Annual bluegrass (Poa annua var. reptans), when grown as a putting green species, is sensitive to winter injury such as ice cover. Inhibiting plant ethylene production could be a way to improve annual bluegrass tolerance of ice encasement. The goals of this study were to determine how winter conditions and ethylene regulatory treatments affect the antioxidant system, fatty acid composition, and apoplastic proteins of annual bluegrass plant tissues. Ethylene-promotive (1-aminocyclopropane-1-carboxylic acid or ethephon) and ethylene inhibition treatments [aminoethoxyvinylglycine (AVG)] were applied to plants in the field during acclimation. Plant plugs were taken and subjected to low temperature (−4 °C) and ice-encasement treatments in growth chamber conditions. Antioxidant activities of ascorbate peroxidase (APX), peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD) were measured along with malondialdehyde content (MDA) and apoplastic protein content in leaf and crown tissue. Saturated and unsaturated fatty acid contents were measured in leaf, crown, and root tissue. Higher unsaturated fatty acids are often associated with greater low-temperature tolerance. Compared with the untreated controls, ethephon-treated annual bluegrass had greater MDA contents, lower POD and SOD activity, and greater saturated and decreased unsaturated fatty acids. Ethylene inhibition treatments caused annual bluegrass to have less saturated fatty acid content and greater unsaturated fatty acid content, a greater content of apoplast proteins, and higher CAT activity when compared with the untreated controls. The activity of APX was greater in AVG-treated annual bluegrass than in controls. Ethylene may reduce physiological health overwinter, and inhibitory treatments may promote winter tolerance by promoting antioxidant activity, apoplast proteins, and the content of unsaturated fatty acids in plant tissues.
Kevin Laskowski and Emily Merewitz
Yingmei Ma and Emily Merewitz
Salt stress is a major problem in turfgrass management. Investigation of metabolites, such as polyamines (PAs) that may improve salt tolerance of turfgrass species, is needed. Two independent growth chamber studies were conducted to evaluate physiological characteristics and changes in PAs, such as putrescine (Put), spermidine (Spd), and spermine (Spm), in response to salt stress in ‘Penncross’ and ‘PsgSLTZ’ creeping bentgrass (Agrostis stolonifera). The study also aimed to determine a method of PA extraction to improve PA yields from creeping bentgrass. Salt solutions were drench applied to plants growing in pure sand daily in a stepwise manner for ≈70 days in both studies. For both cultivars, salt stress caused an increase in leaf Na+ content, percent of electrolyte leakage (EL), and canopy temperature depression (CTD) while it caused a decrease in turf quality (TQ), osmotic potential (Ψs), and K+ and Ca2+ content compared with controls. In the early stages of salt stress, Put content increased in salt-stressed plants compared with controls. Spd content did not change significantly while a transient increase in Spm was observed in the later stage of salt stress. The PA quantification method used in this study included using formic acid during the extraction process, which exhibited enhanced quantification of PAs from creeping bentgrass compared with other methods previously published. Salinity stress upregulated the content of Put and Spm in leaf tissue, which may be involved in salinity tolerance in creeping bentgrass, while Spd accumulation may not be a major salt tolerance mechanism; supplementation with these biochemical compounds could be an alternative to improve creeping bentgrass salt tolerance.
Ice encasement of perennial cool-season turfgrasses is a common problem in many northern regions of the world, and the incidence of ice encasement may increase with climate change. The objective of this review was to discuss recent advances in knowledge of how ice encasement affects turfgrass systems, current knowledge gaps, and current and potential future management strategies that can be used by turfgrass managers to mitigate ice encasement damage to turfgrass species that are sensitive to this stress. Ice encasement is a complex and severe stress, which if prolonged can include low temperatures, anoxia, toxic gases, toxic metabolic by-products, and other complications associated with the stress. Thus, research is needed to specifically identify responses of different turfgrasses to this stress. Species such as annual bluegrass (Poa annua) are widespread in the turfgrass industry but do not have adequate tolerance of ice encasement and extensive plant necrosis can occur. Repairs or renovations of large areas damaged by ice encasement is costly. Research on ice encasement of turfgrass species is needed to provide efficient recommendations and management strategies to the turfgrass industry.
