Understanding photosynthetic performance measured at different

In order to further explore the differences between A/Vcmax25 correlation measured in Australia (0.84) and in Mexico (0.52) (Table 3.11), all data were re-plotted (Figure 3.8). Immediately, two factors become obvious which could underpin the poor correlation between A and Vcmax25 for genotypes measured in Mexico.

Figure 3.8 Assimilation rate (A) as a function of velocity of carboxylation (Vcmax25) for wheat

genotypes grown in different environments and measured at different stages as described in Table 3.1. Symbols are the mean of the traits. Circles delimit the general behaviour of CB_Mex and CA_Mex wheat genotypes.

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First, genotypes measured before anthesis (CB_Mex) had a higher variation in Vcmax25 for a small range of A (Figure 3.8, symbol “×”). As suggested in the literature, A depends on stomatal sensitivity and this is the reason Ci/Ca has been used to select water-use efficient wheat genotypes (Condon et al., 2004). The present study supports this relationship, where gs was highly correlated with A in all experiments (Table 3.12). For experiments in Mexico,

the mean of Ci/Ca was 0.6 (Table 3.7) and in general correlated positively with A (Table 3.12). In the plot (Figure 3.8) it can also be seen that some genotypes with low A had high

Vcmax25. This could be explained by some genotypes displaying low A caused by stomatal closure, while in reality their Rubisco capacity was higher, as evidenced by high Vcmax25. This could explain the low correlation between A and Vcmax25 for CB, CA_Mex (Table 3.12). Table 3.12 Summary of phenotypic correlations with photosynthetic rate (A). Taken from genotypic means from Tables A6, A8, A10, 3.9 and A13.

gs Ci/Ca Vcmax25 J Aus1 0.84*** -0.04 0.97*** 0.92*** Aus2 0.64*** 0.21 0.90*** 0.79*** +NAus2 &Aus3 0.83*** 0.4* 0.82*** 0.87*** CA_Aus3 0.91*** 0.80*** 0.84*** 0.72*** CA_Mex 0.94*** 0.79*** 0.52* 0.55* CB, CA_Mex 0.92*** 0.79*** 0.55** 0.47**

Second, genotypes measured at anthesis (CA_Mex) had higher variation in A for a small range of Vcmax25 (Figure 3.8). This leads to the same explanation from CB_Mex. Genotypes with low A closed their stomata, inferred from their low gs, but it does not mean that these

leaves had lower Rubisco capacity. This confirms that Vcmax25 is not as sensitive to gs as a

simple measurement of A and it is a more robust trait to assess Rubisco capacity, as has been suggested decades ago (Farquhar and Sharkey, 1982). Probably this issue of stomatal limitation is more evident during anthesis than before anthesis because temperature was higher after anthesis (Figure 2.1), and even if the plants were grown under potential yield, measurements in the afternoon and high temperatures caused stomatal closure. The utility of Vcmax25 as a metric for Pc is also supported by the strong correlation between Vcmax25 and Rubisco measured in vitro (Figure 3.5). Perhaps the question that needs further attention is why Vcmax25 and Rubisco in vitro have a curvilinear trend (Figure 3.5).

In Mexico, Vcmax25 did not show significant diversity across wheat genotypes but there was diversity for J and A (Table 3.7). It is possible that the Mexican environment in which these lines were selected and then measured did not permit expression of genetic variation in Vcmax25 or Rubisco levels. This could be to nitrogen leaching that has been reported in that region, because 75% of the nitrogen fertilizer is applied before planting that can be lost

83 in the first post-planting irrigation (Riley et al., 2001). If there was not enough nitrogen in the soil for plant development, perhaps it was not enough nitrogen allocated to leaves, avoiding the maximum quantity of Rubisco protein into leaves and grain (Hawkesford 2013). However, under these conditions, it seems that there is scope for genetic variation in the electron transport rate (Jg) (Table 3.7).

A was also positively related with short flowering time and longer period of grain filling

(Table 3.9), which could be related to sink strength. It has been shown that A changes relatively rapidly depending on the demand from grain filling, and A can drop when sink activity from vegetative growth shows before grains start to grow (King et al., 1967; Evans and Dunstone, 1970; Evans, 1993). In these experiments (CA_Aus and CA_Mex) A was measured 7 to 10 days after anthesis. Some genotypes may have started grain filling earlier than others potentially leading to differences in A due to sink demand. Therefore, while A was representative of the photosynthetically active leaves in a given period from anthesis to grain filling, it may not represent the general genetic potential for photosynthetic capacity, as assessed by Vcmax25. Further research is needed to understand the relationship between

Vcmax25 and sink strength.

There was significant genetic variation for Vcmax25 measured in the CIMCOG Set in Australia. The genotypes were not as vigorous as in Mexico, even if it was attempted to grow the plants under high yield potential conditions; during anthesis temperatures were high, conditions that limited the maximum plant development and could affect expression of maximum Rubisco carboxylation for all genotypes. In regard to potential environmental differences, Mexico belongs to wheat mega- environment 1 and Australia to wheat mega- environment 4 (Fischer et al., 2014). Potentially, these genotypes may have reacted to differences in environmental factors such as photoperiod or light intensity (which are the main differences between the two geographical locations) giving more scope for expression of genotypic diversity in photosynthetic traits in Australia. Differences in photoperiod were observed during wheat selection to increase wheat yield from the middle of Mexico to the north of Mexico (Borlaug, 2007). Solar radiation was higher in Australia than in Mexico (Figure 3.1). Perhaps, such differences influenced changes in leaf size, thickness and leaf nitrogen content.

Interestingly Narea, LMA and SPAD were lower for genotypes grown in Mexico (CA_Mex)

than in Australia (CA_Aus3). Probably different trade-off between leaf structure and photosynthetic traits occurred. For example, in some plants, electron transport is reduced under low irradiance, but it is compensated by investing of more nitrogen in pigment

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protein complexes (Evans, 1989). Also in an experiment comparing a low and a high level of nitrogen nutrition in spinach, at the highest nitrogen level there was an increase of electron transport capacity despite maintaining the same proportion (24%) of thylakoid nitrogen compared to the low nitrogen treatment, and the excess of nitrogen from the high nitrogen treatment was allocated to the soluble protein (Terashima and Evans, 1988). Plants were bigger with larger leaf area evident in Mexico compared to Australia

(unfortunately leaf size was not measured in both locations). Perhaps in Mexico, genotypes had a larger leaf area surface that compensated for a thinner leaf. Having a thin leaf may have limited the chlorophyll and nitrogen per unit leaf area basis. In contrast, plants in Australia increased chloroplast surface with higher LMA to compensate the reduction of leaf size, allowing higher Narea, but it was not enough to increase Rubisco activity.