Effects of gravistimuli on osmoregulation in azuki bean epicotyls

Abstract

The effects of hypergravity on growth and osmoregulation were examined in dark-grown azuki bean epicotyls. Elongation growth of epicotyls was promptly suppressed by hypergravity at 300g. On the contrary, the increase in fresh weight of epicotyls during incubation was not suppressed by hypergravity at 300g at least up to 6h. Also, the level of total osmotic solutes increased during epicotyl growth for 6h, which was not affected by hypergravity. These results suggest that azuki bean epicotyls are capable of maintaining osmoregulation even under 300g conditions for a short period. On the other hand, the increase in fresh weight of epicotyls was suppressed, in addition to suppression of elongation growth, when seedlings were treated with 300g for 24h. The increase in level of total osmotic solutes was also inhibited by 24h hypergravity treatment, which was accounted by the reduced levels of organic solutes, such as sugars and amino acids. Furthermore, the dry weight of seeds decreased during incubation for 24h, but the decrease was inhibited by hypergravity at 300g. Hypergravity treatment at 300g for 24h also increased the pH value of apoplastic solution in epicotyls. Taken together, these results suggest that the translocation of organic solutes from the seed to epicotyls is inhibited by prolonged hypergravity treatment, which may underlie the suppression of epicotyl growth, and that the breakdown of H+ gradient across the plasma membrane in epicotyl cells may be at least partly involved in the reduction of organic solute accumulation under hypergravity conditions.