Effect of chitosan on plant growth, physiological traits, rhizome attributes, and curcuminoids content of turmeric (Curcuma longa L.) under withholding water.

Rhizome yield traits and curcuminoids in turmeric (Curcuma longa L.) rhizome are the major determining factors for its production, especially under water-limited environments. Chitosan, a member of biostimulants, regulates physiological adaptation strategy and works as a chemical elicitor in several plant species under withholding water. The objective of the present study was to assess growth characteristics, physiological adaptation, rhizome attributes, total curcuminoids content, and upregulated expression levels of curcuminoids-related genes in turmeric under water withholding using chitosan biostimulant. The response of two contrasting genotypes, high curcuminoids cv. Surat Thani (ST) and low curcuminoids cv. Pichit (PJT) was evaluated under two water management practices (withholding water for 45 days [WD] and daily irrigation/well-watered [WW] condition) with or without foliar application of chitosan at 20 mg L^-1. Leaf area and pseudostem dry weight in ST grown under WD were significantly decreased by 39% and 26%, respectively, over WW condition without chitosan application, whereas the two water management practices resulted in similar responses in plants treated with 20 mg L^-1 chitosan foliar spray. Fresh weight and dry weight of rhizome under WD were decreased by > 40% over WW treatment. Leaf temperature and crop water stress index were sustained at a low level by 20 mg L^-1 chitosan application, resulting in the preservation of leaf osmotic potential and photon yield of PSII, especially in PJT. In contrast, gas exchange parameters such as transpiration rate, net photosynthetic rate, and stomatal conductance were severely affected by WD, which in turn reduced the amount of total soluble sugar. Under WD, the expression levels of curcuminoids-related genes were increased, whereas total curcuminoids content in the turmeric rhizomes was significantly decreased. The results indicate that foliar application of chitosan as a biostimulant plays a positive role in reducing the harmful impact of drought stress and improving growth characteristics; however, the degree of positive effect is dependent on genotype, application dose, and level of water availability.

Supplementary information: The online version contains supplementary material available at 10.1007/s12298-025-01618-x.