Using nonlinear least squares, V c ,max and R d were estimated first, and the value of R d was used when estimating J. Because calculation of the true V c ,max requires determination of c c , we note that the term determined here from c i and referred to as V c ,max is determined by both the in vivo activity of Rubisco and mesophyll conductance g m.
To identify the transition point between Rubisco and ribulose-1,5-bisphosphate RuBP limitation, we used an approach derived from the recommendations of Gu et al. All possible combinations of A C and A J were fit to each CO 2 response curve, and the best fit was selected based on the minimal value of. The range of leaf VPD during inductions was 1.
In the shade at ambient and higher cuvette c a , g s decreased, minimizing the range of c i that could be obtained and preventing characterization of A J. CO 2 response curves were fit to the data for each 10 s interval of induction. A small number of inadmissible fits were obtained when there was insufficient data to fit both A C and A J ; we re-fit these cases using either A C or A J alongside R d , and chose the best fit based on a comparison of and.
The resulting quadratic was solved for c i at each 10 s interval through induction.
Photosynthesis and Production in a Changing Environment. A field and laboratory manual. Editors: Hall, D.O., Scurlock, J.M.O., Bolhar-Nordenkampf, H.R. Photosynthesis and production in a changing environment: a field and laboratory manual. Responsibility: edited by D.O. Hall [et al.]. Imprint: London ; New.
The time constant for Rubisco activation was determined from the kinetics of A following transitions from low to high light, excluding transient changes occurring during the first minute as described previously [ 23 ]. The model was fit using both nonlinear least squares using data collected from 60 s until s after the change in PPFD , and the linear regression technique described previously [ 23 ], where a plot of against time has slope and intercept. To determine the impacts of Rubisco kinetics on CO 2 assimilation, the response of to PPFD was modelled at approximately 60 s time intervals during a diurnal period.
A PPFD regime was used that predicted light available to the second layer of a crop canopy [ 25 ]. This is justified by the observation that the ears represent the first layer and cause intermittent shading of the flag leaves as the angle of the sun progresses through the day. In the data used, PPFD at a point on the leaf had been predicted using reverse ray tracing, with shade-generating structures in the canopy distributed at random within each layer. Response of net leaf CO 2 uptake A to intercellular CO 2 concentrations c i in flag leaves of wheat at heading-anthesis.
Fitted curves are shown for Rubisco-limited photosynthesis solid lines and RuBP-limited photosynthesis dashed lines. Vertical dotted lines indicate the c i at which limitation of photosynthesis transitions from Rubisco to RuBP regeneration c i,trans , and filled points the steady-state operating values.
This transition was therefore well above the operating c i , i. However, sums of squares SS for residuals of models fit as a single limitation phase were relatively high 6.
For the remainder of the first 10 min following the transition, decreases in c i,trans continued, in concert with increasing V c ,max. The increase in V c ,max was most rapid in the first 4.
The dynamics of this loss may be seen more clearly from a narrower time window around solar noon. At the level of leaf biochemical limitations, the apparent maximum activity of Rubisco V c ,max limits this rate of induction, implying activation of this enzyme as the key factor, rather than regeneration of the RuBP CO 2 acceptor molecule J. In contrast to previous studies [ 4 ], stomatal limitation plays a role in the speed of induction, declining from ca 0.
Combining the ray tracing model of Zhu et al.
This is consistent with previous studies of tobacco, rice and soya bean [ 3 — 5 ]. If g m increased during the course of induction, it would cause part of the apparent increase in V c ,max. As a physical conductance, g m would not vary. However, modelling suggests that in reality it will have some dependence on the positioning of organelles, and in particular the relative localization of chloroplasts and mitochondria, which may change in response to light levels within the leaf [ 26 — 28 ]. It is known that chloroplasts may alter their position with PPFD.
Through its impact on g m , this movement could explain some, but certainly not all, of the change in apparent V c ,max [ 28 ]. Transporters and channels in membranes may change dynamically to affect g m.
