Effect of End-of-day Red Light on Squash Rootstock and Cucumber Hypocotyl Length

Recent advancements in LED technologies have allowed us to better control the light quality that affects the plant morphology without the use of chemical plant growth regulators. Light quality during the day and the end-of-the-day (EOD) influences the physiology and morphology of plants in various ways by changing the phyotochrome form. Previous experiments have shown that application of EOD far-red light produced longer hypocotyl lengths in squash. In vegetable grafting, hypocotyl extension is desired before grafting; however, excessive extension during the post-grafting stage is not acceptable as more compact seedlings are desired for transplanting. In the present experiment we looked at how an application of EOD red light affects morphology and growth of squash rootstock ‘Strongtosa’ (Cucurbita maxima × Cucurbita moschata) and ‘Rembrandt’ cucumbers (Cucurbita sativas). The experiment was performed in a greenhouse from June 16th to July 29th 2015 at an average temperature of 24 °C. The planting density was 625 plants/m² for squash and 173 plants/m² for cucumber. During this time the average photoperiod was 14 h. EOD red light was applied during the twilight period and early night period from 6-9 PM. Due to the lower ratio of red to far-red light during the twilight, red light was expected to increase red to far-red light ratio.  The red photon flux was 3 mmol·m^-2·s^-1 and had a maximum wavelength peak of 632 nm provided by LEDs (HWHM: 16 nm). The application of EOD red light reduced the average squash rootstock hypocotyl length, stem diameter, and dry weight by 0.7 cm (10%), 0.2 cm (6%), and 0.0167 g (13%), respectively, compared with the control without EOD red light treatment. These parameters for cucumber plants were not significantly different from those in the control; presumably due to lower red to far-red light ratio caused by mutual shading as a result of a high planting density relative to their plant size. These results show that the application of EOD red light works but at a limited degree as a suppressor of hypocotyl elongation. This information can be applied in industry practices in order to create a more compact plant.