The balance between R and photosynthesis (A) strongly influences the rate of biomass accumulation of individual plants (Gifford, 2003, Millar et al., 2011). My study shows that N supply did not impact on RD/Ag at the leaf level. There were, however, differences among genotypes for this parameter (Table 5.5). As a consequence of light inhibition of leaf R, RL/Ag was lower (0.03) than RD/Ag (0.04) consistent with past studies (Crous et al., 2012, Weerasinghe et al., 2014). Neither N supply nor genotypic differences influenced RL/Ag. Several past studies have also reported constant ratios for RL/Ag (Ayub et al., 2011, Atkin et al., 2000b, Atkin et al., 2013). When expressed at whole plant level, R/A was largely constant, being about 0.4 across genotypes at early stages of growth. This indicates the interdependence of R and A where R depend on A for substrates and A depends on R for ATP, reducing equivalents and C-skeletons for N assimilation and amino acid biosynthesis (Kromer, 1995, Hoefnagel et al., 1998, Atkin et al., 2000a, Hurry et al., 2005, Noguchi and Yoshida, 2008). This ratio slightly increased up to 0.5 under low N in majority of genotypes due to a reduction in whole plant A of larger plants on the fifth harvest. According to Amthor and Baldocchi (2001) the above ratio (integrated for the whole growing season) for cereals (i.e. maize, rice and wheat) could vary between 0.3-0.6. Taken together, N supply did not influence either RD/Ag or RL/Ag at leaf level. RL/Ag was lower compared with RD/Ag at leaf level due to light inhibition of leaf R. Thus, constant RL/Ag and RD/Ag ratios at leaf level can be used when modelling C fluxes in rice in relation to N supply, yet attention needs to be given for genotypic differences in RD/Ag at leaf level. Constancy of whole plant R/A (0.4) irrespective of N supply can be useful in such modelling in rice during early growth, but not in later growth especially at sub-optimal N supply. The balance between R/A at whole plant level was about ten times higher than in individual leaf. According to Atkin et al. (2007) this difference could be due to three reasons: primarily, R in the stem and roots can increase the whole plant R component compared to an individual leaf. Secondly, the light interception across a canopy at whole plant level is non-uniform due to variations in leaf angles and canopy architecture, thus A at whole plant level is poorly
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represented by a light saturated individual leaf used in gas exchange measurements. Thirdly, presence of both young and old tissue at whole plant level can result variations in gas exchange rates.
5.6 Conclusions
The present study has highlighted the fact that root R is more susceptible to low N conditions compared with leaf R. Biomass allocation to roots per se was not a solid indicator of the contribution of root R to whole plant R etc. While shoots contributed 70% to daily whole plant R, the proportional contribution of each organ to daily whole plant R was independent of N supply. R-N scaling relationship was largely held for leaves and roots with a fairly common slope during steady-state of N supply. R at a common N was higher for roots compared with leaves indicating primary differences in physiological activities among organs. A fundamentally different R-N scaling relationship was formed in roots as a consequence of N cessation reflecting an alteration of energy demand in roots. R-N scaling relationship of leaves was relatively robust to cessation compared with roots. Leaf RD varied by two fold across genotypes within a given N level and low N grown plants exhibited a greater respiratory N use efficiency. Light inhibited leaf R in rice and leaf RL was reduced by low N. Yet, neither RL nor light inhibition of leaf R correlated with leaf Na indicating potential variation in N allocation patterns and activation state of Rubisco across N treatments and genotypes. Genotypic differences were found for RL and light inhibition of leaf R. Variation in light inhibition of leaf R was largely accounted for by leaf RL which was in turn dependent on the activity of Rubisco (either carboxylation or oxygenation) in the light. There was no impact of N supply on the fraction of daily fixed CO2 released by R at the whole-plant level during early growth. However, the above fraction at whole-plant level increased during later growth as a consequence of reduced whole-plant A at low N. Respiration: photosynthesis ratios at leaf level were slightly lower in the light compared to dark, but both remained constant across N supply. Attention needs to be given to genotypic differences when interpreting the above ratio at leaf level in the dark.
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5.7 Future directions
Further work is needed to investigate differences among genotypes and across N treatments for activation state of Rubisco which can help to elucidate the lack of correlation between leaf N and light inhibition despite the contribution of RL to light inhibition and its (RL) close relationship with Rubisco activity.
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