Growth and Physiological Responses of Petunia to Chitosan and Arbuscular Mycorrhizal Fungi Under Sustained Water Content Reduction

Abstract

Reducing irrigation inputs is essential for sustainable container crop production; however, the ability of biostimulants to mitigate the effects of deficit irrigation on ornamental crop quality and postharvest often may be crop-specific, product-specific, and application-specific. Two independent experiments were conducted to evaluate whether chitosan applied as a substrate amendment or arbuscular mycorrhizal fungi (AMF) applied during germination could improve growth, physiology, and postharvest performance of petunia ( Petunia milliflora ‘Picobella TM Pink’) under sustained water content reduction. Plants were grown under three container capacity (CC) treatments (100%, 70%, and 40%) combined with chitosan application timing (no application, week 1, or week 3) or AMF application (with or without). After production, plants were exposed to postharvest environments at 30 °C or 40 °C for 2 weeks. The growth index and canopy area decreased by 10% to 40% relative to plants grown at 100% CC under 70% and 40% CC; however, flower coverage percentage remained unaffected during production. Water use reduced by 20% at 70% CC and by up to 50% at 40% CC, while irrigation water use efficiency (IWUE) was maintained with all CC treatments. In the AMF experiment, plants grown at 40% CC with AMF exhibited the highest IWUE and increased root colonization under severe deficit irrigation. Photosynthetic pigment concentrations were generally maintained under deficit irrigation, whereas malondialdehyde concentrations temporarily increased at week 4 under 40% CC, indicating increased oxidative stress. Neither chitosan nor AMF consistently enhanced plant growth or reduced biochemical stress indicators under the evaluated conditions. Chitosan application timing strongly influenced plant responses, with week 3 applications reducing growth and increasing oxidative stress across CC treatments. During the postharvest evaluation, temperature was the primary factor affecting plant performance, with plants maintained at 40 °C exhibiting lower flower coverage and canopy area compared with plants maintained at 30 °C. The combination of deficit irrigation and chitosan application showed limited potential to improve postharvest heat tolerance, whereas AMF application did not improve postharvest performance. Overall, petunia demonstrated substantial tolerance to sustained deficit irrigation, and 70% CC appeared to be a practical strategy for reducing irrigation inputs while maintaining marketable crop quality.