Levels of apoplastic solutes in developing strawberry fruit

Abstract
Solutes in the apoplast of sink organs can contributeto apoplastic water potential (ψaw) gradients that influence transport of water and assimilates. However, sampling the apoplast solution is often technically difficult. Three methods were used to collect apoplastic solution from developing strawberry fruit for determining apoplastic solute potential (ψas) and sucrose and glucose concentrations. In addition,ψas was was estimated from the difference between ψaw (measured via hygrometry) and fruit xylem pressure potential (ψx-press) (measured with a pressure chamber). As strawberry fruit developed, ψaw decreased linearly whereas ψx-press declined slightly then increased, therefore, the estimated ψaw was also decreased. Liquid was recovered from tissue plugs of ripening fruit using centrifugation.The solute potential (ψs) of these samples from pink and red fruit varied from –0.9 to –1.1 MPa and was similar to that of the bulk fruit solute potential (ψs-bulk).A novel method was developed to sample the apo-plastic solution, with little or no cell damage, by collecting liquid on filter paper discs inserted into the hollow cavity naturally formed in ripening fruit. Here the ψs was about –1.0 MPa, again similar to ψs-bulk, and sucrose and glucose concentrations were each near 50 mM. Solution recovered from ripening fruit by centrifugation and paper discs from the fruit cavity was red, suggesting leakage of anthocyanins into the apoplast. Liquid collected from the pedicel xylem of detached fruit using a pressure chamber was colour-less, had a high ψs (about –0.3 MPa), and low sugar concentrations (≺1mM) through out fruit development, suggesting that this method is not effective for sampling the fruit apoplast. Sugars accounted for onlyabout 35%of the ips of the apoplast of ripening strawberry fruit.