ABA in roots and leaves of flooded pea plants

Roots of potted pea (Piston sativum L. cv. Feltham First) seedlings were flooded with tap water. Within a few hours of the start of the flooding treatment the content of free ABA in roots increased compared to contents of roots of unflooded control plants but this increase was not statistically significant until the beginning of the second day after flooding. Approximately 36 h after first flooding significant increases in the free-ABA content of leaves were detected. This was 14 h after significant increases in the amount of ABA in the roots of the same plants. There was marked diurnal variation in free-ABA content of leaves and roots of plants that had been flooded for several days, with maximum contents recorded 3 h or more after the beginning of the light period. Very rapidly after the lights were switched oft ABA contents declined. On day 3 of the flooding treatment, there was more than a 5-fold decrease in the free-ABA content of leaves within a few hours of the beginning of the dark period. Radio-immunoassay suggested that a very large proportion of the total ABA in the plant was in a bound form. This form of ABA increased substantially as the flooding period progressed. The importance of variation in ABA content for the control of water relations and gas exchange of flooded plants is discussed.