Ethylene, oxygen, carbon dioxide, and temperature in postharvest physiology

Ethylene is biosynthesized from methionine by S -adenosyl- L –methionine (SAM) synthetase, 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS), and ACC oxidase (ACO) and plays a crucial role in ripening and senescence of horticultural crops. Ethylene first binds with ethylene receptors (ETR/ERS) localized on the endoplasmic reticulum (ER) membrane, and the signal is transmitted through a pathway involving CTR1→EIN2→EIN3/EIL→ERFs and then to ethylene- responsive genes, leading to ethylene response. Aminoethoxyvinylglycine (AVG) and 1-methylcyclopropene (1-MCP) are potent inhibitors of ACS activity and ethylene perception, respectively, and are commercially utilized for the control of ethylene in horticultural crops. Temperature is the most influential environmental factor for postharvest control of crops because low temperature suppresses most metabolic process, including respiration, thereby dramatically extending storage and shelf life. In chilling-susceptible crops such as bananas, pineapples, and avocados, exposure to temperatures below a critical limit imposes stress and results in chilling injury, with symptoms of pitting and browning of tissues. In pears and kiwifruit, low temperature modulates fruit ripening in an ethylene-dependent and -independent manner, respectively. Elevated CO₂ and reduced O₂ atmospheres have both beneficial and harmful effects on physiology and quality of crops, depending on concentration, duration, temperature, species, and other factors. Low O₂ conditions reduce rates of respiration and ethylene production in crops and extend storage life. When O₂ concentration drops to a certain limit, anaerobic respiration is induced, resulting in physiological damage. The respiratory response of horticultural crops to elevated CO₂ differs among species and may be mediated by the effect of CO₂ on the synthesis or action of ethylene. Excessive exposure to high CO₂ atmosphere induces a stress response, including a shift from aerobic to anaerobic metabolism in sensitive crops, resulting in CO₂ disorder.