Natural variation in photosynthetic capacity, growth, and yield in 64 field-grown wheat genotypes

Abstract

Increasing photosynthesis in wheat has been identified as an approach to enhance crop yield, with manipulation of key genes involved in electron transport and the Calvin cycle as one avenue currently being explored. However, natural variation in photosynthetic capacity is a currently unexploited genetic resource for potential crop improvement. Using gas-exchange analysis and protein analysis, the existing natural variation in photosynthetic capacity in a diverse panel of 64 elite wheat cultivars grown in the field was examined relative to growth traits, including biomass and harvest index. Significant variations in photosynthetic capacity, biomass, and yield were observed, although no consistent correlation was found between photosynthetic capacity of the flag leaf and grain yield when all cultivars were compared. The majority of the variation in photosynthesis could be explained by components related to maximum capacity and operational rates of CO2 assimilation, and to CO2 diffusion. Cluster analysis revealed that cultivars may have been bred unintentionally for desirable traits at the expense of photosynthetic capacity. These findings suggest that there is significant underutilized photosynthetic capacity among existing wheat varieties. Our observations are discussed in the context of exploiting existing natural variation in physiological processes for the improvement of photosynthesis in wheat.

Keywords: Biomass; Rubisco; natural variation; photosynthetic capacity; wheat; yield..

Fig. 1.

Example of variation observed in data of the response of photosynthetic CO₂ assimilation (A) to different internal CO₂ concentrations (C _i), or A/C _i curve, for four different cultivars. Means of three replicates with standard deviations are shown.

Fig. 2.

Mean and variation of all cultivars for (a) maximum photosynthetic CO₂ assimilation (A _max) at saturating CO₂ concentration, (b) photosynthetic CO₂ assimilation at ambient CO₂ concentration (400 µmol mol^–1 CO₂, A ₄₀₀), and (c) grain yield. Cultivars are ranked according to increasing mean of each parameter. Insets show histograms of frequency distribution of respective parameters with P values for normal distribution.

Fig. 3.

Correlations of photosynthetic parameters operational assimilation rate (A ₄₀₀), maximum carboxylation rate (A _max), maximum velocity of Rubisco carboxylation (V _cmax) and the maximum rate of electron transport demand for RuBP regeneration (J _max) with grain yield and total aboveground biomass. Correlation (r _s), significance (P value), and regression line are given for each figure.

Fig. 4.

(a) Relationship between the maximum rate of electron transport demand for RuBP regeneration (J _max) and maximum velocity of Rubisco for carboxylation (V _cmax) and the relationship between maximum photosynthetic CO₂ assimilation at saturating CO₂ concentration (A _max) and V _cmax. (b) Relationship between the photosynthetic CO₂ assimilation at ambient CO₂ concentration (A ₄₀₀) and the in vitro carboxylation efficiency of Rubisco.

Fig. 5.

(a) Hierarchical clustering of cultivars for photosynthetic parameters, based on Euclidian distances. (b) Histograms of frequency distribution of year of introduction for cultivars per cluster.

Fig. 6.

Means and variation for four clusters of (a) operational assimilation rate (A ₄₀₀), (b) maximum velocity of Rubisco carboxylation (V _cmax), (c) maximum velocity of Rubisco carboxylation (V _cmax), (d) the maximum rate of electron transport demand for RuBP regeneration (J _max), and (e) Leaf Rubisco content. Significant differences are indicated (P<0.05)