The acclimation of photosynthesis in cucumber leaves to different ratios of red and blue light

The scope of processes regulated by blue light, such as stomatal opening and chloroplast movement, has become clearer. However, little quantitative work has been done on the effect of blue light on leaf functioning. We investigated the effect of different ratios of red and blue light (from LEDs) on the photosynthetic characteristics of young cucumber leaves. The plants were grown hydroponically under 100 µmolm-2s-1 irradiance, of which 0, 7, 16, 31 and 49 % was blue. An increasing blue fraction (up to 31%) increased maximum photosynthesis up to four times that produced by red light alone. The photosynthesis rates at growth irradiance increased up to 16% blue (2-fold compared to red alone). The increased photosynthesis was accompanied by an increase in chlorophyll content per cm2, but not per mg dry weight. Thus the leaf weight per cm2 increased accordingly. The low photosynthesis rates at low blue light ratios were not, or at least not solely, due to lower internal CO2 concentrations. At light-limited irradiance levels photosynthesis was measured together with FPSII at 21% and 2% O2. The gross CO2 fixation was identically correlated with the product of FPSII and the absorbed irradiance for all blue light fractions at 2 % O2. At 0% blue and 31% blue the irradiance/FPSII relationships were similar at both 2% and 21% O2 (370 ppm CO2), though between the light treatments the relationships differed greatly. Remarkably, at 7% blue FPSII declined considerably less with increasing irradiance at 2% O2, than it did at 21%. This phenomenon has not yet been explained.