Abstract
Wheat is the second most important direct source of food calories in the world. After considerable improvement during the Green Revolution, increase in genetic yield potential appears to have stalled. Improvement of photosynthetic efficiency now appears a major opportunity in addressing the sustainable yield increases needed to meet future food demand. Effort, however, has focused on increasing efficiency under steady-state conditions. In the field, the light environment at the level of individual leaves is constantly changing. The speed of adjustment of photosynthetic efficiency can have a profound effect on crop carbon gain and yield. Flag leaves of wheat are the major photosynthetic organs supplying the grain of wheat, and will be intermittently shaded throughout a typical day. Here, the speed of adjustment to a shade to sun transition in these leaves was analysed. On transfer to sun conditions, the leaf required about 15 min to regain maximum photosynthetic efficiency. In vivo analysis based on the responses of leaf CO2 assimilation (A) to intercellular CO2 concentration (ci) implied that the major limitation throughout this induction was activation of the primary carboxylase of C3 photosynthesis, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). This was followed in importance by stomata, which accounted for about 20% of the limitation. Except during the first few seconds, photosynthetic electron transport and regeneration of the CO2 acceptor molecule, ribulose-1,5-bisphosphate (RubP), did not affect the speed of induction. The measured kinetics of Rubisco activation in the sun and de-activation in the shade were predicted from the measurements. These were combined with a canopy ray tracing model that predicted intermittent shading of flag leaves over the course of a June day. This indicated that the slow adjustment in shade to sun transitions could cost 21% of potential assimilation.This article is part of the themed issue ‘Enhancing photosynthesis in crop plants: targets for improvement’.
Highlights
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
On shade to sun transitions, this study has shown that several minutes are required for the wheat flag leaf to re-attain maximum photosynthetic efficiency
At the level of leaf biochemical limitations, the apparent maximum activity of Rubisco (Vc,max) limits this rate of induction, implying activation of this enzyme as the key factor, rather than regeneration of the RuBP CO2 acceptor molecule (J ). This was clearly indicated by the fact that ci,trans was well above the actual ci when ca was at the current atmospheric level of 400 mmol mol21 and at the actual greenhouse growth ca of 449 mmol mol21
Summary
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. Using a current cultivar of wheat, this study: (i) determines the speed of adjustment of photosynthesis in the flag leaf on transfer from shade to sun; (ii) infers, by developing dynamic A/ci responses, the in vivo factors determining the speed of adjustment; and (iii) estimates the loss of potential production that may result from this slow adjustment. The potential maximum gross rate of photosynqthffiffieffiffisffiffiiffisffiffiffiffidffiffiffiuffiffiffirffiffiiffinffiffiffigffiffiffiffiffieffiffiaffifficffiffihffiffiffiffiffitffiffiiffimffiffiffiffieffi step (Af*) was predicted as ðfI þ Asat À ðfI þ AsatÞ2 À 4ufIAsatÞ=2u, using parameters from the PPFD response curves fit to steady-state data and setting t 1⁄4 duration of the timestep (s). The value of t 1⁄4 300 s for the decrease was predicted on the basis that 30 min ‘shade’ treatment resulted in a decrease in Vc,max from Vc,max,f to Vc,max,i
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