**3. Discussion**

Previous experiments found that cucumber measured under saturating light and grown under monochromatic red light showed lower photosynthesis than cucumber plants grown under red–blue light, consistent with our findings [10].

The J and Vcmax values calculated suggest that blue light significantly enhances photosynthetic capacity relative to treatments lacking blue light. Since these values are lowest in the R treatment, and significantly higher in the RG and G treatments, we can also conclude that green light improves photosynthetic capacity relative to monochromatic red light. However, because the RG and G treatments have significantly lower J and Vcmax values than treatments containing blue light, the effect of green light must be lesser than that of blue light.

Previously, calculations of Vcmax and J were found to be significantly higher in B than R; however, they found no difference in Vcmax between R and RB, while estimates for Vcmax were significantly higher in RB than R in our experiment [12]. Others calculated Vcmax for a low R:FR treatment as significantly lower than for a high R:FR treatment, while we found Vcmax to be significantly higher in the RGB + FR treatment compared to the RGB treatment [31].

Our findings suggest that green light enhanced net photosynthesis, since values for Vcmax, J, and net photosynthesis were significantly greater in GB relative to B, RG relative to R, and RGB relative to RB (Figure 3). Photosynthesis was also significantly higher in RB relative to B, RG relative to G, and RGB relative to GB, suggesting a red light enhancement. Others have found that a broader spectrum resulted in higher fixation than a red–blue light treatment for tomato and poinsettia but saw no difference in cucumber at ambient CO<sup>2</sup> concentrations [32]. They did observe a difference at elevated CO2, similar to the enhancement we saw for photosynthesis in RGB relative to RB at ambient CO<sup>2</sup> and light levels. Others have found higher fixation in B than R for cucumber while we observed the opposite under ambient conditions [11,12,15,33]. During A vs. C<sup>c</sup> measurements, plants from the B treatment had a higher net photosynthesis level than plants in the R treatment, indicating that the choice of spectral composition, intensity, or both is critically important to comparing net photosynthesis levels between treatments, even when the same light source is used for each treatment.

It is possible that the red and green light enhancement can be described in part by the 'enhancement effect' or 'Emerson effect' which refers to the phenomenon where photosynthesis from combined spectra can be greater than the sum of its parts due to excitation energy distribution between photosystem I and photosystem II [34–36].
