Synthetic auxin 3,5,6-TPA increases fruit size of loquat (Eriobotrya japonica Lindl.) by reducing cell turgor pressure

•3,5,6-TPA applied to loquat at cell enlargement decreases cell sap water potential.•Treated fruit accumulates higher amounts of sugars, via water uptake.•Treated fruit grows faster and becomes larger than untreated fruit.•Treated fruit anticipates the loss of plant hormones, allowing for advancing ripening.•Treated fruit changes colour earlier and can be harvested earlier.

In loquat, the synthetic auxin 3,5,6-trichloro-2 pyridyloxiacetic acid (3,5,6-TPA) applied at 15 mg l−1 at the onset of the linear fruit growth stage or one month later during the active fruit growth period, advanced fruit ripening and harvest. The treatment significantly accelerated fruit growth and enhanced final fruit diameter by 10% compared to the control. The experiment was conducted on adult trees of ‘Algerie’ loquat during two consecutive years. 3,5,6-TPA (as a free acid) was sprayed by handgun to the entire tree until the point of run off, using a randomized design with one-tree plots and 8–10 replications. Cell sap water and osmotic potential were measured at the onset of colour change in treated fruit. Fruit growth rate and carbohydrate concentrations were periodically evaluated and at the first harvest date, fruit colour, fruit weight, plant hormone (ABA, GA, IAA, tZ, and JA) content, total soluble solids, titratable acidity, and number of fruits harvested were also recorded. Results confirmed that the effect of 3,5,6-TPA increasing fruit size is due to a reduction of fruit cell turgor pressure that diminished water potential, improving water uptake and increasing carbohydrate contents.