Analysis of durum wheat photosynthetic organs during grain filling reveals the ear as a water stress-tolerant organ and the peduncle as the largest pool of primary metabolites

Abstract

The pool of carbon- and nitrogen-rich metabolites is quantitatively relevant in non-foliar photosynthetic organs during grain filling, which have a better response to water limitation than flag leaves. The response of durum wheat to contrasting water regimes has been extensively studied at leaf and agronomic level in previous studies, but the water stress effects on source-sink dynamics, particularly non-foliar photosynthetic organs, is more limited. Our study aims to investigate the response of different photosynthetic organs to water stress and to quantify the pool of carbon and nitrogen metabolites available for grain filling. Five durum wheat varieties were grown in field trials in the Spanish region of Castile and León under irrigated and rainfed conditions. Water stress led to a significant decrease in yield, biomass, and carbon and nitrogen assimilation, improved water use efficiency, and modified grain quality traits in the five varieties. The pool of carbon (glucose, glucose-6-phosphate, fructose, sucrose, starch, and malate) and nitrogen (glutamate, amino acids, proteins and chlorophylls) metabolites in leaf blades and sheaths, peduncles, awns, glumes and lemmas were also analysed. The results showed that the metabolism of the blades and peduncles was the most susceptible to water stress, while ear metabolism showed higher stability, particularly at mid-grain filling. Interestingly, the total metabolite content per organ highlighted that a large source of nutrients, which may be directly involved in grain filling, are found outside the blades, with the peduncles being quantitatively the most relevant. We conclude that yield improvements in our Mediterranean agro-ecosystem are highly linked to the success of shoots in producing ears and a higher number of grains, while grain filling is highly dependent on the capacity of non-foliar organs to fix CO₂ and N. The ear organs show higher stress resilience than other organs, which deserves our attention in future breeding programmes.

Keywords: Carbon metabolism; Drought; Field trial; Mediterranean region; Nitrogen metabolism; Source–sink dynamics.

Fig. 1

Scheme of the experimental setup and analyses carried out. a Five durum wheat varieties were b grown under two water regimes (irrigated and rainfed conditions). c Climatic conditions during the 2018/2019 crop season are shown: daily maximum (red line), mean (grey line), and minimum (dark blue line) temperatures, relative humidity (light blue line), and water inputs (precipitation as light green bars, and with support irrigation as dark green bars). d A detailed list with the protocols performed at different growth stages and plant levels is included, e with an emphasis on the sampled organs

Fig. 2

a Grain yield (GY), b C isotope composition (δ¹³C) in grains at harvest, and c relative water content (RWC) in flag leaves at ear emergence, anthesis and mid-grain filling (Zadoks 55, 65 and 75, respectively), in five durum wheat varieties (MEX, Mexa; EUR, Euroduro; DRI, Don Ricardo; KNI, Kiko Nick; HAR, Haristide) under rainfed and irrigated conditions. For each comparison of means, letters are significantly different (ns, not significant; *, P < 0.05; **, P < 0.01; ***, P < 0.001; two-way ANOVA, TUKEY test)

Fig. 3

Spectral vegetation indices at a the canopy and b leaf levels in five durum wheat varieties (MEX, Mexa; EUR, Euroduro; DRI, Don Ricardo; KNI, Kiko Nick; HAR, Haristide) under rainfed and irrigated conditions. a A GreenSeeker spectroradiometer was used to determine the Normalised Difference Vegetation Index (NDVI) at six growth stages: tillering, stem elongation, flag leaf sheath extension, boot swollen, ear emergence, anthesis, and mid-grain filling (Zadoks 25, 35, 40, 45, 55 and 65, respectively); while b a DUALEX leaf-clip sensor estimated the chlorophyll (Chl), flavonoid, and anthocyanin contents and the N balanced index (NBI) at ear emergence, anthesis and mid-grain filling stages. All values are dimensionless, specific for each device. For each comparison of means, letters are significantly different (ns, not significant; *, P < 0.05; **, P < 0.01; ***, P < 0.001; two-way ANOVA, TUKEY test)

