Mechanistic understanding of photosynthesis in wheat

In document Screening genetic variation for photosynthetic capacity and efficiency in wheat (Page 68-72)

CHAPTER 3 GENETIC VARIATION FOR PHOTOSYNTHETIC CAPACITY

3.4.2 Mechanistic understanding of photosynthesis in wheat

The velocity of carboxylation (Vcmax) represents Rubisco activity in vivo inferred from non- destructive measurements on living plants, but it can also be measured in vitro from leaf samples. Rubisco is present in leaves as an abundant component of chloroplasts and chloroplast and Rubisco account for a high percentage of leaf nitrogen. Anatomical parameters such as leaf mass per unit dry area (LMA) can reflect high leaf

density/thickness and higher chloroplast area. For this reason, in this section Rubisco in

vivo and Rubisco in vitro are analysed as a function of LMA and nitrogen per leaf area (Narea).

3.4.2.1 Rubisco in vivo

When LMA and Vcmax25 were compared across genotypes, a large range for Vcmax25 was observed over a relatively small range of LMA (Figure 3.3.a). Plants grown with low nitrogen (EVA -N_Aus1) seemed to have similar leaf density/thickness as the high

nitrogen treatment (EVA +N_Aus1). Similar behaviour was observed for BYPB -N_Aus2 and BYPB +N_Aus2. These observations raise the question of whether Rubisco

measured in vitro behaves similarly at high and low fertilizer treatment.

Figure 3.3 Vcmax25 as a function of a) leaf mass area (LMA), and b) nitrogen (Narea) for wheat

genotypes grown in different environments and measured at different stages as described in Table 3.1. Symbols represent means of each genotype. The arrow represents the diversity of Vcmax25 for a given LMA or Narea.

69 At low leaf N, there was a positive correlation between Vcmax25 and Narea, but at higher

nitrogen content the correlation was low, which suggests that there is significant variability in Vcmax25 for a given nitrogen investment at high leaf nitrogen contents per unit leaf area (Figure 3.3.b). Potentially, this parameter, Vcmax25/Narea, could be used to uncouple

measurements of Vcmax25 from leaf nitrogen, and this trait is used to explore photosynthetic efficiency (Peff) across wheat genotypes.

3.4.2.2 Rubisco in vitro

Four wheat genotypes and one triticale genotype from the BYPB set were selected based on Peff (Vcmax25/Narea) to measure leaf Rubisco in vitro (by the CABP binding assay).

Genotypes from EVA and BYPB were ranked based on the mean of repetitions. For the BYPB set, the means were considered from genotypes measured in the glasshouse (Aus2) and field (Aus3) (Figure 3.4).

Figure 3.4 Diversity of photosynthetic efficiency represented by Vcmax25/Narea of the early

vigour set (EVA +N_Aus1) and BUNYIP Set measured in the glasshouse (BYPB

+N_Aus2) and in the field (BYPB_Aus3). Genotypes labelled were selected for Rubisco in vitro measurements of Rubisco. Genotypes were ranked based on the mean of repetitions. For the BYPB set, the means were considered from genotypes measured in the glasshouse (Aus2) and field (Aus3).

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The BYPB set was measured in the cabinet (see Chapter 2), in the glasshouse with two levels of nitrogen (Aus2), and in the field (Aus3) so genotypes from this set were chosen to measure leaf Rubisco in vitro. The genotypes (Espada, Merinda, Mace, Drysdale and

Hawkeye) were selected to cover the range between EVA and BYPB, from lower to higher

Vcmax25/Narea (Figure 3.4).

Measurements of Rubisco in vitro (CABP binding) were compared to Rubisco in vivo (Vcmax25) and related to LMA and Narea using leaf samples from genotypes measured in the

growth cabinet, glasshouse, and field.

Vcmax25 showed a positive relationship with Rubisco in vitro although it was not directly proportional. Consequently, Vcmax25 per Rubisco declined as Rubisco content per unit leaf area increased (Figure 3.5).

Results of this experiment show that LMA did not correlate with the Rubisco content in

vitro. Rubisco content varied almost threefold while LMA varied between 45.8 to 65 g m-2

(Figure 3.6.a). This confirms the relationship within a given experiment already shown in Figure 3.3.a. Leaf nitrogen showed a positive correlation with Rubisco in vitro and diversity for a given Narea (Figure 3.6.b), in general similar to Vcmax25 (Figure 3.3.b).

Figure 3.5 Maximum carboxylation rate in-vivo (Vcmax25) as a function of leaf Rubisco

content. Genotypes sampled in the cabinet, glasshouse and field, n=41. The shaded band indicates the level of uncertainty of the regression line.

71 Figure 3.6 Leaf Rubisco content as a function of a. leaf mass area (LMA), b. leaf nitrogen per area (Narea). Data from four wheat genotypes and a triticale measured and sampled in

the cabinet, glasshouse and field, n=41.

From Vcmax25, representing the carboxylation of Rubisco in vivo, and Rubisco catalytic sites in

vitro representing Rubisco content, it is possible to calculate the catalytic turnover rate for

Rubisco carboxylation (kcatc). This is shown for each genotype in relation to the leaf nitrogen content (Figure 3.7). Even if these five genotypes represent the extremes from EVA and BYPB set, it was difficult to distinguish genetic diversity for kcatc. Overall, kcatc was not affected by fertilizer treatment. However, kcatc was reduced in Espada samples from fertilized compared to unfertilized plants and when comparing kcatc in relation to leaf nitrogen, Espada, Mace and Merinda showed a tendency for kcatc to decrease at higher Narea.

Drysdale and Hawkeye showed no trend with Narea.

Figure 3.7 Catalytic turnover rate of Rubisco carboxylation (kcatc) as a function of nitrogen

per leaf area (Narea) in four wheat genotypes and a triticale (Hawkeye). Black dot are plants

without fertilizer. Blue dots are plants with fertilizer. Samples from plants grown in the glasshouse or in the field are not differentiated. Dotted line is tobacco kcatc 3.5 mol

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In summary both in vivo and in vitro measurements of Rubisco showed that Rubisco is not correlated with LMA. This indicates that anatomical constraints have flexibility to enable significant variation in Rubisco content. Regardless of whether Rubisco was derived from gas exchange measurements (Figure 3.3.b) or quantified in vitro (Figure 3.6.b). Rubisco is strongly related with leaf nitrogen. It seems that there is variation in Rubisco activity per unit leaf nitrogen Vcmax25/Narea that can be exploited. Vcmax25/Narea can be used as one

measure of photosynthetic efficiency. However, no evident differences in kcatc were detected across 5 genotypes selected on the basis variation in Vcmax25/Narea.

In document Screening genetic variation for photosynthetic capacity and efficiency in wheat (Page 68-72)