Sanalkumar Krishnan and Emily B. Merewitz
Salinity stress is becoming more prevalent in turfgrass management with the increasing use of recycled water for irrigation. Creeping bentgrass (Agrostis stolonifera) is a cool-season turfgrass species that contains significant cultivar variation in salt stress tolerance, but the mechanism related to this cultivar variation is not well understood. Our objectives were to determine whether differential hormone content could play a role in cultivar variation of salt responses and to evaluate whether cell viability assays using dye techniques could differentiate salt stress damage levels in turfgrass species. Therefore, a growth chamber study with potted plants was conducted to evaluate salt ion concentrations, physiological responses, and hormone analysis [abscisic acid (ABA), indole-3-acetic acid (IAA), jasmonic acid (JA), salicylic acid (SA), zeatin riboside (ZR), and ethylene] at 4, 8, and 12 dS·m−1 in relatively salt-tolerant ‘Mariner’ compared with salt-sensitive ‘Penncross’ creeping bentgrass. A hydroponics-based growth chamber study was performed for evaluation of whether dead-cell stains coupled with image analysis could be a quick method for indicating cell viability variation between cultivars. Greater salt tolerance was evident in ‘Mariner’ at 12 dS·m−1, which showed significantly lower electrolyte leakage, higher leaf relative water content (RWC), osmotic potential, photochemical efficiency, and photochemical yield compared with ‘Penncross’. A higher K+ and lower Na+ content was maintained in leaves of ‘Mariner’ compared with ‘Penncross’ while roots of ‘Mariner’ maintained higher Ca2+ content under stressed and nonstressed conditions. Phytohormone levels showed a decline in salt-stressed roots compared with nonstressed plants but ‘Mariner’ roots were able to maintain levels higher than ‘Penncross’. ‘Mariner’ leaves showed an increased accumulation of ABA, JA, SA, and ZR while roots maintained higher IAA and SA compared with ‘Penncross’. The results suggest that ‘Mariner’ was able to mitigate salt stress by better ion regulation and differential regulation of hormones compared with ‘Penncross’. ‘Mariner’ leaves and roots showed significantly lower dead cells compared with ‘Penncross’ under salt stress. The results suggest that staining for cell viability could be a useful technique for studying turfgrass stress or other cellular responses.
Sanalkumar Krishnan and Emily B. Merewitz
Polyamines (PAs), spermine (Spm), and spermidine (Spd) may enhance the abiotic stress tolerance and growth of creeping bentgrass (Agrostis stolonifera). Growth chamber studies were conducted to investigate the effect of PA application on the physiological response and hormone content in creeping bentgrass ‘Penn-G2’ under drought. Spm (1 mm) and Spd (5 mm) were applied exogenously under drought or well-watered conditions. PA-treated plants maintained significantly higher turf quality (TQ), relative water content (RWC), photochemical efficiency, and membrane health while maintaining lower canopy temperature. Spm at the 1-mm rate had a 2.46-fold higher osmotic adjustment (OA) at 10 d compared with control plants. A greater content of gibberellic acid (GA) isoforms (GA1, GA4, and GA20) were observed compared with controls during both studies for PA-treated plants under drought. After 7 days of drought stress in Expt.1, GA1 levels were 3.26 higher for Spm 1-mm-treated plants compared with drought controls. GA4 contents were 69% and 65% higher compared with drought-stressed-untreated plants for Spd 5-mm application after 9 and 11 days. Higher levels of GA20 were observed at 10 days (Spd 5 mm, 108.9% higher) due to PA treatment compared with drought controls. In addition to differential regulation of GA isoforms, we observed enhanced abscisic acid (ABA) due to PA application; however, not on a consistent basis. This study showed that PA application may play a role in GA1, GA4, and ABA accumulation in creeping bentgrass ‘Penn G-2’ under drought stress.
