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Although heat stress injury is known to be associated with membrane dysfunctions, protein structural changes, and reactions of activated forms of oxygen, the underlying mechanisms involved are poorly understood. In this study, the relationships between thermotolerance and hydrogen peroxide (H₂O₂) defense systems, radical scavenging capacity [based on 1,1-diphenyl-2-picrylhydrazyl (DPPH) reduction], and protein aggregation were examined in vinca [Catharanthus roseus (L.) G. Don `Little Bright Eye'], a heat tolerant plant, and sweet pea (Lathyrus odoratus L. `Explorer Mix'), a heat susceptible plant. Vinca leaves were 5.5 °C more thermotolerant than sweet pea leaves based on electrolyte leakage analysis. Vinca leaf extracts were more resistant to protein aggregation at high temperatures than sweet pea leaf extracts, with precipitates forming at ≥40 °C in sweet pea and at ≥46 °C in vinca. Vinca leaves also had nearly three times greater DPPH radical scavenging capacity than sweet pea leaf extracts. Two enzymatic detoxifiers of H₂O₂, catalase (CAT) and ascorbate peroxidase (APOX), demonstrated greater activities in vinca leaves than in sweet pea leaves. In addition, CAT and APOX were more thermostable in vinca, compared with sweet pea leaves. However, tissue H₂O₂ levels did not differ between controls and tissues injured or killed by heat stress in either species, suggesting that H₂O₂ did not play a direct role in acute heat stress injury in vinca or sweet pea leaves. Greater thermotolerance in vinca, compared with sweet pea, was associated with greater DPPH radical scavenging capacity, indicating that AOS other than H₂O₂ may be involved in acute heat stress injury.

The flowering response of Dianthus gratianopolitanus Vill. ‘Bath's Pink’ was characterized after varying durations at vernalizing temperatures. Genetically identical clonally propagated plants were treated at 5 °C for 3, 6, 9, 12, or 15 weeks in Expt. I; at 0, 5, or 10 °C for 2, 4, 6, or 8 weeks in Expt. II; and at 0, 5, 10, or 15 °C for 1, 2, 4, 6, or 8 weeks in Expt. III. Dianthus gratianopolitanus ‘Bath's Pink’ exhibited a quantitative vernalization response after treatment at 0 to 10 °C and did not vernalize after 8 weeks at 15 °C, which was the longest duration tested. Complete flowering was achieved after 4 or more weeks at 0 °C, 3 or more weeks at 5 °C, and 8 weeks at 10 °C. Based on time to anthesis and node number at anthesis, the flowering response was saturated after vernalization treatment at 0 °C for 4 or more weeks and 5 °C for 3 or more weeks. However, maximum flowers at anthesis were produced after 8 weeks at 0 °C and 6 or more weeks at 5 °C. Flowering was delayed after the 8-week treatment at 10 °C compared with 6 or more weeks at 0 °C and 4 or more weeks at 5 °C. Based on the minimum vernalization duration required to achieve the maximum flowering response, the order of efficacy of vernalizing temperatures was 5 °C > 0 °C ≫ 10 °C.

The objective of this study was to characterize the influence of vernalizing temperatures and durations based on different flowering responses of Campanula ‘Birch Hybrid’. Clonally propagated plants of Campanula ‘Birch Hybrid’ were exposed to −2.5, 0, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, or 20 °C for 0, 3, 5, 7, 9, or 12 weeks and were subsequently grown at 20 °C in a greenhouse. Campanula ‘Birch Hybrid’ exhibited a near-obligate vernalization requirement, and all flowering responses studied were influenced by the treatment temperatures, durations, and their interactions. The minimum and maximum cardinal temperatures for vernalization were <0 °C and between 15 and 17.5 °C, respectively. The range of optimal vernalizing temperatures (T_opt) varied based on the flowering response assessed. For instance, T_opt for flowering percentage ranged between 2.5 to 7.5 °C, while T_opt for number of open flowers was 0 to 12.5 °C when plants were vernalized for 5 weeks. T_opt for flowering time also varied when analyzed as rate to flower, time to flower from the end of temperature treatments, total time to flower measured from the start of temperature treatments, and thermal time to flower. For example, after 12 weeks of treatment, T_opt for thermal time to flower was 0 to 2.5 °C yet shifted to 2.5 to 12.5 °C for total time to flower. Because the flowering response being assessed altered the T_opt, this study reiterates the significance of considering all relevant flowering responses while developing and interpreting vernalization models.

