Effect of ABA, GA and fusicoccin on the transmembrane potential during early phases of germination in radish seeds

In the first stages of germination of seeds of the radish (Raphanus sativus) an outflow of K takes place which is immediately followed by its reincorporation; substances which stimulate germination accelerate the reincorporation of K while germination inhibitors slow it down. During early stages of germination the capacity to incorporate K develops precociously and parallel to the germination of seeds treated with phytoregulators. Further-more, a Mg K -dependent ATP-ase activity linked to the membrane, probably invol-ved in transport of K , is found to develop in step with germination. Germination of the seed thus seems to be correlated with the increase in activity linked to the plasmatic membrane. ( 1 ) In order to obtain information as to the state of organization and function of the plasmatic membrane, a study was made of the variations of tra,:!smembrane electric potential during early stages of germination l:oth in seeds treated with phytoregulators and in untreated seeds. PD ( potentiaI difference ) measurements were made on seeds soaked at 0 C and made to germinate in water at 26 C under agitation after removal of the coats. Such measurements were in all cases preceded by a period of equilibration of 30 min in KCI 0.5mM pH 6 with or without the presence of the phytoregulators used. Preliminary tests were made measuring the PO of cells of the embryonal axis, exc!uding the apical zones, and of the cortical cells of the cotyledons. The results showed that the PD of the cortical cells (2nd-3rd layer) both of the embryonal axis and of the cotyledons ' reaches practically the same value; later studies were made on the cortical cells of the embryonal axis from 2 to 4mm from the root apex since this area permits greater reproducibility and stability of electrophysiologic measurements . During early stages of germination in water the PD rises from a value of -120 mV to -150 mV at 5 h. This successively falls back to stable values around -120 mV after 15 h. In seeds trated with FC ( fusicoccin ), a germination-stimulating phytoregulator which raises K uptake and protons extrusion of protons into the medium, the pattern of PD values observed during germination is similar to that observed in controls but with markedly-higher absolute values; a value of -175 mV after 5 h and over long time intervals ( 15 h) the value becomes stab le around -145 mV . GA3, in agreement with observations for K uptake, would not appear to produce substantial changes in patterns of PO values with respect to controls in the time interval studied. Treatment from the start with ABA in concentrations sufficient to inhibit germination and K uptake also prevents hyperpolarization which is to be observed in controls during the first few hours; PO is practically co nstant in time a t va lues around -110 mV. Removal of ABA from the medium rapidly (in 20 min) produces hyperpolarization o f the PO up to approximately the levels found in the controls which have only been soaked: the increase observed appears to be linked to the favourable effect on seed germination o f the removal of the ABA. These findings can be summed up in the observation that the hyperpolarization which accompanies the early stages of germination is suppressed when germination is inhibited by ABA, whilst on the other hand the initial hyperpolarization and PO level are found to be greater when germination is stimulated by FC. Comparison o f such data with findings relating to K + uptake show that the initial rise of negative potential is not due to loss of K by the cells of the seed. In fact, in seeds grown in ABA the initial hyperpolarization does not take place, and the reincorporation of K is also suppressed. On the contrary it would seem that hyperpolarization can rather be attributed to the activation of an energy-dependent electrogenic mechanism, which is presumably also responsible for the increase of K uptake. (1) Cocucci,S.and Cocucci,M. Plant Science Letters 10,85 (1977) ( 2 ) Lado,P.,Rasi-Caldogno,F.and Colombo,R. Physiol.Plant. 34,359 (1975) 67.