS) and benzyladenine (BA) and hand pinching were evaluated across two seasons (Spring and Summer 2014). There were three pinching treatments: one, two, or three pinches. Plant growth regulators (PGRs) were applied at 400, 800, 1600, or 2400 ppm DS or 600, 1000, 1250, or 1750 ppm BA. Both studies included an untreated control. Red firespike treated with all concentrations of BA and 1600 and 2400 ppm DS had increased branching compared with the control, except 1000 ppm BA in Expt. 1. Pinching did not affect the number of branches. Dikegulac sodium at 1600 and 2400 ppm and all concentrations of BA resulted in shorter plants than the control. Phytotoxicity was observed in plants treated with 1600 or 2400 ppm DS. In both experiments, DS at 1600 and 2400 ppm had the least plant dry weight compared with the control. Treatment with BA at 1750 ppm resulted in greatest leaf area compared with control. Dikegulac sodium at 800 ppm increased the number of flowers compared with control. Pinching and BA did not affect number of inflorescences. All concentrations of BA and DS delayed flowering, except 1000 ppm BA. Plants treated with 800, 1600, and 2400 ppm DS had shorter inflorescences compared with control plants. Benzyladenine decreased the length of the inflorescence at high concentrations, 1250 and 1750 ppm. Pinching treatments did not affect inflorescence length.

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Amir Rezazadeh, Richard L. Harkess and Guihong Bi

This study evaluated the effects of paclobutrazol (PBZ) and flurprimidol on the morphological and physiological characteristics of potted red firespike (Odontonema strictum) under drought stress. PBZ and flurprimidol were applied as a soil drench at 0.24 mg/pot. Untreated plants acted as a control. The plants were exposed to drought stress 2 weeks after plant growth regulator (PGR) application. Another group of plants treated with PGRs was watered regularly. A reduction in plant height, plant diameter, and growth index (GI) was observed in plants treated with PGRs and drought stress 5 weeks after beginning the study. Drought stress reduced plant height by 45% compared with control. Flurprimidol under drought stress decreased plant diameter and GI by 36% and 76%, respectively, compared with the control. The least leaf area and plant dry weight were observed in plants drenched with flurprimidol and grown under drought stress. Drought stress also delayed flowering and the number of plants flowering. Plants treated with PBZ had the highest photosynthesis rate, 54% more than untreated plants under drought stress alone. The lowest stomatal conductance (g S) was measured in plants under drought stress alone or drought plus PBZ. Application of PBZ-enhanced red firespike drought tolerance reducing adverse effects of water stress on photosynthesis during the experiment.

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Amir Rezazadeh, Richard L. Harkess and Guihong Bi

The effect of plant growth regulators (PGRs) on growth and flowering of potted red firespike (Odontonema strictum) were examined in two experiments. In Expt. 1, foliar spray applications of daminozide, uniconazole, paclobutrazol, or flurprimidol or media drenches of paclobutrazol or flurprimidol were applied. In Expt. 2, foliar spray application of daminozide or media drenches of paclobutrazol or flurprimidol were further tested for efficacy of height control. Both studies included an untreated control. In Expt. 1, drench applications of paclobutrazol and flurprimidol resulted in plants 65% or 46% to 62% shorter than control, respectively. Paclobutrazol and flurprimidol drenches also decreased overall plant growth by 81% to 88% and 74% to 84%, respectively, compared with the control plants. PGRs did not affect number of inflorescences; however, paclobutrazol and flurprimidol delayed flowering 23 to 31 days. In Expt. 2, plants treated with flurprimidol or paclobutrazol drenches were shorter than the control. The greatest reduction in total plant growth occurred using a flurprimidol drench at 0.47 mg/pot, which resulted in plants 78% smaller than the untreated control. Paclobutrazol and flurprimidol increased the time to flowering 11 to 27 days and 10 to 26 days, respectively. The most attractive and well-shaped plants were achieved with flurprimidol applied at 0.24 mg/pot or applications of paclobutrazol at 0.35 mg/pot.

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Eric T. Stafne, Amir Rezazadeh, Melinda Miller-Butler and Barbara J. Smith

White drupelet disorder (WDD) is a problem that occurs during the ripening stage in some blackberry (Rubus subgenus Rubus) cultivars. Although berries affected with a few white drupelets may taste fine, they are unpleasant aesthetically, and this may lead to negative consumer perceptions and economic ramifications. During 2 years of observational studies and field trials (2016 and 2017), we evaluated changes in three susceptible cultivars in Mississippi affected by WDD. ‘Chickasaw’, ‘Kiowa’, and ‘Sweetie Pie’ berries were harvested twice per week and assessed for WDD. Weather conditions differed substantially during harvest in 2016 and 2017, with 2017 being cooler and rainier which resulted in a lower incidence of WDD. Compared with 2016, in 2017, the overall percentage of berries exhibiting WDD dropped from 22% to 12% for ‘Sweetie Pie’, 6% to 3% for ‘Chickasaw’, and 8% to 3% for ‘Kiowa’. The soluble solids concentration was highest in ‘Sweetie Pie’, 11.9% and 9.5% for 2016 and 2017, respectively. For all cultivars examined, the soluble solids concentration of extracted white drupelets was substantially lower than regular drupelets. The value for skin break force for white drupelets was higher than that for black drupelets, 0.99 N vs. 0.29 N, respectively. In 2017, an about 30% shadecloth treatment had a significantly positive impact by decreasing WDD symptoms in all cultivars by 63% when compared with non-shaded plants, but soluble solids concentration was lower. Our results indicate that rain and shadecloth decrease symptoms of WDD. Therefore, growers may be able to use overhead irrigation and shade to reduce WDD symptoms.