Age-dependent changes of the ion content and the circadian leaf movement period in the Phaseolus pulvinus

The circadian movement of the lamina of primary leaves of Phaseolus coccineus L. depends on circadian changes of the K+, Cl- and (depending on the Cl- availability) malate content in the swelling and shrinking motor cells of the laminar pulvinus. After sowing in soil, the laminar pulvinus develops within about 26 days. When the leaves emerge from the soil (about 6 days after sowing) and the pulvinus starts with the diurnal movement (about 9 days after sowing) the pulvinar dimensions are about half of those of the mature pulvinus. The anatomical structure, however, is basically the same as in the developed pulvinus. In soil-grown plants, the K+, Cl- and malate content as well as the period length of the circadian leaf movement rhythm change in the developing pulvinus. In the embryo of the dry seed, the Cl- content is low (0·03 mmol g-1 DW), the K+ content, however, 22-fold higher than the Cl- content. When the leaves emerge from the soil, the pulvinar K+ and Cl- content is the same as in the whole embryo of the dry seed. In the developing pulvinus the K+ content increases by a factor of 2 and the Cl- content by a factor of 41 in the mature pulvinus. The pulvinar malate content increases between the 6th and 10th day from about 40 to 180 µmol g-1 DW, then decreases until the 17th day and remains thereafter on a low level (around 80 µmol g-1 DW). These results indicate that the Cl- availability increases in the developing pulvinus with age. It explains furthermore why in young leaves malate was found as counterion to K+ in the osmotic leaf movement motor, in older ones, however, Cl-. The circadian leaf movement starts 9 days after sowing. The period length decreases during the development of the pulvinus from 31·3 to 28·6 h in leaves of intact soil-grown plants. In leaves which were cut from the plants and immersed with their petioles in distilled water, the age dependent decrease of the period length is also found. However, the period lengths are shorter by more than 1 h than in the leaves of intact plants. The increasing Cl- availability in the developing pulvinus does not seem to be the cause for the age dependent shortening of the period length, because the period length in 22 days old Cl--deprived pulvini is the same as in 22 days old pulvini with a high Cl- content.