Ear photosynthesis in C3 cereals and its contribution to grain yield: methodologies, controversies, and perspectives

Abstract

In C3 cereals such as wheat and barley, grain filling was traditionally explained as being sustained by assimilates from concurrent leaf photosynthesis and remobilization from the stem. In recent decades a role for ear photosynthesis as a contributor to grain filling has emerged. This review will analyze several aspects of this topic: (i) methodological approaches for estimation of ear photosynthetic contribution to grain filling; (ii) the existence of genetic variability in the ear contribution, and evidence of genetic gains in the past; (iii) the controversy about the existence of C4 metabolism in the ear; (iv) the response of ear photosynthesis to water deficit; and (v) morphological and physiological traits possibly related to ear temperature and thermal balance of the ear. The main conclusions are: (i) there are a number of methodologies to quantify ear photosynthetic activity (e.g. gas exchange, chlorophyll fluorescence) and the ear contribution to grain filling (individual ear shading, ear emergence in shaded canopies, isotope composition); (ii) the contribution of ear photosynthesis seems to have increased in modern wheat germplasm; (iii) the ear contribution to grain filling increases under resource-limitation conditions (water deficit, defoliation, pathogen infection); (iv) there is genetic variability in the ear contribution in wheat, opening the possibility to use this trait to ameliorate grain yield; (v) current evidence supports the existence of C3 metabolism rather than C4 metabolism; (vi) the ear is a ‘dehydration avoider organ’ under drought; (vii) thermal balance in the ear is a relevant issue to explore, and more research is needed to clarify the underlying morphological and physiological traits.

Genetic Yield Gains and Changes in Morphophysiological-Related Traits of Winter Wheat in Southern Chilean High-Yielding Environments

“…During grain filling, leaves of the canopy bottom are senescenced, while the flag leaf and the penultimate leaf remain green for a longer time. Thus, the flag leaf is the main photosynthetic organ to support assimilates to the grain, besides the non-foliar tissues (i.e., spikes) and redistribution of assimilates stored in the stem ( Simkin et al, 2020 ; Elazab et al, 2021 ; Tambussi et al, 2021 ).…”

Photosynthetic Efficiency in Flag Leaves and Ears of Winter Wheat during Fusarium Head Blight Infection

“…In C3 cereals, such as wheat, the flag leaf is considered to be the main photosynthetic tissue, but the ear is also photosynthetically active and can contribute to final grain yield, especially under unfavorable growth conditions [27]. The evaluation of photosynthetic efficiency of flag leaves and ears of wheat in this study was based on the measurement of chlorophyll a fluorescence rise kinetics (OJIP kinetics) combined with a multiparametric analysis of the recorded fluorescence transients (JIP-test).…”

Photosynthetic organs contributions to grain yield genetic gains in Chilean winter wheat

“…The chamber enclosed the ear in a vertical position to avoid breaking the peduncle during the measurements. The solar radiation inside the greenhouse is known to be heterogeneous (i.e., the ear intercepts both zenithal and lateral light) and not intact directly to the ear as in the leaf cuvette (Tambussi et al., 2021). Thus, a led‐light source contained the whole natural light spectrum (red, blue, yellow, white, infrared, and ultraviolet) was placed in one side of the ear chamber, which supported the ear chamber with a PPFD of 1,000 μmol m −2 s −1 at 10 cm apart from the ear chamber (Supplemental Figure S1) to mimic ear photosynthesis at a saturation PPFD of around 1,500 μmol m −2 s −1 PPFD.…”

“…They confirmed the validity of the source manipulation techniques for studying ear photosynthesis in many accessions. Other techniques were proposed for the high throughput phenotyping of photosynthetic organs contributions, such as using the stable carbon isotope (δ 13 C) signatures in the grains dry matter vs. the δ 13 C signatures in other organs (i.e., peduncle, flag, rachis, glumes, and awns), which quantify the different organ photosynthetic contribution to grain filling (Araus et al., 1993; Hafsi et al., 2000; Jia et al., 2015; Jiang et al., 2021; Merah & Monneveux, 2015; Merah et al., 2018; Tambussi et al., 2021). Using the chlorophyll (Chl) fluorescence technique was also proposed to indirectly assess the photosynthetic activity of different plant organs (Lu & Lu, 2004; Maydup et al., 2014).…”