The Napier grass continues to be a popular forage crop in Nakhonsawan Province and other areas in the northern and east-northern parts of Thailand. Increasing the growth and biomass of Napier grass growing in low-quality sandy soil will increase the economic value of this plant. The stem cutting of two cultivars of Napier grass, cv. Pakchong 1 (Pennisetum purpureum x Pennisetum americanum) and sweet grass (Pennisetum purpureum cv. Mahasarakham), were exposed to two plant growth regulators, gibberellic acid and salicylic acid, and two application methods, soaking for 24 hr and watering after inoculation into the soil, were studied. For one plant growth regulator test, the most appropriate concentration of gibberellic acid and salicylic acid were 0.01 and 100 mg/L, respectively, for the soaking and watering method. There was an interaction between the combination of plant growth regulator type and application method for both cultivars. The best stimulation effect for sweet grass was soaking with 100 mg/L salicylic acids only. The response of Napier grass cv. Pakchong 1 was different. Soaking with 0.01 mg/L gibberellic acid and watering with 0.01 mg/L gibberellic acid was suitable for increasing shoot and root length while soaking with 0.01 mg/L gibberellic acid and watering with 100 mg/L salicylic acid was suitable for increasing fresh and dry weight. This information will be useful for stimulating the growth of Napier grass in agricultural purposes.

• Beiraghdar, M., Yazdapoor, S., Naderi, D., & Zakerin, A. (2014). The effects of various salicylic acid treatments on morphological and physiological features of zoysia grass (Zoysia species). Journal of Novel Applied Sciences, 3(9), 984-987.

• Calvelo-Pereira, R., Monterroso, C., & Macías, F. (2010). Phytotoxicity of hexachlorocyclohexane: Effect on germination and early growth of different plant species. Chemosphere, 79(3), 326-333. https://doi.org/10.1016/j.chemosphere.2010.01.035

• Chouychai, W., & Somtrakoon, K. (2022). Potential of plant growth regulators to enhance arsenic phytostabilization by Pennisetum purpureum cv. Mott. Pertanika Journal of Tropical Agricultural Science, 45(3), 835-851. https://doi.org/10.47836/pjtas.45.3.18

• Chouychai, W., Kruatrachue, M., & Lee, H. (2015). Effect of plant growth regulators on phytoremediation of hexachlorocyclohexane-contaminated soil. International Journal of Phytoremediation, 17(11), 1053-1059. https://doi.org/10.1080/15226514.2014.989309

• Daud, Z., Hatta, M. Z. M., Kassim, A. S. M., Aripin, A. M., & Awang, H. (2014). Analysis of Napier grass (Pennisetum purpureum) as a potential alternative fibre in paper industry. Materials Research Innovations, 18(Sup 6), S6-18-S6-20. https://doi.org/10.1179/1432891714Z.000000000925

• Dawood, M. G., Sadak, M. S., Bakry, B. A., & Karamany, M. F. E. (2019). Comparative studies on the role of benzoic, t-cinnamic, and salicylic acids on growth, some biochemical aspects, and yield of three flax cultivars grown under sandy soil conditions. Bulletin of the National Research Centre, 43, 112. https://doi.org/10.1186/s42269-019-0152-4

• El Karamany, M. F. E., Sadak, M. S., & Bakry, B. A. (2019). Improving quality and quantity of mungbean plant via foliar application of plant growth regulators in sandy soil conditions. Bulletin of the National Research Centre, 43, 61. https://doi.org/10.1186/s42269-019-0099-5

• Graebe, J. E. (1987). Gibberellin biosynthesis and control. Annual Review of Plant Physiology, 38, 419-465. https://doi.org/10.1146/annurev.pp.38.060187.002223

• Hadi, F., Bano, A. & Fuller, M. P. (2010). The improved phytoextraction of lead (Pb) and the growth of maize (Zea mays L.): The role of plant growth regulators (GA3 and IAA) and EDTA alone and in combinations. Chemosphere, 80(4), 457–462. https://doi.org/10.1016/j.chemosphere.2010.04.020

• Hosseini, S. M., Kafi, M., & Arghavani, M. (2015). The effect of salicylic acid on physiological characteristics of Lolium grass (Lolium perenne cv. “Numan”) under drought stress. International Journal of Agronomy and Agricultural Research, 7(1), 7-14.

• J⊘rgensen, S. T., Pookpakdi, A., Tudsri, S., Stölen, O., Ortiz, R., & Christiansen, J. L. (2010). Cultivar-by-cutting height interactions in Napier grass (Pennisetum purpureum Schumach) grown in a tropical rain-fed environment. Acta Agriculturae Scandinavica Section B – Soil and Plant Science, 60(3), 199-210. https://doi.org/10.1080/09064710902817954

• Khan, M. I. R., Iqbal, N., Masood, A., Per, T. S., & Khan, N. A. (2013). Salicylic acid alleviates adverse effects of heat stress on photosynthesis through changes in proline production and ethylene formation. Plant Signaling and Behavior, 8(11), e26374. https://doi.org/10.4161/psb.26374

• Kowitwiwat, A., & Sampanpanish, P. (2020). Phytostabilization of arsenic and manganese in mine tailings using Pennisetum purpureum cv. Mott supplemented with cow manure and acacia wood-derived biochar. Heliyon, 6(7), e04552. https://doi.org/10.1016/j.heliyon.2020.e04552

• Liong, Y. Y., Halis, R., & Mohamed, R. (2013). Chemical characterization of Imperata cylindrica (‘Lalang’) and Pennisetum purpureum (Napier grass) for bioethanol production in Malaysia. Pertanika Journal of Tropical Agricultural Science, 36(S), 109-116.

