Incident Far‐Red Photons Drive Leaf Photosynthesis Less Efficiently Than PAR Light, but Are More Effective in Promoting Growth

Recently, far‐red light (FR) in the range 700–750 nm has been reported to have similar photosynthetic efficiency as photosynthetically active radiation (PAR, 400–700 nm), when supplied in combination with PAR. We aimed to investigate if adding FR to PAR is equally efficient in promoting photosynthesis as adding PAR, and if long‐term acclimation to FR would change the short‐term response to FR. Lettuce plants were grown in a climate chamber at two levels of PAR (200 and 400 μmol m −2 s −1 , red/blue light), and at each PAR level there were also treatments with 25% of PAR or FR added. In all six treatments, response curves of leaf net photosynthesis rate (P n ) to different intensities of PAR or PAR + FR were determined. Adding FR to PAR increased P n , but this was only 39%–64% of the increase seen under additional PAR, due to lower absorption of FR than PAR. Absorbed PAR and FR photons had similar photosynthetic efficiency. Leaves grown under FR showed acclimatory responses, such as reduced photosynthetic capacity and pigmentation, but the instantaneous photosynthesis response to FR was unaffected. FR had strong positive effects on growth: Partly substituting PAR by FR increased the radiation use efficiency of growth even when expressed per unit of absorbed radiation.