Three ornamental grasses, each within the families Cyperaceae [leatherleaf sedge (Carex buchananii), ‘Frosted Curls’ sedge (Carex comans), and ‘Toffee Twist’ sedge (Carex flagellifera)] and Poaceae [‘Rosea’ pampas grass (Cortaderia selloana), ‘Gracillimus’ miscanthus (Miscanthus sinensis), and muhly grass (Muhlenbergia capillaris)], received two foliar sprays 2 weeks apart of benzyladenine (BA) at 500 or 1000 mg·L⁻¹, trinexapac-ethyl (TE) at 220 mg·L⁻¹, or uniconazole at 20 or 40 mg·L⁻¹. The influence of these spray applications on plant height and tiller number was assessed 0, 2, 4, and 8 weeks after the initial treatment (WAIT). Benzyladenine applications did not suppress the height of leatherleaf sedge or ‘Gracillimus’ miscanthus, yet did suppress the height of the other ornamental grasses by <15% compared to the controls, depending on the concentration used and the time. Applications of BA increased tiller production only in ‘Toffee Twist’ sedge at 2 and 4 WAIT compared to the controls; however, at 8 WAIT, this increase was diminished. Depending on the species, uniconazole suppressed the height of the Cyperaceae grasses by 11% to 22% compared to the controls at 8 WAIT. In Poaceae species, uniconazole suppressed the height of only ‘Rosea’ pampas grass by up to 32% compared to the controls. Uniconazole applications did not increase the tillering of any ornamental grasses tested, except ‘Toffee Twist’ sedge at 8 WAIT. Within Cyperaceae, TE suppressed the height of only ‘Toffee Twist’ sedge compared to the controls, while TE effectively controlled the height of all Poaceae grasses. Based on the species and time, TE application elicited up to 37% height suppression compared to the controls of Poaceae grasses, while it did not influence the tiller number of any ornamental grasses in this study.

Floriculture crop species that are inefficient at iron uptake are susceptible to developing iron deficiency symptoms in container production at high substrate pH. The objective of this study was to compare genotypes of iron-inefficient calibrachoa (Calibrachoa ×hybrid Cerv.) in terms of their susceptibility to showing iron deficiency symptoms when grown at high vs. low substrate pH. In a greenhouse factorial experiment, 24 genotypes of calibrachoa were grown in peat:perlite substrate at low pH (5.4) and high pH (7.1). Shoot dry weight, leaf SPAD chlorophyll index, flower index value, and shoot iron concentration were measured after 13 weeks at each substrate pH level. Of the 24 genotypes, analysis of variance (ANOVA) found that 19 genotypes had lower SPAD and 18 genotypes had reduced shoot dry weight at high substrate pH compared with SPAD and dry weight at low substrate pH. High substrate pH had less effect on flower index and shoot iron concentration than the pH effect on SPAD or shoot dry weight. No visual symptoms of iron deficiency were observed at low substrate pH. Genotypes were separated into three groups using k-means cluster analysis, based on the four measured variables (SPAD, dry weight, flower index, and iron concentration in shoot tissue). These four variables were each expressed as the percent reduction in measured responses at high vs. low substrate pH. Greater percent reduction values indicated increased sensitivity of genotypes to high substrate pH. The three clusters, which about represented high, medium, or low sensitivity to high substrate pH, averaged 59.7%, 42.8%, and 25.2% reduction in SPAD, 47.7%, 51.0%, and 39.5% reduction in shoot dry weight, and 32.2%, 9.2%, and 27.7% reduction in shoot iron, respectively. Flowering was not different between clusters when tested with ANOVA. The least pH-sensitive cluster included all four genotypes in the breeding series ‘Calipetite’. ‘Calipetite’ also had low shoot dry weight at low substrate pH, indicating low overall vigor. There were no differences between clusters in terms of their effect on substrate pH, which is one potential plant iron-efficiency mechanism in response to low iron availability. This experiment demonstrated an experimental and statistical approach for plant breeders to test sensitivity to substrate pH for iron-inefficient floriculture species.