Evidence of photosynthetic acclimation to self-shading in sugarcane canopies

Abstract

Increasing the efficiency of photosynthesis in sugarcane canopies is the key for improving crop yield. Herein, we evaluated the photosynthetic performance along the canopy of ten sugarcane cultivars and three Saccharum species. Canopy morphological traits were evaluated, and leaf gas exchange was measured in the first (sun-exposed, +1) and the fourth (shaded, +4) fully expanded leaves and under low- and high-light conditions. Similar photosynthetic capacity was found in leaves +1 and +4 under high light in genotypes with a high leaf area index and a high fraction of the sky blocked by the foliage (> 85%). Interestingly, such canopy characteristics cause low light availability to leaves +4, suggesting the photosynthetic acclimation of these leaves to self-shading in some genotypes. We highlight IACCTC06-8126 and CTC4 as those genotypes with higher canopy photosynthetic capacity, presenting high leaf area, high photosynthetic rates in sun-exposed leaves, and high responsiveness of shaded leaves to increasing light availability.

Keywords: Saccharum spp; light; photosynthesis; plant canopy.

Fig. 1. Leaf area index (LAI) (A), tillering (B), visible sky ratio – fraction of the sky that is not blocked by the foliage (C), and photosynthetic photon flux density on leaf +4 (Q) (D) of thirteen sugarcane genotypes. Different letters indicate statistical differences between genotypes (BF₁₀ > 3, n = 4).

Fig. 2. Photosynthetic rate (P_N) (A), stomatal conductance (g_s) (B), instantaneous CO₂ quantum efficiency (Φ_CO2) (C), and instantaneous carboxylation efficiency (k) (D) in leaves +1 and +4 of thirteen sugarcane genotypes under high light [index ‘H’, Q = 2,000 μmol(photon) m^–2 s^–1]. * indicates a difference between leaves +1 and +4 (BF₁₀ > 3, n = 4).

Fig. 3. Ratio of photosynthesis between leaves +4 and +1 (P_N^+4H:+1H) of thirteen sugarcane genotypes under high light [index ‘H’, Q = 2,000 μmol(photon) m^–2 s^–1]. Different letters indicate statistical differences between genotypes (BF₁₀ > 3, n = 4).

Fig. 4. Ratios of photosynthesis (P_N^+4L:+4H), stomatal conductance (g_s^+4L:+4H), instantaneous CO₂ quantum efficiency (Φ_CO2^+4L:+4H), and instantaneous carboxylation efficiency (k^+4L:+4H) in leaf +4 of thirteen sugarcane genotypes under low [index ‘L’, Q = 200 μmol(photon) m^–2 s^–1] and high [index ‘H’, Q = 2,000 μmol(photon) m^–2 s^–1] light. Different letters indicate statistical differences between genotypes (BF₁₀ > 3, n = 4).

Fig. 5. Correlation of thirteen sugarcane genotypes, based on Spearman's coefficient (P<0.05). Photosynthetic rate (P_N), stomatal conductance (g_s), instantaneous CO₂ quantum efficiency (Φ_CO2), and instantaneous carboxylation efficiency (k) and ratios considering leaves +1 and +4 and light level [low (L) or high (H)]; photosynthetic photon flux density (Q), leaf area index (LAI), the proportion of the sky that is not blocked by the foliage (Sky ratio), mean tilt angle of the foliage (MTA) and tillering.