Changes in photosynthetic rates and growth following root treatments of tomato plants with phytohormones

The effects of root applications of kinetin, gibberellic acid (GA₃) and indoleacetic acid (IAA) on photosynthesis was measured using an open infrared CO₂ gas-exchange system. There was a 30-35% increase in the photosynthetic rates (mg CO₂/dm²/hr) of attached leaves within 8 hr following root treatment with 0.47 μM kinetin. On a short-term basis (up to 2 days) 0.47 μM kinetin was shown to have the optimal stimulatory effect on photosynthesis, relative growth rate (RGR) and total plant dry weight. If the roots were in contact with 0.47 μM kinetin for longer than two days there was severe branching of the root system and growth was severely decreased. When plants were left in contact with the kinetin treatment for up to 7 days the optimal stimulatory concentration was considerably lower (0.0047 μM) . Plants receiving a 4, 8, or 12 hr pulse with 0.47 μM kinetin to the roots exhibited higher leaf photosynthetic rates than the control. Plants receiving an 8 or 12 hr pulse with 0.47 μM kinetin maintained photosynthetic rates higher than the control for the duration of the experiment (8 days) while the 4 hr pulse remained higher than the control for only 5 days. A sharp decrease in the photosynthetic rate, RGR and total plant dry weight was observed two days following continual treatments with 0.47 μM kinetin to the roots. At low light levels there was approximately a 100% increase in the photosynthetic rate two days following treatment with 0.47 μM kinetin while at a saturating irradiance there was a 30 to 35% increase. Indoleacetic acid either showed no effect on the photosynthetic rate, RGR and total plant dry weight or an inhibitory effect was observed. Either GA₃ or kinetin alone were shown to stimulate photosynthesis, RGR and total plant dry weight, however, when GA₃ at a 1.4 μM concentration was applied in combination with kinetin at a 0.0047 μM concentration to the roots of tomato plants there was no additive effect. In all cases kinetin dramatically reduced leaf resistance whereas GA₃ had no effect. By supplying either GA₃ or kinetin to the roots of tomato plants a highly reproducible stimulation in the photosynthetic rate, RGR and total plant dry weight can be achieved at physiologically relevant concentrations, whereas IAA appears to have an inhibitory effect.