Germination and Growth Responses of Trifoliate Orange (Poncirus trifoliata L.) Seeds to Glycerol-Chitosan-Based Biostimulant Treatments

Trifoliate orange (Poncirus trifoliata L.), a citrus rootstock valued for its cold hardiness and resistance to broad biotic stress, produces desiccation-sensitive seeds whose successful germination requires optimized pre-sowing treatments for reliable sexual propagation. This study investigated whether chitosan-based coating systems, incorporating glycerol as a plasticizer combined with either an amino acid biostimulant (betaine-proline) or gibberellic acid (GA3), could improve germination kinetics and seedling establishment. Seeds from mature fruits were subjected to 18 treatments across Petri dish and soil bioassays. In Petri dishes, the germination percentage remained uniformly high (86.67–100%), with GA3-containing formulations promoting shoot elongation and proline-betaine combinations favoring root development. In soil, treatments primarily modulated germination kinetics: GA3 combined with chitosan (1.5–2%) increased the germination speed by approximately 3.0–3.4 times relative to the control, while 1.5% chitosan with proline-betaine and glycerol yielded the fastest germination overall. Biomass responses were strongly formulation-dependent: 2% chitosan with GA3 maximized shoot and total dry weight, whereas 2% chitosan with proline-betaine and glycerol substantially improved root dry biomass and overall seedling quality. Glycerol exerted context-dependent effects, enhancing amino acid formulations while reducing performance in GA3 mixtures. These findings support the use of optimized chitosan-based treatments as effective tools for accelerating emergence and improving seedling robustness in rootstock production.