Increased photosynthetic rates following gibberellic acid treatments to the roots of tomato plants

This study was conducted to evaluate the effects of root applications of gibberellic acid (GA₃) on photosynthesis in tomato plants grown hydroponically. Photosynthetic rates (mg CO₂/dm²/hr) determined using an open infrared CO₂ gas exchange system showed a 40–50% increase within 5 hr after treatment with a 1.4 µM gibberellic acid (GA₃) to their roots. The effect was shown to persist for the duration of the experiment (9 days). Plants receiving pulses of 1.4 µM GA₃ to the roots for 1, 4, 8 or 12 hr exhibited significantly higher photosynthetic rates than the control for 6 days following treatment. By day 9 however, there was no significant difference. Continual treatments with 1.4 µM GA₃ to the roots maintained the photosynthetic rate significantly higher than the control for the duration of the experiment. Interestingly, at the lower light levels the percent stimulation was more dramatic. There was approximately a 90% increase in the photosynthetic rate at 80 µE m^-2 s^-1 while at saturating light conditions (560 µE m^-2 s^-1) there was approximately a 40% increase over the control rate. The light saturation point for both treated and control plants was 240 µE m^-2 s^-1. Applications of physiologically relevant concentrations of GA₃ to the roots of tomato plants stimulates photosynthesis more consistently than that achieved by previous studies involving foliar absorption.Approved for publication on May 28, 1981 as paper number 6242 in the Journal series of the Pennsylvania Agricultural Experiment Station.