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ABA and Shading Induce ‘Bartlett’ Pear Abscission and Inhibit Photosynthesis but Are Not Additive

Einhorn, T. C., Arrington, M.
Journal of plant growth regulation 2018 v.37 no.1 pp. 300-308
abscisic acid, additive effect, canopy, carbohydrates, carbon dioxide, carbon dioxide fixation, fruit drop, fruit set, gas exchange, leaves, pears, photosynthesis, rootstocks, shade, stomatal conductance, stomatal movement, trees
Exogenous application of abscisic acid (ABA) can cause fruit abscission. ABA indirectly reduces carbon assimilation via stomatal closure, but may also induce abscission directly through hormone action. Variable thinning of pear suggests an effect of light environment on ABA efficacy. We tested the hypothesis that combinations of ABA and shade will be additive in inducing fruit abscission if they affect different physiological processes. A two-way factorial design was used to evaluate ABA (0, 125 ppm) and shading (0, 44, 77%) on gas exchange and fruit set of 10-year-old ‘Bartlett’ pear trees on OH × F 97 rootstock. Within 6 h from the application of ABA (applied at petal fall), entire canopies of treated and non-treated trees were shaded for 15 days. Single-leaf stomatal conductance (g ₛ) was controlled solely by ABA, whereas photosynthesis (Pn) was regulated by ABA and shade. Under increasing shade, however, Pn inhibition was not increased by ABA. Internal CO₂ of leaves was consistently highest for 77% shade, indicating non-stomatal effects. Fruit set was significantly reduced by both ABA and shade, but a significant interaction suggested that ABA played a negligible role in abscission as shading increased. The data suggest that ABA and shading inhibited Pn by different mechanisms; shading limited light reactions and ABA reduced CO₂ supply. When 125 ppm ABA was combined with high levels of shading, shading placed greater limits on Pn. Had ABA elicited a direct hormonal response to induce fruit abscission, an additive effect of ABA and shade would have been observed. Based on these data, the major role of ABA on pear thinning may be carbohydrate deficit.