Abstract
BackgroundIn wheat (Triticum aestivum L), the flag leaf has been thought of as the main source of assimilates for grain growth, whereas the peduncle has commonly been thought of as a transporting organ. The photosynthetic characteristics of the exposed peduncle have therefore been neglected. In this study, we investigated the anatomical traits of the exposed peduncle during wheat grain ontogenesis, and we compared the exposed peduncle to the flag leaf with respect to chloroplast ultrastructure, photosystem II (PSII) quantum yield, and phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) activity.ResultsTransmission electron microscope observations showed well-developed chloroplasts with numerous granum stacks at grain-filling stages 1, 2 and 3 in both the flag leaf and the exposed peduncle. In the exposed peduncle, the membranes constituting the thylakoids were very distinct and plentiful, but in the flag leaf, there was a sharp breakdown at stage 4 and complete disintegration of the thylakoid membranes at stage 5. PSII quantum yield assays revealed that the photosynthetic efficiency remained constant at stages 1, 2 and 3 and then declined in both organs. However, the decline occurred more dramatically in the flag leaf than in the exposed peduncle. An enzyme assay showed that at stages 1 and 2 the PEPCase activity was lower in the exposed peduncle than in the flag leaf; but at stages 3, 4 and 5 the value was higher in the exposed peduncle, with a particularly significant difference observed at stage 5. Subjecting the exposed part of the peduncle to darkness following anthesis reduced the rate of grain growth.ConclusionOur results suggest that the exposed peduncle is a photosynthetically active organ that produces photosynthates and thereby makes a crucial contribution to grain growth, particularly during the late stages of grain-filling.
Highlights
In wheat (Triticum aestivum L), the flag leaf has been thought of as the main source of assimilates for grain growth, whereas the peduncle has commonly been thought of as a transporting organ
The upper part of the peduncle develops leaf-like autotrophic carbohydrate metabolism when it is exposed to high irradiance [12], accounting for a high proportion of the photosynthesis of the stem [5,7]
The average stomatal density reached 80.23 ± 9.41 mm-2 on the exposed part of the peduncle (Figure 1E and 1F). This measure was significantly higher than the density measured on the adaxial face of the flag leaf (58.14 ± 7.25 mm-2) and the density recorded on the abaxial face of the flag leaf (45.65 ± 6.43 mm-2)
Summary
In wheat (Triticum aestivum L), the flag leaf has been thought of as the main source of assimilates for grain growth, whereas the peduncle has commonly been thought of as a transporting organ. Green leaves have commonly been considered the primary source of photosynthate production In addition to their leaves, higher plants can potentially use almost any vegetative or reproductive structure to perform photosynthetic CO2 assimilation for growth and development [1,2,3]. The exposed peduncle has been identified as one of the photosynthetically active organs in wheat, the structure of the chloroplasts and the functional events related to photosynthesis and the translocation of photosynthates during the period of grain-filling have not precisely characterised. Evidence from ultrastructural observations and photochemical assays is needed to elucidate the mechanisms by which the exposed peduncle produces photoassimilates for grain development
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