Fruit ripening gas - ethylene



Ethylene (C2H4, also known as ethene) is a gaseous organic compound that is the simplest of the alkene chemical structures (alkenes contain a carbon-carbon double bond). Ethylene is the most commercially produced organic compound in the world and is used in many industrial applications. Ethylene is also a gaseous plant hormone.



The hormone effects of ethylene on general plant growth were first noted in 1864 when leakage from gas street lighting systems caused stunting and deformation of nearby plants. In 1901 Neljubow identified the active component of the gas to be ethylene but it was not until 1934 that Gane identified that plants could synthesise ethylene and in 1935 Crocker proposed ethylene to be the hormone responsible for fruit ripening and senescence of vegetative tissues.



Research has since demonstrated that ethylene has an important role in many plant development processes, including seed germination, vegetative growth, leaf abscission, flowering, senescence and fruit ripening. Ethylene also plays a role in response to water stress, chilling and mechanical injury.



Ethylene and fruit ripening



Early examples of the human utilisation of ethylene to enhance fruit ripening include the ancient Egyptian practice of gashing figs to enhance ripening responses. The ethylene produced by the injured fruit tissue triggers a broader ripening response. Similarly, the ancient Chinese practice of burning incense in closed rooms with stored pears (ethylene is released as an incense combustion by-product) stimulates ripening of the fruit. The idiom ‘one bad apple spoils the barrel’ is based upon the effect of one apple ripening (or rotting) and emitting ethylene which accelerates the ripening and senescense of apples stored with it.



Considerable research has since been conducted into the effects of ethylene on fruit ripening. Strategies to minimise fruit exposure to external sources of ethylene and treatments for managing the internal ethylene concentration are the key to commercial optimisation of the storage life and eating quality of many fruits. Understanding the fundamental relationship between ethylene and fruit respiration rates during ripening is necessary in order to manage the harvesting, storage and distribution processes.



Ripening and respiration



Ripening is a term applied to fruit that describes the transition from physiological maturity to senescence (ageing and death of the plant tissues). It is a developmental stage evolved to facilitate reproduction by preparing the seed-bearing organ for detachment from the plant. Ripening is the start of significant biochemical and physiological transformations, such as changes in skin colour, internal flesh softening, aroma development and sweetening. Ripening generally begins after fruit has reached maximum size and is physiologically mature.



At physiological maturity fruit have accumulated a range of complex molecules in the form of carbohydrates, proteins, lipids and organic acids. Once detached (harvested) from the plant the fruit continues as a living organism but can no longer draw on water and nutrient from the plant to supply its energy needs and complete the ripening processes. The fruit remains metabolically active and respiration now relies on these accumulated complex molecules.



Respiration is a process of oxidative breakdown (catabolism) of complex molecules into simpler molecules, yielding energy, water, carbon dioxide and simpler molecules needed for other cellular biochemical reactions required for ripening. The respiration rate per unit of fruit weight is (as a general rule) highest in immature fruit, with the respiration rate declining with age. Thus respiration rate of fruit is an indicator of overall metabolic activity level, progression of ripening and potential storage life of the fruit (i.e. a low respiration rate means that the energy reserves will take longer to be consumed and the fruit can be stored for longer).



Some fruits show a significant variation to the pattern of declining respiration rate during their ripening. They exhibit a distinct increase in respiration rates (a respiratory climacteric) of varying intensity and duration, commensurate with ripening. Fruit that exhibit this characteristic increase in respiration rate are classified as ‘climacteric’ whereas fruit that follow the pattern of steadily declining respiration rate through ripening are classified as ‘non-climacteric’.