• Mapato, C., & Wanapat, M. (2018). New roughage source of Pennisetum purpureum cv. Mahasarakham utilization for ruminants feeding under global climate change. Asian-Australasian Journal of Animal Sciences, 31(12), 1890-1896. https://doi.org/10.5713/ajas.18.0210

• Ologidi, C. G., Tanee, F. B. G., & Agbagwa, I. O. (2022). Petroleum hydrocarbons reduction by selected tropical grass species in oil-based drill cuttings contaminated soil. International Journal of Phytoremediation, 25(6), 728-736. https://doi.org/10.1080/15226514.2022.2104808

• Ramadhan, A., Njunie, M. N., & Lewa, K. K. (2015). Effect of planting material and variety on productivity and survival of Napier grass (Pennisetum purpureum Schumach) in the coastal lowlands of Kenya. East African Agricultural and Forestry Journal, 81(1), 40-45. https://doi.org/10.1080/00128325.2015.1040647

• Reddy, K. O., Maheswari, C. U., Shukla, M., & Muzenda, E. (2014). Preparation, chemical composition, characterization, and properties of Napier grass paper sheets. Separation Science and Technology, 49(10), 1527-1534. https://doi.org/10.1080/01496395.2014.893358

• Sihag, S., Brar, B., & Joshi, U. N. (2019). Salicylic acid induces amelioration of chromium toxicity and affects antioxidant enzyme activity in Sorghum bicolor L. International Journal of Phytoremediation, 21(4), 293-304. https://doi.org/10.1080/15226514.2018.1524827

• Somtrakoon, K., & Chouychai, W. (2022a). Effect of salicylic acid and calcium chloride on growth of corn under water stress condition. The Journal of Agricultural Sciences - Sri Lanka, 17(2), 350-359. http://doi.org/10.4038/jas.v17i2.9747

• Somtrakoon, K., & Chouychai, W. (2022b). Gibberellic acid treatment improved pyrene phytoremediation efficiency of ridge gourd (Luffa acutangula (L.) Roxb.) in soil. Soil and Sediment Contamination: An International Journal, 31(2), 253-263, https://doi.org/10.1080/15320383.2021.1926419

• Souri, M. K., & Tohidloo, G. (2019). Effectiveness of different methods of salicylic acid application on growth characteristics of tomato seedlings under salinity. Chemical and Biological Technologies in Agriculture, 6, 26. https://doi.org/10.1186/s40538-019-0169-9

• Taheri, S., Arghavani, M., & Mortazavi, N. (2017). Morphophysiological evaluation of Bermuda grass under salicylic acid treatment in water deficit conditions. Iranian Journal of Horticultural Science, 48(2), 431-442. https://doi.org/10.22059/ijhs.2017.221966.1140

• Thongruang, S., Kleawkleaur, K., Prombut, P., & Manatrinon, S. (2021). Comparisons in yields, forage characteristics, sweetness and nutritive values of sweet grass (Pennisetum purpureum cv. Mahasarakham) and Napier Pak Chong 1 grass (Pennisetum purpureum x Pennisetum americanum) at different cutting ages. Khon Kaen Agriculture Journal, 49(5), 1092-1102. https://doi.org/10.14456/kaj.2021.97

• Xu, Z.-M., Mei, X.-Q., Tan, L., Li, Q.-S., Wang, L.-L., He, B.-Y., Guo, S.-H., Zhou, C., & Ye, H.-J. (2018). Low root/shoot (R/S) biomass ratio can be an indicator of low cadmium accumulation in the shoot of Chinese flowering cabbage (Brassica campestris L. ssp. chinensis var. utilis Tsen et Lee) cultivars. Environmental Science and Pollution Research, 25, 36328-36340. https://doi.org/10.1007/s11356-018-3566-x

• Yakoubi, F., Babou, F. Z., & Belkhodja, M. (2019). Effects of gibberellic and abscisic acids on germination and seedling growth of okra (Abelmoschus esculentus L.) under salt stress. Pertanika Journal of Tropical Agricultural Science, 42(2), 847-860.

• Yasuda, M., Ishii, Y., & Ohta, K. (2014). Napier grass (Pennisetum purpureum Schumach) as raw material for bioethanol production: pretreatment, saccharification, and fermentation. Biotechnology and Bioprocess Engineering, 19, 943-950. https://doi.org/10.1007/s12257-014-0465-y