Leaf-colour modification impact rice photosynthesis at different scales

Background

The improvement of rice yields in the past years is largely dependent on agronomic practices (such as heavy nitrogen application) and selection of cultivars with high harvest index. However, nitrogen fertilizer input comes at the cost of serious environmental pollution while the harvest index of rice is now approaching a plateau. Nowadays, great efforts have been devoted to exploring options of improving photosynthesis, so as to shift the yield potential of rice. One of the novel options that have been recently proposed is to modify leaf colour to improve canopy photosynthesis with an optimized distribution of incoming light over the depths of the canopy. However, manipulation of leaf colour means an alternation of nitrogen investment in chlorophyll, which will lead to a series of subsequent uncertainties across different scales. 

Project description

Leaf-colour modification contrasts with a default concept from empirical cultivation that the greener the rice leaves are, the higher the photosynthesis and yield will be. After many generations of breeding, the chlorophyll content in rice leaves has reached the redundant state, which probably gives rise to the suboptimal conversion efficiency from photosynthetic photon flux density to biomass for rice canopy. In response to this problem, a robust strategy that reduces canopy chlorophyll content for improved photosynthetic efficiency and maximum carbon gain in crop canopy has been proposed. However, manipulation in leaf colour means an alternation of the nitrogen investment in chlorophyll, from the perspective of leaf physiology; this change, its possible resulting redistribution of leaf nitrogen to other photosynthetic proteins (e.g., Rubisco or other soluble protein), and the impacts of these changes on whole-leaf photosynthesis remain unknown. Furthermore, canopy photosynthesis depends not only on light distribution, but also on the distribution of leaf nitrogen and photosynthetic proteins over the layers of canopy. Finally, the pale-leaf idea differs considerably from the traditional concept in crop physiology that advocates “stay-green” traits for improved grain filling duration. Hence, whether reducing leaf chlorophyll content will indirectly influence the grain filling duration and associated source-sink relationships during grain filling remain unclear. 

At leaf scale, we aim to examine:

1. the relationship between non-photochemical quenching and reduced chlorophyll content;
2. whether reducing leaf chlorophyll content might improve the nitrogen partitioning and thus leaf photosynthesis, and if so, whether these effects depend on cultivar backgrounds; and
3. whether reducing leaf chlorophyll content would influence leaf morphology.

At canopy scale, we aim to:

1. quantify the ability of canopy nitrogen acclimation to the light environment by calculating the ratios of nitrogen extinction coefficient (KN) to light extinction coefficient (KL) among genotypes with contrasting chlorophyll content;
2. examine whether and how canopy photosynthesis and dry matter accumulation can be improved by leaf-colour modification; and
3. identify the beneficial traits improving the crop yield and biomass production after leaf-colour modification.

At crop level, we aim to compare the sink-source differences in the variant and its default genotype by quantifying the carbon and nitrogen partitioning among source and sink organs. 

Finally, A mathematical crop model (i.e., GECROS), as a powerful tool for scaling from leaf to canopy levels and ultimately to crop productivity, will be used to investigate the extent to which the advantageous traits from leaf-colour variants can contribute to canopy photosynthesis and grain yield.