‘Pathumthani1’ rice is a new fragrant rice (Oryza sativa L. var. indica) cultivar in Thailand, which is mainly grown in the
central part of Thailand. Polymeric and oligomeric chitosan with 80 and 90% degree of deacetylation (DD) dissolved
in 1% (v/v) acetic acid were tested at 5–80 mg/L for the ability to stimulate plant growth by seed-soaking and foliar
spraying of 14-d-old seedlings. Although all four types of chitosan inhibited rice seedling growth, this was likely to be
due to the negative growth effects of the acetic acid solvent. Lactic acid showed a markedly lower toxicity to the rice
seedlings than acetic acid and citric acid. Accordingly, using lactic acid as the chitosan solvent, polymeric chitosan with
a 90% DD at an applied dose of 40 mg/L was found to be the best plant growth stimulator, resulting in an 11% and 12%
increase in the shoot and root fresh weights, respectively, and a significant increase in the plant height and root length of
21-d-old seedlings compared to that of the water control. However, neither of the oligomeric chitosan forms at the same
concentration exerted any significant growth enhancement effects, but rather they were inhibitory or neutral. Whilst lactic
acid may then be an alternative solvent for chitosan application in agriculture, it seems likely that the solvent, as well as
type and concentration of chitosan, is required to be optimized for each plant and cultivar prior to application.