Drought continues to be a major limiting factor for creeping bentgrass (Agrostis palustris Huds. A.) quality and persistence on golf course fairways, greens, and tees. Little breeding specifically aimed at improving bentgrass drought resistance has been completed. However, a number of reports indicate that treatment with natural products such as seaweed extracts and humic acids improve cool‐season grass drought resistance possibly by hormonal up‐regulation of plant defense systems against oxidative stress. This study was conducted to determine the response of exogenous natural product treatment of three creeping bentgrass cultivars subjected to drought. ‘Penn G‐2’, ‘L‐93’, and ‘Penncross’ creeping bentgrass were treated with seaweed extract (SWE) at 0.5 kg ha−1, humic acid (HA; 80% a.i.) at 1.5 kg ha−1, alone or in combination, and maintained in a greenhouse at approximately field capacity (−0.01 MPa) or allowed to dry until near the permanent wilting point (−1.5 MPa). Unashed samples of SWE and HA contained 66 μg g−1 and 57 μg g−1 zeatin riboside (ZR), respectively, while ashed samples contained no detectable cytokinins as determined by enzyme‐linked immunosorbent assay (ELISA). There were no significant differences between cultivars in response to drought, except for ZR concentration, which was higher in Penn G‐2 than in L‐93 or Penncross foliage. Turf quality and photochemical efficiency began to decline 14 d into the dry‐down for the control and at 21 d in the natural product‐treated bentgrass. The combination of HA + SWE enhanced root mass (21–68%), and foliar α‐tocopherol (110%) and ZR (38%) contents. This is the first known report indicating that these natural products contain cytokinins and that their application resulted in increased endogenous cytokinin levels, possibly leading to improved creeping bentgrass drought resistance.

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10 grass species examined. Others reported that drought induced a significant increase in the antioxidant status This study was conducted to determine whether the plant endogein plants (Moran et al., 1994; Mukherjee and Choudhuri, nous antioxidant concentration is responsive to exogenous hormonecontaining products (HCPs) in two turfgrass species subjected to 1983; Smirnoff and Colombe, 1988 ). drought. Two-week-old seedlings of tall fescue (Festuca arundinacea Proper application of certain HCPs not only can im-Schreb.) and creeping bentgrass (Agrostis palustris Huds. A.) were prove turfgrass growth but also can enhance stress tolertreated with two HCPs, seaweed extract (SWE) at 326 g ha Ϫ1 or humic ance (Schott and Walter, 1991; Schmidt and Zhang, acid (HA, 25% a.i.) at 5 L ha Ϫ1 , applied alone or in combination and 1997). Seaweed extract and HA contain organic comgrown under either Ϫ0.03 or Ϫ0.5 MPa soil moisture for 5 wk. Growth pounds that generate auxin-or cytokinin-like activity. and antioxidant status of leaves were determined subsequently. The When these materials are applied to plants, they en-HCP treatments significantly improved leaf water status (LWS) and hance plant tolerance to salinity, drought, chilling, and shoot and root growth of the grasses grown under high (Ϫ0.03 MPa) other environmental stresses; increase antioxidant activand low (Ϫ0.5 MPa) soil moisture. ␣-Tocopherol concentration inity; and improve turfgrass quality (Crouch and Van Stacreased significantly and ascorbic acid concentration remained unchanged for drought-stressed compared with nonstressed turfgrass. den, 1993; Fagbenro and Agboola, 1993;Finnie and Van The HCP treatments significantly increased ␣-tocopherol and ascorbic Staden, 1985;Nabati, 1991;O'Donnell, 1973; Nelson acid concentration of the grasses grown under high and low soil moisand van Staden, 1984; Schmidt and Zhang, 1997; Zhang ture. Positive correlation between antioxidants and shoot or root and Schmidt, 1999).

The presence of biologically active substances (BAS) in biosolids may enhance plant stress tolerance and growth, but the underlying mechanisms are not well understood. This greenhouse study investigated the effects of untreated biosolids, containing 2.1 μg indole‐3‐acetic acid (IAA) g−1, and tryptophan‐treated biosolids, containing 15.4 μg IAA g−1, on tall fescue [Lolium arundinaceum (Schreb.) Darbysh.] drought resistance. Treatments included a fertilizer control, indole‐3‐butyric acid (IBA) at 2 μM (hormone control), treated biosolids (2.95 g kg−1 soil), and untreated biosolids (2.97 g kg−1 soil). Nitrogen availability was equalized among treatments, and tall fescue physiological responses were measured under well‐watered or moisture stress conditions. Treated biosolids improved turfgrass quality and root mass under both soil moisture regimes and delayed leaf wilting under moisture stress. At the end of the second drought cycle (2 February), treated biosolids improved quality and photochemical efficiency by 18 and 26% relative to the control. Treated biosolids increased leaf IAA by 122 and 52%, respectively, and trans‐zeatin riboside by 100 and 189%, respectively, during each of two maximum drought periods. Leaf tissue increases of IAA and cytokinin were associated with greater content of these hormones in the soil. The data suggest that biosolids application may alter growth hormone (IAA and cytokinin) content and improve plant drought resistance.

BackgroundSwitchgrass (Panicum virgatum L.) is a warm-season C4 grass that is a target lignocellulosic biofuel species. In many regions, drought stress is one of the major limiting factors for switchgrass growth. The objective of this study was to evaluate the drought tolerance of 49 switchgrass genotypes. The relative drought stress tolerance was determined based on a set of parameters including plant height, leaf length, leaf width, leaf sheath length, leaf relative water content (RWC), electrolyte leakage (EL), photosynthetic rate (Pn), stomatal conductance (gs), transpiration rate (Tr), intercellular CO2 concentration (Ci), and water use efficiency (WUE).ResultsSRAP marker analysis determined that the selected 49 switchgrass genotypes represent a diverse genetic pool of switchgrass germplasm. Principal component analysis (PCA) and drought stress indexes (DSI) of each physiological parameter showed significant differences in the drought stress tolerance among the 49 genotypes. Heatmap and PCA data revealed that physiological parameters are more sensitive than morphological parameters in distinguishing the control and drought treatments. Metabolite profiling data found that under drought stress, the five best drought-tolerant genotypes tended to have higher levels of abscisic acid (ABA), spermine, trehalose, and fructose in comparison to the five most drought-sensitive genotypes.ConclusionBased on PCA ranking value, the genotypes TEM-SEC, TEM-LoDorm, BN-13645-64, Alamo, BN-10860-61, BN-12323-69, TEM-SLC, T-2086, T-2100, T-2101, Caddo, and Blackwell-1 had relatively higher ranking values, indicating that they are more tolerant to drought. In contrast, the genotypes Grif Nebraska 28, Grenville-2, Central Iowa Germplasm, Cave-in-Rock, Dacotah, and Nebraska 28 were found to be relatively sensitive to drought stress. By analyzing physiological response parameters and different metabolic profiles, the methods utilized in this study identified drought-tolerant and drought-sensitive switchgrass genotypes. These results provide a foundation for future research directed at understanding the molecular mechanisms underlying switchgrass tolerance to drought.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0342-8) contains supplementary material, which is available to authorized users.