Therefore, the lag attributed to Rubisco activation could in reality be a combination this activation with an increase in g m. Previous research has shown a strong correlation between the speed of induction and the activation of Rubisco, in particular, the enzyme Rubisco activase [ 3 , 5 ]. However, stomatal opening appears to depend strongly on photosynthesis in the mesophyll [ 29 , 30 ]. Thus, there may be some dependency of the speed of stomatal opening on the speed of Rubisco activation in the mesophyll. Assuming c c in the shade is sufficient to support rapid carbamylation of Rubisco, increasing the speed of activation might increase the speed of stomatal opening.
Decreases in activation linked with de-carbamylation as a result of low CO 2 availability [ 31 ] are unlikely in this scenario. The rate of Rubisco activation during induction is thought to respond to CO 2 availability [ 32 ], consistent with greater availability of CO 2 driving Rubisco carbamylation and minimizing alternative reactions reviewed in [ 33 ]. Calculating V c ,max in a dynamic analysis averages across measurements that may reflect different activation states.
We anticipate that experimentation and modelling to understand how c a affects Rubisco activation state during induction will improve our understanding of the induction process, and the potential feedbacks due to mesophyll and stomatal conductance responses. Importantly, this research shows that the speed of non-steady-state adjustment of photosynthesis to light fluctuations in the field, regardless of underlying cause, will strongly affect flag leaf photosynthesis.
In turn, this will decrease the supply of assimilate for the developing grain. Although, only the flag leaf was examined here, the same lags in induction will likely apply to all leaves of the plant. Thus, the growth and production that supports the development of the plant to flowering and seed fill will be affected.
Increasing the rate of induction following shade to sun transitions under typical field conditions during grain filling would decrease the impact of a significant limitation, and therefore represents an excellent target through which increases in productivity would be obtained. The gains in productivity could be of similar magnitude to those observed by bioengineering a faster rate of adjustment to sun to shade transitions [ 1 ].
Acceleration might be achieved by over-expressing the amount of Rca [ 5 ], by targeted amino acid substitutions of Rca [ 35 ], altered ratios of alpha and beta forms [ 35 ], or by exploring the natural variation in speed of adjustment apparent in soya bean [ 4 ]. The results presented here suggest that these changes have the potential to open an important new route, through photosynthesis, to a much needed yield jump for wheat. We thank Dr Elizabete Carmo-Silva for helpful discussions on Rubisco activation and for supplying us with the wheat plants used here, and we thank Prof.
There are no discussion topics on this book yet. The analysis of chlorophyll fluorescence and the activity of SBPase indicated that the enhancement of photosynthesis in salt stress was not related to the function of PSII but to the activity of SBPase. In turn, this will decrease the supply of assimilate for the developing grain. The model was fit using both nonlinear least squares using data collected from 60 s until s after the change in PPFD , and the linear regression technique described previously [ 23 ], where a plot of against time has slope and intercept. Additionally, P max was lower in NPT at all points during development, suggesting an intrinsic limitation on photosynthesis. These were combined with a canopy ray tracing model that predicted intermittent shading of flag leaves over the course of a June day. The rate of Rubisco activation during induction is thought to respond to CO 2 availability [ 32 ], consistent with greater availability of CO 2 driving Rubisco carbamylation and minimizing alternative reactions reviewed in [ 33 ].
Xinguang Zhu for providing raw data on light within canopies from his simulation study [ 25 ]. Lancaster University provided the financial support necessary to undertake this work. National Center for Biotechnology Information , U. Published online Aug Samuel H. Taylor 1 and Stephen P. Long 1, 2, 3. Stephen P. Author information Article notes Copyright and License information Disclaimer. Accepted Apr 2. This article has been cited by other articles in PMC. Associated Data Supplementary Materials Original data from each plant sampled. Abstract Wheat is the second most important direct source of food calories in the world.
Keywords: food security, Rubisco, Rubisco activase, photosynthetic induction, wheat, crop yield improvement. Introduction Leaves of crops in the field experience frequent fluctuations in light, moving from shade to full sunlight, and vice versa, as clouds obscure the sun or as leaves go into the shade of other leaves, stems and floral structures.
Material and methods a Plant material and growth conditions A bread-making quality wheat Triticum aestivum L. Results a Factors limiting photosynthesis in wheat, cv. Open in a separate window. Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6.