Rapid irradiance fluctuations in a greenhouse: Effects of diffuse glass on shadeflecks

The intensity of sunlight incident on leaves often fluctuates, affecting physiological processes and plant growth at various temporal and spatial scales. However, sunlight is often diffuse due to clouds and aerosols, and the extent to which the characteristics of fluctuating light intensity change under diffuse compared to direct light is not well quantified. Making use of a glass that converts ca. 45 % of incoming light into diffuse light in a commercial tomato greenhouse, light intensity above the crop was recorded at high frequency (10 Hz) for 4.5 months under both clear and diffuse glass. Dips in light intensity below an upper, calculated baseline intensity were marked as shadeflecks, and their daily number, duration, frequency, amplitude and light integral were recorded. Diffuse glass reduced the number of shadeflecks (44 day −1 vs. 112 day −1 under direct glass), and increased their average length (460 s vs. 250 s per shadefleck). Under both glass types, most shadeflecks were very short (<1 s), and were fewer and weaker in winter than in spring and summer. Short shadeflecks (0.1–0.4 s length) occurred 60–110 % more often under direct than under diffuse glass. It was concluded that glass which makes approximately half of all incoming light diffuse reduces the number of shadeflecks, and tends to increase their length as well as reduce their amplitude. However, despite these effects, fluctuations in light intensity are still surprisingly many and short under diffuse glass. • First study to quantify shadeflecks under diffuse glass using rapid measurements. • Under both clear and diffuse glass, most shadeflecks were short (<1 s). • There were fewer and longer shadeflecks under diffuse compared to clear glass.