Coordination of leaf photosynthetic traits and chloroplast relocation of the C4 plant Setaria palmifolia

Abstract

Setaria palmifolia (Poaceae), an NADP-malic enzyme type C4 grass, is distributed from the understory to the margins of forests. To reveal how this C4 plant acclimates to habitats with contrasting light regimes, we documented the behavior of chloroplast relocation and determined patterns of chloroplast relocation and photosynthetic characteristics in response to the light of S. palmifolia grown under different light regimes. Through the observation of leaf sections and measurement of leaf transmittance (T), we confirmed that mesophyll (M) cells of S. palmifolia performed avoidance and accumulation responses, resulting in an increase and decrease in T, respectively. Measurements of gas exchange revealed that the leaf photosynthetic characteristics of S. palmifolia were modulated by growth light. Shade-grown plants significantly increased dry-weight-based chlorophyll (Chl) contents and specific leaf area (SLA), indicating increased light capture. These plants, however, displayed a reduced light-saturated photosynthetic rate (Pmax) and light saturation point (LSP) compared to unshaded plants. The patterns of chloroplast relocation were closely aligned with the photosynthetic response. Shade-grown plants displayed significantly faster kinetics, a greater extent of chloroplast relocation, and lower incident light for the induction of the avoidance response than unshaded plants. The coordination patterns of chloroplast relocation and photosynthetic response suggest that chloroplast relocation in S. palmifolia complements its leaf photosynthetic responses to light availability. The ability to adjust its photosynthetic behavior and perform chloroplast relocation may enable this C4 species to thrive in habitats with various light conditions.