Suboptimal Acclimation of Photosynthesis to Light in Wheat Canopies

Abstract

Photosynthetic acclimation (photoacclimation) is the process whereby leaves alter their morphology and/or biochemistry to optimize photosynthetic efficiency and productivity according to long-term changes in the light environment. The three-dimensional architecture of plant canopies imposes complex light dynamics, but the drivers for photoacclimation in such fluctuating environments are poorly understood. A technique for high-resolution three-dimensional reconstruction was combined with ray tracing to simulate a daily time course of radiation profiles for architecturally contrasting field-grown wheat (Triticum aestivum) canopies. An empirical model of photoacclimation was adapted to predict the optimal distribution of photosynthesis according to the fluctuating light patterns throughout the canopies. While the photoacclimation model output showed good correlation with field-measured gas-exchange data at the top of the canopy, it predicted a lower optimal light-saturated rate of photosynthesis at the base. Leaf Rubisco and protein contents were consistent with the measured optimal light-saturated rate of photosynthesis. We conclude that, although the photosynthetic capacity of leaves is high enough to exploit brief periods of high light within the canopy (particularly toward the base), the frequency and duration of such sunflecks are too small to make acclimation a viable strategy in terms of carbon gain. This suboptimal acclimation renders a large portion of residual photosynthetic capacity unused and reduces photosynthetic nitrogen use efficiency at the canopy level, with further implications for photosynthetic productivity. It is argued that (1) this represents an untapped source of photosynthetic potential and (2) canopy nitrogen could be lowered with no detriment to carbon gain or grain protein content.

Figure 1.

Overview of the reconstruction process. A, Original photograph. B, Point cloud reconstruction using stereocameras (Wu, 2011). C, Output point cloud. D, Mesh following the reconstruction method (Pound et al., 2014). E, Final canopy reconstruction. The multicolored disc in A to C is a calibration target used to optimize the reconstruction process and scale the final reconstructions back to their original units.

Figure 2.

Example canopy reconstructions from front and top-down views. A to C, Preanthesis. D to F, Postanthesis. A and D, Parent line. B and E, Line 1. C and F, Line 2.

Figure 3.

Progressive lowering of the canopy position in a canopy results in the reduction in daily integrated PPFD (μmol m⁻² s⁻¹) but also in the pattern and incidence of high-light events within the canopy. The left side shows a representative reconstructed preanthesis wheat canopy with a single plant in bold. Maximum PPFD ranges are color coded. The right side shows PPFD during the course of a day at nine representative and progressively lower canopy positions (the height of each canopy location from the ground is given in the top left corner of each graph) calculated using ray-tracing techniques.

Figure 4.

Fitted light response curves (LRCs) for preanthesis in the Parent line (A), Line 1 (B), and Line 2 (C) and for postanthesis in the Parent line (D), Line 1 (E), and Line 2 (F). Layers are top (black), middle (dark gray), and bottom (light gray).

Figure 5.

Whole-canopy acclimation model output (blue) versus gas-exchange measurement (red) graphs. The acclimation model was run at 250 locations throughout the canopy depth to predict the optimal P_max at each location dependent upon the light environment that it experienced, calculated via ray tracing. The time-weighted average (Eq. 4) was fixed at τ = 0.2. This is an exponentially decaying weight used to represent the fact that photosynthesis is not able to respond instantaneously to a change in irradiance levels. If τ = 0, a plant will be able to respond instantaneously to a change in irradiance, whereas if τ > 0, the time-weighted average light pattern will relax over the time scale τ. Model results are compared with field-measured gas exchange. A to C, Preanthesis. D to F, Postanthesis. A and D, Parent line. B and E, Line 1. C and F, Line 2.

Figure 6.

Relationships between photosynthesis (P_max taken from fitted LRCs) and Rubisco properties (V_cmax from fitted ACi curves and Rubisco/TSP amount) throughout the canopy depth. A, P_max and Rubisco content. B, P_max and V_cmax. C, P_max and TSP. D, V_cmax and Rubisco content. Black (circles) represents the Parent line, dark gray (triangles) represents Line 1, and light gray (upside-down triangles) represents Line 2.