Melatonin chitosan microparticles decrease degradation and increase drought resistance properties of melatonin

Objectives Melatonin is capable of increasing drought tolerance of crops but it is easily degraded when exposed to environment. We tried to protect melatonin from degradation by coating it with chitosan, and tested the drought resistance property of the prepared melatonin chitosan particles (MP-MT).

Methods The MP-MTs were prepared by cross-linking chitosan, sodium tripolyphosphate, and pectin through electrostatic gravitational force to form hybridized particles first, then melatonin was encapsulated into the particles in a certain mass ratio. The optimum addition rate of melatonin was 5 mg of MP-MTs. The surface morphology and functional groups of MP-MT were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The release rate of melatonin was tested by dissolving MT-MPs in pH 5.0 and pH 7.0 phosphate buffer solutions. The photodegradation of melatonin was tested by exposing MP-MT to sunlight. A drought resistance pot experiment was conducted using wheat seedlings with two leaves and one shoot as test materials. The drought stress was kept at 40% of maximum substrate water holding capacity, and 1.0 g/L melatonin solution (MT1.0), chitosan (MP), and 0.5, 1.0, and 1.5 g/L of MT-MP were used as treatments. The growth of plants and roots, leaf SPAD value, antioxidant enzyme activity, and malondialdehyde content of wheat seedlings were investigated on the 8^th day after treatment.

Results The SEMs showed that the MP-MT particles had a rougher surface than melatonin. FTIR results revealed the presence of characteristic peaks of chitosan and melatonin in MP-MT. The release rate of MP-MT was 65.43% and 50.13% at pH 5.0 and pH 7.0 in phosphate buffer solution for 36 h, respectively. The photodegradation rate of melatonin in MP-MTs was 143.37% lower than normal melatonin after 4 h of light exposure. Chitosan did not show any effect on wheat seedlings growth. Further, compared with MT1.0 treatment, the dry weight of wheat seedlings treated with three MP-MT treatments increased by 4.50%–22.73%, the root length increased by 15.92%–32.56%, the activity of peroxidase increased by 19.56%–20.34%, and the content of malondialdehyde decreased by 9.77%–12.30% (P<0.05).

Conclusions The encapsulation of melatonin with chitosan/sodium tripolyphosphate/pectin particles decreases the degradation of melatonin through slow release and retention of melatonin, and thus significantly increases the drought resistance effect of melatonin. Encapsulation also elicited the same drought resistance property with melatonin at lower application rate, making it suitable for practical application in agriculture.