Influence of paclobutrazol and substrate on daily evapotranspiration of potted geranium

The application of plant growth regulators and the use of different culture media are common practices in potted ornamental crops. We report the results of a study to evaluate the effect of two substrates (peat moss or coconut fiber) and spraying 30 ppm paclobutrazol (PAC) on water resource management of zonal geranium (Pelargonium × hortorum L.H. Bailey) seedlings growing in a greenhouse during spring–summer. Growth, water consumption, leaching, evapotranspiration rate (ET), water use efficiency (WUE) and leaf water potential were determined. Plant growth was greater in peat moss (peat) than in coconut fiber (coconut), while the use of PAC decreased plant growth in both substrates, although more markedly in peat. Coconut produced a higher leaching fraction than peat in both PAC-treated plants and non-treated plants, although PAC application increased the leaching fraction in both substrates. Water consumption was lower in coconut than in peat and decreased in response to PAC. Peat was more effective at improving the water-use efficiency and increased the electrical conductivity in the leachate, while PAC decreased the latter in this same substrate. Daily ET showed a maximum of 200 mL/pot in peat and a minimum of 60 mL/pot in the coconut-PAC treatment. During the experimental period, accumulated ET values were higher in peat than in coconut, while the application of PAC reduced the value in peat but increased it in coconut. The evolution of ET per hour during the day showed that the highest rate (23 mL/h) was registered at midday in peat, while about 90% of ET occurred between 10 and 18 h in all treatments. When a cycle of water stress and recovery was applied, the peat induced the highest ET values of all treatments during the first days of drought, but the lowest ET after a week of drought. PAC modified this response, favoring a more balanced behavior of ET, although after the second cycle of drought this effect disappeared. Based on these results, the best strategy in normal irrigation conditions would be to use coconut fiber and not apply PAC, because this provides plants of excellent commercial quality with low water consumption. In plants at moderate water deficit PAC played a more important role than the substrate, improving the water status of plants with both substrates used. However, in the face of severe water stress the effect of PAC disappeared and peat acted as a better substrate than coconut fiber.