Emily B. Merewitz and Sha Liu
Naturally derived products that may enhance the functionality of fertilizers or other agricultural inputs are needed to reduce inputs associated with stress damage and increase the sustainability of turfgrass management. Damage to high-value creeping bentgrass (Agrostis stolonifera) turf areas caused by heat stress is a widespread problem. This study aimed to evaluate multiple, diverse treatments that may illicit antioxidant responses in plants, melatonin, rutin, and Si, when applied as foliar pretreatments to heat stress. Creeping bentgrass plants were grown in growth chambers at optimal (23 °C) or heat stress conditions (35 °C). Turfgrass quality, chlorophyll content, leaf electrolyte leakage, photochemical efficiency, lipid peroxidation, antioxidant enzyme activity, and fatty acid content were measured to determine the effects of foliar treatments on heat stress responses. Melatonin, Si, and rutin were all found to improve some or all of the physiological parameters measured in the study, but only melatonin and Si reduced lipid peroxidation, increased antioxidant enzyme activity, and altered fatty acid contents. Melatonin- and Si-treated plants had greater superoxide dismutase and peroxidase activity and increased the content of the unsaturated fatty acid, linoleic acid, in creeping bentgrass leaves during heat stress compared with controls. Rutin improved turf quality and reduced electrolyte leakage during heat stress, but the mechanism associated with these changes is unclear because no changes were found in antioxidant enzyme activities or fatty acids. Melatonin and Si treatment promoted antioxidant enzyme activity and linoleic acid content of leaves, which have been associated with the improved heat tolerance of creeping bentgrass plants.
Sanalkumar Krishnan, Yingmei Ma, and Emily Merewitz
Mowing frequencies are associated with differences in disease susceptibility of turfgrasses, but how hormones respond to mowing practices are not fully understood. Two independent growth chamber experiments were conducted to determine how leaf trimming and heat stress play a role in modulating endogenous hormones within creeping bentgrass (Agrostis stolonifera) leaf tissues. The study also aimed to evaluate whether there are hormone changes at 0, 15, and 30 minutes after leaf trimming (wounding). The effects of trimming and temperature on sod plugs of creeping bentgrass ‘Penncross’ and ‘Penn-G2’ were investigated under optimal conditions (23/20 °C day/night) and heat stress (30/25 °C day/night). Plants were 1) untrimmed and sampled by plucking at the leaf base, 2) untrimmed and sampled by cutting at 0, 15, and 30 minutes, or 3) trimmed once every 3 days. Salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), and indole-3-acetic acid (IAA) were generally greater in untrimmed plants compared with plants trimmed every 3 days under high temperature conditions. Zeatin riboside (ZR) was lower in untrimmed plants compared with plants trimmed every 3 days. JA and gibberellic acid (GA) accumulated to greater levels in the plants after 15 and 30 minutes of sampling. Polyamines (PAs) exhibited a transient increase in putrescine (Put) due to wounding. The results demonstrate the importance of research practices that consider the timing of sampling turfgrass plants for hormone analysis, help elucidate why mowing practices may play a role in stress susceptibility, and may be applicable to various studies related to leaf wounding.
