Turfgrasses receive a high number of cultural inputs on a frequent basis for adequate performance and functionality. One of the inputs required for all turfgrass areas is frequent mowing. The most highly managed turfgrass areas, such as golf course putting greens or fairways, can be mowed as often as three times per week, daily, twice daily, or other similar rates (Turgeon, 2011). The importance of mowing practices is not reflected in the recent literature available related to how mowing practices affect turfgrass biochemistry and physiology. With modern advances in equipment and techniques to isolate and detect molecular and physiological changes, a greater understanding of the response of turfgrass species to mowing is possible and is needed. Turfgrass scientists too often have to rely on comparing physiological or hormonal responses to other crop species that are culturally managed highly differently than turfgrass species. No other major crop species is consistently mown as often as a turfgrass stand. Mowing practices alter turfgrass growth and architecture, but how hormones associated with growth changes such as cytokinins, GAs, and auxins in response to mowing are not well documented. Hormone responses in turfgrass species is a critical issue in turfgrass management, due to the widespread use of plant growth regulators and other substances altering plant growth physiology. An understanding of hormone responses is also critical for the development of germplasm with tolerance to abiotic and biotic stresses. Therefore, whether mowing causes hormonal fluctuations within the plant and whether the frequency of mowing has an effect on hormone profiles are two questions that need to be answered.
Mowing height and frequencies have been investigated for effects on turfgrass traits such as turfgrass performance, disease severity, and other treatments such as tolerance to abiotic stresses. Two major diseases, anthracnose (Colletotrichum cereal) and dollar spot (Sclerotinia homeocarpa), have been primarily focused on for evaluation of mowing practices on disease incidence and progression. Inguagiato et al. (2009) found that the hemibiotrophic anthracnose, a parasitic organism in living tissue that also can live in dead tissue, was significantly affected by mowing frequencies and heights. Overall, they concluded that raising the mowing height very small amounts can reduce disease severity. Mowing twice per day reduced disease severity compared with once per day on a limited number of days. However, more frequent mowing (6 d/week) compared with (2 d/week) increased disease occurrence of dollar spot on creeping bentgrass (Putman and Kaminski, 2011). Other reports are consistent in that low mowing heights tend to increase disease severity of anthracnose (Backman et al., 2002; Uddin and Soika, 2003). All of these diseases are most problematic in summer months under high temperatures (Turgeon, 2011). The major cause for mowing height differences are still not well understood and are often attributed to reduced photosynthetic potential, reduced carbohydrate reserves, reduced rooting, and other effects caused by extremely low mowing heights (Beard, 2002). Mowing frequency differences have been attributed to mechanical factors such as dew removal to reduce the wet environment often desired by turfgrass pathogens (Delvalle et al., 2011). Perhaps hormone regulation may play an important role in modulating turfgrass responses to disease incidence under varying heights of cut or frequencies of cut under optimal or high temperature conditions.
Mechanical plant leaf wounding causes both a local and systemic response. The major responses are for wound healing, reallocation of energy resources, or to prevent further attack, such as further insect herbivory or fungal invasion. These responses all require significant modification of many phytohormones and energy to be performed. The preventative responses may only serve as a benefit to the plant if the plants are subsequently attacked by a pathogen (Sanchez-Serrano, 2010). Jasmonic acid is the major hormone involved in mechanical wound responses as it highly regulates wound-induced gene expression leading to induced systemic resistance (ISR) and plays a major role in tolerance to necrotrophic pathogens. ISR is a mechanism that primes plants for enhanced disease or stress tolerance following a select set of environmental exposures, such as exposure to certain microbes or chemicals (Fu and Dong, 2013). Thus, determining whether turfgrasses may moderate these responses due to frequent mowing or continuously express these biochemical pathways is needed. If moderation of JA-regulated wound responses occur to save energy, frequently mowed turfgrass plants may be more susceptible to necrotrophic pathogens. Many necrotrophic pathogens, pathogens that kill host tissues, of highly managed turfgrasses do plague the turfgrass industry, such as dollar spot. SA acts antagonistically to JA or as an inhibitor to wounding responses in plant leaves. SA controls the systemic acquired resistance (SAR) pathway in order for a plant response to pathogen attack. SAR involves a local signal in response to pathogen attack that is converted to systemic defense mechanisms, such as the accumulation of pathogenesis-related proteins (Doares et al., 1995). How SA may be regulated by mowing practices or turfgrass leaf wounding has not yet been investigated.
In addition to other hormone responses due to mowing, how PAs may be associated with wounding responses of plants, particularly of turfgrasses is not well understood. PA biosynthesis and regulation of PA content is closely associated with the phytohormone pathways of SA and JA. PAs are also known to have plant growth regulator function and are classified by some as plant hormones (Davies, 1995). PAs have been shown to play a role in the defense against pathogens (Rossi et al., 2015) and may be directly involved in wounding responses (Lulai et al., 2015; Perez-Amador et al., 2002). Therefore, elucidation of PA responses to mowing is an initial step that could improve our understanding of growth regulation, pathogen defense, and other attributes such as abiotic stress related to mowing practices or leaf wounding. Determining whether PAs may respond to mowing will allow for more accurate comparisons inter- and intraexperimentally and enhance our ability to interpret results within the study of PAs in grass species. These results may also be useful for forage grass species or various sources of wounding stress other than mowing. In addition, how the major hormones described in this paper respond to heat stress in turfgrass species is not well documented.
Therefore, the objectives of the study were to evaluate hormone profiles in response to different leaf trimming practices to simulate mowing frequency under optimal or high temperature conditions and to understand hormone regulation in the early response to leaf wounding. The results of the study are important to better understand and interpret hormonal responses for the promotion of turfgrass science research and management.
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