Paclobutrazol Application Improves Grain 2AP Content of Thai Jasmine Rice KDML105 under Low-Salinity Conditions

KDML105 rice (Oryza sativa L.) cultivar, also known as the jasmine rice, is the most valuable fragrant rice of Thailand. The aroma is attributed to a highly 2-acetyl-1-pyrroline (2AP) compound, which is abundantly produced when rice plants are cultivated in the infertile saline soil in northeastern Thailand. The area suitable for production of KDML105 has been limited, and the farmers suffer low grain yield as a trade-off for the premium quality. In this study, we investigated whether foliar application of paclobutrazol (PBZ), which is a plant growth retardant, could mimic the effect of salt stress in promoting the 2AP content of the Thai fragrant rice. PBZ was sprayed on KDML105 rice leaves during the vegetative or reproductive growth stages. PBZ application up to 150 ppm during the vegetative stage was found to have mild effects on growth performances and yield components compared to the reproductive stage. The 100 ppm PBZ foliar spray during the vegetative stage was also found to increase the levels of 2AP and proline, which might be directed toward the 2AP production, in KDML105 rice grains to nearly the levels found in rice cultivated under 0.4% NaCl. The effects of PBZ were found on other tested aromatic verities: PT1 and RD43, and a non-aromatic variety: RD31. However, the amylose content of RD31 was significantly reduced in response to the PBZ application. These results showed that PBZ application is a promising strategy for improving grain quality of fragrant rice, although the effects may vary among rice genetic backgrounds.