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