Light Intensity and Relative Humidity Effects on the Regrowth of Newly Grafted Tomato Plants

Grafting successfully and efficiently requires levels of light, temperature, and humidity that promote vascular reconnection and plant growth. Tomato plants are routinely grafted successfully in practice; however, it may be possible to further optimize the process with a heightened understanding of the separate and interactive effects of key environmental variables on plant growth during the healing phase. Therefore, we completed two studies involving a total of eight combinations of light intensity and relative humidity at standard healing temperatures. The use of light-emitting diodes (LEDs) and climate-controlled chambers was central to this purpose. Because they light with less heat than conventional sources, LEDs may be more significant in commercial grafting operations in the future. LEDs are also useful experimentally because they emit customized spectra at highly-controlled intensities. In both studies, four week-old ‘Cherokee Purple’ and ‘Maxifort’ tomato seedlings were splice-grafted and placed under LEDs emitting a 20% red, 20% white and 60% blue mixed spectrum for 12 hours each day. Study 1 light levels were 2–5, 45–65, 140–160, or 290–310 μmol·m^-2·s^-1, all at 25±4 °C and 79±8% relative humidity (RH). Study 2 conditions were the four combinations of 45–65 or 240–260 μmol·m^-2·s^-1 light with 65% or 90% RH, all at 26/20 °C. We hypothesized that these conditions would affect at least one of the nine components of plant growth we tracked after grafting, beginning with plant survival and concluding with stem and leaf traits at 7–10 days after grafting. In both studies, above-ground dry weight and compactness (above-ground dwt/plant height) increased with light intensity. No across-study trends were evident in the other seven measured variables. In Study 2, leaf fresh weight, leaf area relative growth and specific leaf area were influenced by the light x RH interaction. RH affected these variables only at 45–65 but not at 240–260 μmol·m^-2·s^-1, with the combination of 45–65 μmol·m^-2·s^-1 at 90% RH always having the largest value. These results suggest that the common practice of placing newly-grafted plants in low light conditions should be reexamined, especially if alternative light sources are available.