Light quality in and between tomato plant rows in a greenhouse

Recently in some large scale greenhouses, young tomato seedlings have been planted between mature crop rows to enable year-round production. Young seedlings receive light intercepted and transmitted/reflected by leaves of mature crops. Light quality (spectrum) changes by these procedures and affects the seedling growth. In particular, red/far red ratio is of main concern, because this ratio affects the stem extension rate via the change in the phytochrome photostationary state (P-fr/P-total). However, there have been no reports on the light quality in and between tomato crop rows. Measurements of light quality in and between mature tomato crop rows were conducted in a semi-arid greenhouse in Tucson, Arizona on a clear day and profiles of R/FR ratio and P-fr/P-total were calculated. When the direct solar radiation penetrated into the canopy in parallel to the row, photon flux density (PFD) in FR at the height of 2.20 m in the passage between the rows was larger than that at the canopy height (3.10 m). Gradual decrease of R/FR with the depth into the canopy was observed in the passage, but in the rows, R/FR took the minimum value at the middle of the height. In the passage, Pfr/Ptotal was almost constant with regard to the height when the direct solar radiation ran parallel to the row, but at other periods slightly decreased with the depth into the canopy from 0.7 at the canopy height to 0.6 at the ground. In the rows, P-fr/P-total took the minimum at the middle of the height.