Elevated [CO₂] Affected Fluctuating Light Acclimation in Cucumber Plants by Changes in Specific Leaf Area and Photosynthetic Efficiency

Plants continuously acclimate to natural fluctuations in light intensity, and this may be modulated by elevated CO₂ (e[CO₂]) concentrations. How strongly such a combination affects plant and leaf morphology, anatomy, and photosynthetic biochemistry is unknown. We grew cucumber (Cucumis sativus) under sinusoidal (SN) and randomly fluctuating light (FL) intensities, at two CO₂ levels (440 μmol mol⁻¹, a[CO₂]; 860 μmol mol⁻¹, e[CO₂]), and conducted an extensive analysis of photosynthesis, leaf anatomy, plant morphology, and biomass. Under a[CO₂], plants demonstrated larger morphological and anatomical plasticity to FL than SN-grown plants, with larger enhancements of stem height, leaf area, and specific leaf area (SLA). Surprisingly, leaf density instead of thickness explained the 70% higher SLA under FL than SN. Despite these changes, dry biomass was 25% lower under FL than SN, presumably due to lower photosynthetic efficiency and consequently lower diurnal photosynthesis under growth conditions (A_diurnal). e[CO₂] reduced the negative effect of FL on dry biomass, but was still 11% lower under FL compared to SN. This could be attributed to adjustments in SLA, enhanced shoot biomass, and A_diurnal under FL by e[CO₂]. Overall, the negative impact of FL was partially mitigated under e[CO₂] by adjustments in plant morphology and anatomy.