Effect of plant growth regulators on postharvest calyx retention of citrus fruit

Abstract

Abscission is a naturally occurring physiological process that enables plant organ separation in response to various developmental, hormonal and/or environmental signals. Postharvest calyx abscission can adversely affect fruit quality and diminish the cosmetic appearance. Synthetic auxins are applied to impede abscission, of which 2,4-dichlorophenoxy acetic acid (2,4-D) is the most widely used in citriculture. However, due to restrictions imposed on the use of 2,4-D, it is imperative to investigate potential alternatives with relatively lower toxicities and similar performance concerning citrus postharvest quality maintenance. This study aimed to evaluate the relative efficacy of five plant growth regulators [amimoethoxyvinyl-glycine (AVG), 3,5,6-trichloro-2-pyridyloxyacetic acid (3,5,6-TPA), 2-(4-amino-3,5-dichloro-6-fluoropyridin-2yl) oxyacetic acid (Fluroxypyr), s-ethyl-4-chloro-O-tolilooxytioacetate (MCPA) and gibberellic acid (GA3)] to minimise postharvest calyx senescence and abscission, with Ethephon as the negative control. All of the auxin treatments positively affected the calyx condition when compared to the control fruit. This study showed potential alternatives for 2,4-D that elicit the same effect on quality parameters, however, citrus type and cultivar influenced susceptibility to calyx senescence and response to plant growth regulators. 3,5,6-TPA showed similar effects to 2,4-D in maintaining calyx condition and overall fruit quality for ‘Nules Clementine’ mandarin. AVG and Fluroxypyr showed comparable effects to 2,4-D in minimising calyx senescence while upholding the commercial quality of other parameters for ‘Esbal Clementine’ mandarin and ‘Washington’ navel orange. MCPA maintained external and internal quality parameters equivalent to 2,4-D in ‘Eureka’ lemon. Ethephon’s effect on calyx condition generated a new theory where ethylene and programmed cell death might be involved in an alternative, generalised senescence route, resulting in calyx fusion.