Vijaya Shukla, Yingmei Ma, and Emily Merewitz
Polyamines (PAs) such as spermidine (Spd), spermine (Spm), and putrescine are involved in various biological functions including abiotic stress response. Whether PAs play an important role in cool-season turfgrass tolerance of drought stress is not well investigated. We have conducted a series of growth chamber (GC) studies including one hydroponic and two soil-based GC studies with creeping bentgrass (Agrostis stolonifera) ‘Penncross’ and ‘Penn-G2’ to determine whether exogenous application of PAs may affect plant growth and stress tolerance. Application of relatively low concentrations of Spd (500 or 750 μM) or Spm (500 μM) promoted tillering rates under optimal growth conditions in hydroponics. The same levels of PA treatments moderated the damages associated with drought stress in the soil-based GC studies. The most notable differences in drought response associated with PA treatment were increased membrane health. This was observed as greater photochemical efficiency, higher quantum yield, less electrolyte leakage, and less lipid peroxidation (malondialdehyde content) in PA-treated plants compared with control plants. The relatively low level of exogenous PAs used in this study did not have a major effect on plant water relations under drought stress. Canopy temperatures and soil moisture content were unaffected by any PA treatment; however, on some days during early drought stress, relative water content was significantly higher in PA-treated plants compared with controls. PA could play a major role in protecting photosynthetic and cellular membranes during drought stress of creeping bentgrass.
Emily B. Merewitz, Thomas Gianfagna, and Bingru Huang
Drought stress is a widespread abiotic stress that causes a decline in plant growth. Drought injury symptoms have been associated with an inhibition in cytokinin (CK) synthesis. The objectives of this study were to investigate whether expression of a gene (ipt) encoding the enzyme adenine isopentenyl phosphotransferase for CK synthesis ligated to a senescence-activated promoter (SAG12) or a heat shock promoter (HSP18.2) would improve drought tolerance in creeping bentgrass (Agrostis stolonifera) and to examine shoot and root growth responses to drought stress associated with changes in endogenous production of CK, and the proportional change in CK and abscisic acid (ABA) due to ipt transformation. Most SAG12-ipt and HSP18.2-ipt transgenic lines exhibited significantly higher turf quality, photochemical efficiency, chlorophyll content, leaf relative water content, and root:shoot ratio under drought stress than the null transformant or the wild-type ‘Penncross’ plants. Transgenic lines that had better growth and turf performance generally had higher CK content and a higher CK-to-ABA ratio, although the direct correlation of CK and ABA content with individual physiological parameters in individual lines was not clear. Our results demonstrated that expressing ipt resulted in the improvement of turf performance under drought stress in creeping bentgrass in some of the transgenic plants with SAG12-ipt or HSP18.2-ipt, which could be associated with the suppression of leaf senescence and promoting root growth relative to shoot growth due to the maintenance of higher CK level and a higher ratio of CK to ABA.
Xiuju Bian, Emily Merewitz, and Bingru Huang
Understanding factors influencing drought resistance traits is important for improving turfgrass growth in water-limited environments. The objectives of this study were to examine effects of a plant growth regulator, trinexapac-ethyl (TE), on turf growth and water use for creeping bentgrass (Agrostis stolonifera L.) exposed to drought stress, and to determine changes in the accumulation of solutes involved in osmotic adjustment associated with TE application. Plant foliage of cultivar L-93 was sprayed with 1.95 mL·L−1 of TE at 0.113% a.i. 14 days before and at the initiation of drought stress. TE-treated and untreated plants were exposed to well-watered or drought stress conditions for 28 days in a growth chamber. TE-treated plants exhibited a reduced rate of water depletion from the soil as demonstrated by higher soil water content, lower evapotranspiration rates, and higher leaf relative water content during 28 days of drought stress compared with non-TE-treated plants. During the later phase of drought stress, TE-treated plants had a greater reduction in leaf ψS at full turgor or greater osmotic adjustment, which was associated with increased accumulation of soluble sugars and inorganic ions (Ca and K) in leaves of TE-treated plants. Proline content increased in response to drought stress, but was unaffected by TE application, suggesting that it may not contribute to the effects of TE on osmotic adjustment. TE-treated plants maintained significantly higher turf quality and leaf photochemical efficiency under drought stress. The results suggest that the promotive effects of TE application on turf growth during drought stress were associated with the reduction in water depletion or lower water use and increases in osmotic adjustment due to the accumulation of inorganic solutes and soluble sugars.