Optimise 2,4-D Applications and Investigate Alternatives for Citrus Calyx Retention

Abscission is a naturally occurring physiological process that enables plant organ separation in response to various developmental, hormonal and/or environmental stimuli. Synthetic auxins, such as 2,4-dichlorophenoxy acetic acid (2,4-D), are incorporated into postharvest practices to impede citrus calyx deterioration. However, restrictions on 2,4-D use require alternative chemicals and practices. The aim was to assess the efficacy of alternative agro-chemicals, re-evaluate the role of 2,4-D in the packline and determine the impact of postharvest practices, such as degreening and storage, on the calyx condition. Citrus type and cultivar influenced the calyx’s susceptibility to senescence, abscission, and response to plant growth regulators. For mandarin and navel, orange 3,5,6-trichloro-2-pyridyloxyacetic acid (3,5,6-TPA), amimoethoxyvinyl-glycine (AVG) and 2-(4-amino-3,5-dichloro-6-fluoropyridin-2yl) oxyacetic acid (Fluroxypyr) showed similar effects on the calyx to 2,4-D and s-ethyl-4-chloro-O-tolilooxytioacetate (MCPA) for lemon. At shorter storage periods (< 20 days), less 2,4-D can be used, while citrus stored for longer periods (> 30 days) should receive the standard 2,4-D dose. Degreening adversely affected fruit quality when not countered with a preceding 2,4-D drench. Prolonging degreening increased calyx deterioration, especially when 48 hours was exceeded, suggesting a possible ethylene tolerability threshold. Fruit storage at –0.5 °C, 2 °C and 4 °C maintained calyx condition and minimized weight loss, although 2 °C and 4 °C showed greater calyx conservation. Temperature regulation became increasingly important at longer storage durations. Macroscopic calyx evaluations showed that citrus type and cultivar had a degree of influence over calyx abscission sensitivity and progression. Reduced reliance on 2,4-D is possible if degreening could be avoided. However, when color development is inadequate and degreening is required in the early season, this auxin is recommended.