Expansion, apoplastic solutes, and sugar uptake in developing strawberry fruit

Graduation date: 1996Identification of factors that affect water and sugar transport in strawberry could aid\ud in attempts to influence fruit size and quality. Apoplastic water potential (Ψaw) gradients\ud within the strawberry plant, that could influence water and assimilate transport, were\ud identified. Fruit expansion was very sensitive to water stress and occurred when the Ψaw\ud of leaves was higher than that of fruit. Green-white fruit underwent osmotic adjustment in\ud drought stressed plants, but not to levels to where expansion could be maintained during\ud daytime.\ud Solutes in the apoplast of developing strawberry fruit may contribute significantly to\ud a Ψaw gradient within in the plant. We utilized four techniques to determine fruit apoplastic\ud solute potential (Ψas). First, Ψas was estimated from the difference between Ψaw (measured\ud via psychrometry) and the xylem water potential (Ψx)(measured with a pressure chamber);\ud secondly, xylem exudate was collected via a pressure chamber; thirdly, liquid was\ud recovered by centrifugation of plugs of fruit tissue; and finally, apoplastic solution was\ud collected on paper disks by a novel method developed to sample the open pith cavity of\ud ripening strawberry fruit with little or no cell damage. All methods, except the xylem\ud exudate method, suggested moderate levels of apoplastic solutes (more negative Ψas) in\ud ripening fruit, including concentrations of sucrose and glucose each near 50 mM.\ud Turgor-sensitive sugar uptake from the apoplast of sink tissues may represent a\ud point of regulation in translocation of sucrose by influencing the rate of uptake of sucrose\ud from the sink apoplast. To determine whether Ψaw of bathing solutions (and thereby cell\ud turgor) influenced sugar uptake by fruit cortex tissue from the apoplast, disks were cut from\ud green-white fruit and placed into incubation solutions at various osmolarities. Lowering\ud cell turgor decreased in vitro uptake of C¹⁴-sucrose and C¹⁴-glucose by tissue disks.\ud Strawberry fruit cells apparently do not possess a sugar uptake system that is stimulated by\ud a reduction in turgor, as found in some plants