Sugar uptake into strawberry fruit is stimulated by abscisic acid and indoleacetic acid

Changes in sugar uptake into strawberry fruits with maturation and the hormonal effect on uptake mechanisms, though important to fruit development, are not known. Therefore, the kinetics of sugar uptake into strawberry (Fragaria × ananassa Duch cv. Nyoho) fruit tissue and the effects of abscisic acid (ABA) and indoleacetic acid (IAA) on the mechanism of uptake were investigated at 25 and 35 days after pollination (DAP). Uptake of ¹⁴C-sugar was measured over the concentration range of 2 to 30 mM. Uptake kinetics showed a biphasic response to increasing external concentration of ¹⁴C-sugars, and indicated the presence of p-chloromercuribenzenesulfonic acid (PCMBS)-sensitive and PCMBS-insensitive uptake. The K_m value for each sugar was in the range of 10 to 20 mM. Stage of development had no effect on K_m, but V_max for glucose decreased with maturation. Further, sucrose was not taken up through a PCMBS-sensitive transport at 35 DAP. ABA, especially 10 uM, at 25 DAP stimulated uptake of all sugars, mostly through enhanced PCMBS-insensitive uptake but not PCMBS-sensitive uptake. In contrast to ABA, stimulation of sugar uptake by IAA was most effective at 1 μM. The PCMBS-insensitive uptake of each sugar was also stimulated by IAA. Further, the PCMBS-sensitive uptake of glucose was enhanced. The developmental change of PCMBS-sensitive sugar uptake and the effect of ABA and IAA on uptake mechanism in this study are considered to be important in influencing the development and enlargement of fruits.