Fluctuations in light intensity modulate ion fluxes from grape berry mesocarp: Direct evidence from microelectrode ion flux estimations

Canopy architecture and light spectral composition have long been considered as important factors determining grape juice acidity and K⁺ concentrations. In this study, we provide direct evidence that fluctuations in intensity of light incident on berry tissue may significantly modulate net ion fluxes from and into the berry mesocarp. The MIFE technique was used to quantify fluxes of K⁺, H⁺, and Ca²⁺ in Pinot Noir and Emperor grape berries. Transition from dark to light induced complex multiphase kinetics of K⁺ flux resulting in significant shift towards net K⁺ uptake by the peripheral mesocarp tissue. When light intensity was modulated in a square-wave manner with a period of 5 to 15 min (mimicking periodicity of natural variations during a typical autumn day in Hobart), K⁺ flux oscillated with the same frequency and a magnitude of about 100 nmol/m²s. Changes in H⁺ and Ca²⁺ fluxes showed high positive (R = 0.8) and negative (R = 0.9) correlation, respectively, with those in K⁺. To the best of our knowledge, these data from excised fruit provide the first reported evidence that light directly incident on bunches may modulate apoplasmic pH and K⁺ concentration in grape. Possible underlying mechanisms are discussed.