Net CO2 Assimilation Rate of Grapevine Leaves in Response to Trunk Girdling and Gibberellic Acid Application

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Abstract
Net CO2 assimilation rate (A), stomatal conductance (gs), and weight per unit leaf area (W) were determined on Thompson Seedless grapevines [Vitis vinifera] grown in the field. Treatments included fruit set applications of gibberellic acid (40 milligrams gibberellic acid (GA3) per liter) to vines, shoots and clusters, alone and in combination with trunk girdling. Leaf A and gs were measured prior to and 3, 6, and 13 days after fruit set. Weight per unit leaf area was determined on leaves collected subsequent to gas exchange measurements. Leaf A of girdled vines was reduced approximately 30% when compared to the control 13 days after treatment. The reduction in A due to girdling was not as great when vines were sprayed with GA3. GA3 sprays alone had no significnat effect on A. Stomatal conductance was reduced by girdling 13 days after treatment. Weight per unit leaf area was 17% greater for trunk girdled vines when compared to the controls. Results indicate GA3 affected net CO2 assimilation rate only on girdled vines, a treatment which increased weight per unit leaf area.