Fig. 4

a Principal component analysis (PCA) of dry weight (DW), water content (WC), and C and N metabolites expressed as concentrations. b PCA of C and N metabolites expressed as total content per organ. The C metabolites include glucose (Glc), glucose-6-phosphate (Glc6P), fructose (Fru), sucrose (Suc), starch, and malate, while the N metabolites include glutamate (Glu), total amino acids (aa), proteins, and chlorophylls a (Chla), b (Chlb) and total (Chltotal). Six photosynthetic organs (leaf blades and sheaths, peduncles, awns, glumes and lemmas) of five durum wheat varieties (MEX, Mexa; EUR, Euroduro; DRI, Don Ricardo; KNI, Kiko Nick; HAR, Haristide) under rainfed and irrigated conditions were considered. The measurements were carried out at anthesis (Zadoks 65) and mid-grain filling (Zadoks 75)

Fig. 5

Effect of water regime on dry weight (DW), water content (WC), and C metabolites [glucose (Glc), glucose-6-phosphate (Glc6P), fructose (Fru), sucrose (Suc), starch, and malate] in six photosynthetic organs during anthesis (Zadoks 65) and mid-grain filling (Zadoks 75). The boxplots were built per organ using the data from five durum wheat varieties using the percentage of change in rainfed compared to irrigated conditions for every variety. Symbols on the right of each figure indicate the significance of the water regime effect according to the two-way ANOVA in Supplementary Table S1 (ns, not significant; *, P < 0.05; **, P < 0.01; ***, P < 0.001; two-way ANOVA, TUKEY test)

Fig. 6

Effect of water regime on N metabolites [glutamate (Glu), total amino acids (aa), proteins, and chlorophylls a (Chla), b (Chlb) and total (Chltotal)] in six photosynthetic organs during anthesis (Zadoks 65) and mid-grain filling (Zadoks 75). The boxplots were built per organ using the data from five durum wheat varieties by using the percentage of change in rainfed compared to irrigated conditions for every variety. Symbols on the right of each figure indicate the significance of the water regime effect according to the two-way ANOVA in Supplementary Table S1 (ns, not significant; *, P < 0.05; **, P < 0.01; ***, P < 0.001; two-way ANOVA, TUKEY test)

Fig. 7

Differences in the content of metabolites related to C metabolism [glucose (Glc), glucose-6-phosphate (Glc6P), fructose (Fru), sucrose (Suc), starch, and malate] per organ and water regime. The analyses were performed in six photosynthetic organs during anthesis (Z65) and mid-grain filling (Z75). For each comparison of means, letters are significantly different as detailed in Supplementary Table S2 (P < 0.05; two-way ANOVA, TUKEY test)

Fig. 8

Differences in the content of metabolites related to N metabolism [glutamate (Glu), total amino acids (aa), proteins, and total chlorophylls (Chltotal)] per organ and water regime. The analyses were performed in six photosynthetic organs during anthesis (Z65) and mid-grain filling (Z75). For each comparison of means, letters are significantly different as detailed in Supplementary Table S2 (P < 0.05; two-way ANOVA, TUKEY test)

Fig. 9

Total content of sugars, amino acids (aa), and proteins per organ (or group of organs) and water regime during anthesis (Zadoks 65) and mid-grain filling (Zadoks 75). The comparison of the flag leaf blade with the organs with a function more related to transport and storage (sheath + peduncle) and the ear organs (awn + glume + lemma) has been highlighted. Sugar content is the sum of glucose, glucose-6-phosphate, fructose, sucrose, starch, and malate. Each box represents the average ± standard deviation of five field-grown durum wheat varieties. For each comparison of means, letters are significantly different (P < 0.05; two-way ANOVA, TUKEY test)