Growth Cycles in Lemna gibba Cultures and Their Effects on Growth Rate and Ultrastructure

Abstract A long‐established axenic culture of Lemna gibba G3 was maintained in exponential growth phase under controlled conditions. Weekly analyses for 2 years showed that the individual plants of the Lemna gibba clone fluctuated between two forms. One extreme consisted of plants light in weight, small in size, and with a high relative growth rate (RGR), the other of heavy, large, and more slowly growing plants. At intervals plants having intermediate characteristics dominated in the stock culture. Indication of an annual growth‐cycle was also found. The magnitude of growth response (weight, RGR, area, and dry matter content) after treatment with abscisic acid (ABA), 6‐benzylaminopurine (BAP), and a combination of the two was different for low‐weight and heavy plants. The heavy plants were more sensitive to ABA and BAP treatment than the low‐weight ones. The accumulation of starch was least in small untreated plants and greatest in ABA treated plants. Large electron transparent globules were found in the chloroplasts of the ABA treated plants and in heavy plants regardless of how they had been treated. The different physiological and ultrastructural characteristics of the two forms of Lemna plants probably reflect an ageing‐rejuvenation cycle. Emphasis is placed on the importance of this cycle when Lemna is used as a model plant in physiological experiments.