Rapid irradiance fluctuations occur in a greenhouse: Quantification and implication

Several vital processes in crop canopies and their microclimate, such as canopy photosynthesis and transpiration, are affected by fluctuations in irradiance, here called lightflecks. These lightflecks occur both in the field and inside greenhouses, although their dynamics may be different, due to an absence of wind in the greenhouse. When modelling plant and climate processes, these dynamic conditions are often ignored. The current study reports (1) high frequency (i.e. 1s interval) observations of lightflecks inside a greenhouse during several months and (2) a quantification of the effect of these lightflecks on leaf-level photosynthesis, transpiration and energy balance in rose by comparing results from a dynamic and a steady-state biophysical model. We found that lightflecks in the greenhouse were short, typically <5s. Increasing complexity in irradiance fluctuation patterns (i.e. from very artificial to measured time series) reduced the difference between dynamic and steady-state simulations, e.g. for photosynthesis this difference decreased from 50% to 5%; for water-use efficiency the difference decreased from 70% to 5%. The relative contribution of the increase in light sum and fewer delayed responses by compensating drops in light level through additional light supply on plant processes was different. For example, photosynthesis was equally affected by both the increase in light sum and fewer delayed responses (i.e. 19% and 15% respectively), whereas water-use efficiency was mostly a result of less delayed plant responses (i.e. 9% compared to 1%). Our results demonstrate that lightflecks occur frequently in the greenhouse, and have strong effects on crop photosynthesis, water-use efficiency, and microclimate conditions.