Anecdotal evidence of ethylene effects on plants was noted in the late nineteenth and early twentieth centuries. Escapes of gas from street lamps were observed to defoliate city trees. Subjective evidence, from kerosene heaters used to degreen citrus and cross ripening of commodities shipped in vessels with apples, suggested that a gaseous hormone exists. Burg and Burg (1965) found that ethylene is the "fruit-ripening hormone" when they used gas chromatography to demonstrate that an increase in ethylene evolution occurs prior to the respiratory climacteric in apples.
Although the primary focus of ethylene research has been in fruit maturation and ripening, it also plays a role in regulating vegetative growth. In dark-grown seedlings, ethylene controls plumular expansion and hypocotyl development. Ethylene is also produced in response to tissue wounding. Klein and Faust (1978) demonstrated an enhancement in ethylene levels in shoots subjected to severe summer tipping. Ethylene also increases in response to branch bending. As such, it suppresses extension growth and stimulates radial growth, stiffening limbs. When applied exogenously to apple trees, ethylene can suppress shoot growth and stimulate flowering in the subsequent season.
252 CONCISE ENCYCLOPEDIA OF TEMPERATE TREE FRUIT MODE OF ACTION
Our view of the mode of action of plant hormones has changed markedly in the past two decades. After the discovery of cytokinins, and the advent of widespread use of micropropagation, the hormonal balance theory was in favor (Dilley, 1969). That hypothesis pitted promoters against inhibitors in regulation of fruit and fruit tree growth and development. With recent advances in molecular biology, it is recognized that hormones are messengers that work to induce changes in response through specific developmental signals and pathways. Hormones facilitate the synthesis of new messages and enzymes to carry out developmental changes.
During the past century, scientists have identified five plant hormones. These highly powerful organic molecules regulate cell division, cell expansion, and cell differentiation. At the organ level, they also regulate numerous developmental processes. The results of basic research provide insights into the complex mechanisms controlling tree and fruit growth and development. From these studies, many commercial methods for improving productivity and quality have been developed.
Related Topics: CARBOHYDRATE PARTITIONING AND PLANT GROWTH; FLOWER BUD FORMATION, POLLINATION, AND FRUIT SET; HIGH-DENSITY ORCHARDS; PLANT GROWTH REGULATION; POSTHARVEST FRUIT PHYSIOLOGY; TRAINING AND PRUNING PRINCIPLES
Burg, S. P. and E. A. Burg (1965). Ethylene action and ripening of fruits. Science 148:1190-1196.
Chan, B. and J. C. Cain (1967). Effect of seed formation on subsequent flowering in apple. Proc. Amer. Soc. Hort. Sci. 91:63-68.
Dilley, D. R. (1969). Hormonal control of fruit ripening. HortScience 4:111-114.
Klein, J. D. and M. Faust (1978). Internal ethylene content in buds and woody tissues of apple trees. HortScience 13:164-166.
Sachs, T. and K. V. Thimann (1967). The role of auxins and cytokinins in the release of buds from dominance. Amer. J. Bot. 54:136-144.
Westwood, MelvinN. (1993). Temperate-zone pomology: Physiology and culture. Portland, OR: Timber Press.
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