Time as a growth and development factor of plants not only in in vitro cultures
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The paper deals with time as a physical (space-time coordinate with specific properties and measure methods) as well as biological concept. Biological time is running differently taking in account various organizational levels like cell, cell organelles, whole organism and, finally, population or species. Time is a factor that regulates plant ontogenesis. In many plant species, time regulates seed dormancy and plant vernalization, photoperiodism and circadian rhythms. Time sets in motion the 'biological clock' which controls physiological and biochemical processes in plant, particularly the rate of enzymatic reactions, oscillation processes (circadian or annual rhythms), as well as internal 'cell clock' deciding upon the length of cell life as well as the rate of cell ageing processes. Senescence or death of particular cells do not mean death of the whole organism. In numerous plant species, death of individual cell is even a factor determining survival of the whole plant. The plant senescence is regulated by phytohormones such as abscisic acid, cytokinins or auxins. Time gathers new meaning in in vitro cultures. It may differ with respect to stable cell suspension culture in many cases maintaining an ability for cell multiplication for many years as compared to another fast regenerating cultures. The influence of various stress factors under in vitro conditions enables the'switching on' the clock controlling the processes of cell multiplication and differentiation.
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Franciszek Dubert, Instytut Fizjologii Roslin, Polska Akademia Nauk, ul. Niezapominajek 21, Krakow, Poland