Measurement of trans-membrane and trans-tissue voltages in the Shiraz berry mesocarp

Abstract

AIM: In mid to late ripening, sugar and potassium (K+) accumulation into the berry slows and is eventually completed1. K+ is the most abundant cation in the berry, undertaking important physiological roles. During late ripening, Shiraz mesocarp cells die within the central region of the berry. The cessation of K+ import may be a contributing factor to this loss in cell vitality. Many K+ trans-membrane transporters and channels are regulated by the membrane voltage (Vm). We thus measured trans-membrane voltage (Vm) and trans-tissue voltages (Vt) in the mesocarp during Shiraz berry development.

METHODS: Vm measurement Shiraz berries, grown in Coombe vineyard at the University of Adelaide, were sampled weekly from the completion of véraison to the late-ripening stage. To assess Vm, the microelectrode was inserted through the berry skin and into mesocarp. During injection, voltage signals and the corresponding depths of the micropipette tip were recorded. Vt measurement The Vt was measured by a similar method described above without micropipette injection. A small piece of skin was removed, allowing the measurement of Vt from the pedicel to the mesocarp surface. Living berries and dead berries from véraison and late-ripening stage were used. Dead berries were measured after freezing overnight followed by thawing.

RESULTS: Vm The voltages became less negative with increasing tissue depth. This may be attributed to the more severe hypoxia within deeper regions of the berry2. Voltage responses were detected in both living berries and dead berries in the late-ripening stage, with similar profiles. This indicates that other structures or factors contributed to the voltage detected by this method. Vt In living berries, the Vt values were more negative in véraison berries than those in late-ripening berries. This trend was not observed in dead berries. There was no significant difference between the Vt values measured from living berries and dead berries in late-ripening stage.

CONCLUSIONS

The uneven distribution of the Vm between berry compartments may be correlated with oxygen concentration, which could impact on K+ transport within berries. The declined Vm and Vt in the late ripening berries could be associated with the cessation of K+ import into berries.