Enlightening ad- and abaxial photosynthesis: integrating bidirectional illumination in a tomato FSPM

Light sources for greenhouse crops are usually located above the canopy, whether this is solely solar radiation or supplementary light. Therefore, mostly the adaxial side of leaves is illuminated.
A new lighting strategy entails intracanopy lighting with light emitting diodes (LEDs), to decrease the vertical light gradient in the canopy and reduce light losses due to canopy reflection. With intracanopy lighting (see figure) a substantial proportion of light is captured by the abaxial side of leaves.
Leaf anatomical and optical properties result in a gradient of light absorption within the leaf, which is often steeper than the photosynthetic capacity profile. The impact of abaxial lighting on these gradients and the consequences for crop performance are not known yet.

This research aims to elucidate effects of intracanopy lighting on tomato crop photosynthesis. It addresses the quantification of bidirectional light on absorption and photosynthesis at the leaf level and at the whole canopy level by experimenting and functional-structural plant modelling.

To determine light response curves of tomato leaves a growth chamber experiment was conducted. Carbon fixation was measured with an adapted leaf chamber to enable varying ratios of simultaneous ad- and abaxial illumination. Additionally, optical characteristics of tomato leaves were measured to provide data on the absorption under altered light directions. Photosynthesis rates of tomato leaves and canopy were simulated using Farquhar’s biochemical model of photosynthesis in a tomato FSPM in the GroImp platform. Based on the measured data, the photosynthesis model was calibrated and optical properties for the ad- and abaxial sides were implemented.

Photosynthesis for ad- and abaxial illumination differed in the measured tomato leaves. The optical properties and the photosynthesis model of tomato leaves in the FSPM showed the effects of bidirectional light from intracanopy lighting on photosynthesis at leaf and canopy level. The developed FSPM was also used to assess the impact of intracanopy lighting strategies. Given the difference in optical and photosynthetic properties of the ad- and abaxial side of tomato leaves, the ratio of adaxial/abaxial absorbed light has important implications for total crop photosynthesis.