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  • Author or Editor: Michael P. Fuller x
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Extrinsic ice nucleating agents (such as ice-nucleation-active bacteria, dew, etc.) significantly limit the ability of herbaceous plants to supercool. It is believed that with an absence of these extrinsic nucleating agents, a plant could supercool to less than -4 °C. Other evidence, however, indicates that intrinsic nucleating agents may limit the extent of supercooling. Infrared video thermography was used to study freezing in young, `Rutgers' tomato (Lycopersicon esculentum L.) plants and to determine if a hydrophobic barrier on the plant surface could prevent extrinsic nucleating agents such as Ice+ bacterial strain (Cit7) of Pseudomonas syringae Van Hall from initiating freezing within a plant. Freezing tests were conducted in a programmable freezing chamber, a radiative frost chamber, and outdoors. Freezing was visualized and recorded on videotape using an infrared radiometer. Freezing of the plants was induced extrinsically by application of droplets (5 to 7 μL) of water containing Cit7. To provide a barrier to the action of extrinsic ice nucleating agents, an emulsion of hydrophobic kaolin (aluminum silicate mineral) was applied to the plant surface before application of an extrinsic nucleating agent. Results indicate that dry, young tomato plants can supercool to as low as -6 °C whereas plants having a single droplet of Cit7 would freeze at -1.5 to -2.5 °C. Application of the hydrophobic barrier blocked the effect of Cit7 and allowed whole plants to also supercool to -6 °C, despite the presence of frozen droplets on the leaf surface. When whole plants were sprayed with water and Cit7 using an aerosol sprayer and exposed to -3 °C, plants coated with the hydrophobic particle film exhibited significantly less foliar injury then nontreated plants. Similar results were obtained using the radiative frost chamber. Experiments conducted under natural frost conditions also resulted in less injury to the coated plants. The hydrophobic kaolin particle film performed better at preventing plants from freezing due to extrinsic ice nucleation than nonaltered, hydrophyllic kaolin alone or an antitranspirant with putative frost protection properties.

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Factors that determine when and to what extent a plant will freeze are complex. Although thermocouples have served as the main method of monitoring the freezing process in plants, infrared (IR) thermography offers distinct advantages and the use of this latter technology has provided new insights on the processes of ice nucleation and propagation. This technology is based on the fact that freezing is an exothermic event. The temperature and spatial resolution of a high-resolution IR camera has enabled researchers to clearly define initial sites of nucleation as well as monitor the ice front as it spreads into surrounding tissues. Ice nucleation is induced by both extrinsic and intrinsic nucleators. Ice nucleation-active bacteria and moisture are two major extrinsic agents. In herbaceous plants, the influence of extrinsic ice nucleators on ice nucleation can be moderated by thick cuticles or the application of synthetic hydrophobic barriers. The situation in woody plants, however, is different. Woody plants appear to possess native, intrinsic nucleating agents that are as active as many extrinsic agents. However, the identity of the intrinsic nucleating agents in woody plants is not known. Despite the presence of intrinsic nucleating agents, barriers exist in woody plants that inhibit growth of ice from older stems into primary, lateral appendages. This is important because many tissues in woody plants that are frost-sensitive are flowers and primary, elongating shoot tissues that arise from buds attached to older stems. Pictures derived from video segments of the freezing process and data on the ability to block nucleation through the use of hydrophobic kaolin are provided.

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Black cumin (Nigella sativa) is an important medicinal plant in the pharmacological industry. It is cultivated on a commercial scale, but its seeds have a slow, unsynchronized germination rate. Enhancing seed germination is crucial for improving the production of black cumin. The influence of presowing treatments [gibberellic acid (GA3), potassium nitrate, salicylic acid, and stratification at 4 °C] on seed germination was assessed. Seed germination was determined daily for 30 days, and germination parameters, including final germination percentage (FGP), corrected germination rate, number of days to reach 50% of FGP, and seedling length vigor index, were evaluated. Endogenous contents of GA3 and abscisic acid (ABA) in nonstratified and stratified seeds were estimated using high-performance liquid chromatography (HPLC) and seedling growth was determined in 45-day-old seedlings. All presowing treatments tended to boost early germination for the first 10 days compared with the control. Low concentrations of GA3 at 0.25 g·L−1 also increased FGP (80%) compared with the control group (65.55%). Stratification for 4 weeks provided the greatest FGP value at 95.56%, and stratification for 3 weeks proved to be the most effective treatment for optimal seedling growth. Sodium dodecyl sulphate–polyacrylamide gel electrophoresis patterns of stratified seeds revealed the alteration in intensities of 13 bands and the appearance of a new band (180 kDa) indicating a change in the synthesis of proteins during stratification. Moreover, stratification modulated the endogenous GA3 and ABA contents of black cumin seeds, which alleviated the physiological dormancy and resulted in high and synchronized seed germination.

Open Access