Fluctuating Light Takes Crop Photosynthesis on a Rollercoaster Ride

Abstract

Crops are regularly exposed to frequent irradiance fluctuations, which decrease their integrated CO₂ assimilation and affect their phenotype.

Figure 1.

Sunflecks in two crop canopies. A, Irradiance fluctuations above and below a durum wheat and white mustard canopy, logged at 1 s resolution. B, Fraction of the total number of sunflecks as a function of sunfleck duration; calculations based on data displayed in A. Photosynthetically active irradiance (PAR; 400–700 nm) was logged using two LI-190R quantum sensors (Li-Cor Biosciences) and an LI-1400 (Li-Cor) data logger. Data were recorded 10 cm above the ground for measurements below canopies and just above canopies for 6 h (11:00–17:00) on two consecutive days (May 26 and 27, 2017) in Wageningen, The Netherlands (51.97 °N, 5.67 °E, 12 m above sea level). The two days were cloudless with average wind speeds of 3.5 m s⁻¹ and 4.2 m s⁻¹, respectively. In the absence of sunflecks, the irradiance measured below the canopy was 2.4% and 3.7% of above-canopy PAR for white mustard and wheat, respectively, indicating full canopy closure. To detect sunflecks, a baseline was constructed by interpolating PAR values in the absence of sunflecks and defining a sunfleck as the absolute change in PAR with respect to the baseline >10 µmol m⁻² s⁻¹ (this was larger than the measurement error).

Figure 2.

Schematic depiction of dynamic reactions of leaf photosynthetic processes to irradiance fluctuations. The leaf is initially adapted to shade (50 µmol m⁻² s⁻¹), then exposed to strong irradiance (1,000 µmol m⁻² s⁻¹) for 60 min, after which it is shaded again for 35 min. Displayed are net photosynthesis rate (A; red line, continuous), stomatal conductance (g_s; blue line, dots), substomatal CO₂ partial pressure (C_i; green line, long dashes), nonphotochemical quenching (NPQ; gray line, short dashes), and the electron transport efficiency of PSII (Φ_PSII; black line, long dashes and dots). These values are representative of Arabidopsis Col-0, grown in climate chambers at a constant irradiance of 170 µmol m⁻² s⁻¹.