The yield potential ( Y p ) of a grain crop is the seed mass per unit ground area obtained under optimum growing conditions without weeds, pests and diseases. It is determined by the product of the available light energy and by the genetically determined properties: efficiency of light capture ( e i ), the efficiency of conversion of the intercepted light into biomass ( e c ) and the proportion of biomass partitioned into grain ( h ). Plant breeding brings h and e i close to their theoretical maxima, leaving e c , primarily determined by photosynthesis, as the only remaining major prospect for improving Y p . Leaf photosynthetic rate, however, is poorly correlated with yield when different genotypes of a crop species are compared. This led to the viewpoint that improvement of leaf photosynthesis has little value for improving Y p . By contrast, the many recent experiments that compare the growth of a genotype in current and future projected elevated [CO 2 ] environments show that increase in leaf photosynthesis is closely associated with similar increases in yield. Are there opportunities to achieve similar increases by genetic manipulation? Six potential routes of increasing e c by improving photosynthetic efficiency were explored, ranging from altered canopy architecture to improved regeneration of the acceptor molecule for CO 2 . Collectively, these changes could improve e c and, therefore, Y p by c . 50%. Because some changes could be achieved by transgenic technology, the time of the development of commercial cultivars could be considerably less than by conventional breeding and potentially, within 10‐15 years.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2005.01493.x

Can improvement in photosynthesis increase crop yields

We review the current understanding of the temperature responses of C(3) and C(4) photosynthesis across thermal ranges that do not harm the photosynthetic apparatus. In C(3) species, photosynthesis is classically considered to be limited by the capacities of ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco), ribulose bisphosphate (RuBP) regeneration or P(i) regeneration. Using both theoretical and empirical evidence, we describe the temperature response of instantaneous net CO(2) assimilation rate (A) in terms of these limitations, and evaluate possible limitations on A at elevated temperatures arising from heat-induced lability of Rubisco activase. In C(3) plants, Rubisco capacity is the predominant limitation on A across a wide range of temperatures at low CO(2) (<300 microbar), while at elevated CO(2), the limitation shifts to P(i) regeneration capacity at suboptimal temperatures, and either electron transport capacity or Rubisco activase capacity at supraoptimal temperatures. In C(4) plants, Rubisco capacity limits A below 20 degrees C in chilling-tolerant species, but the control over A at elevated temperature remains uncertain. Acclimation of C(3) photosynthesis to suboptimal growth temperature is commonly associated with a disproportional enhancement of the P(i) regeneration capacity. Above the thermal optimum, acclimation of A to increasing growth temperature is associated with increased electron transport capacity and/or greater heat stability of Rubisco activase. In many C(4) species from warm habitats, acclimation to cooler growth conditions increases levels of Rubisco and C(4) cycle enzymes which then enhance A below the thermal optimum. By contrast, few C(4) species adapted to cooler habitats increase Rubisco content during acclimation to reduced growth temperature; as a result, A changes little at suboptimal temperatures. Global change is likely to cause a widespread shift in patterns of photosynthetic limitation in higher plants. Limitations in electron transport and Rubisco activase capacity should be more common in the warmer, high CO(2) conditions expected by the end of the century.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2007.01682.x

The temperature response of C(3) and C(4) photosynthesis.

Most plants show considerable capacity to adjust their photosynthetic characteristics to their growth temperatures (temperature acclimation). The most typical case is a shift in the optimum temperature for photosynthesis, which can maximize the photosynthetic rate at the growth temperature. These plastic adjustments can allow plants to photosynthesize more efficiently at their new growth temperatures. In this review article, we summarize the basic differences in photosynthetic reactions in C3, C4, and CAM plants. We review the current understanding of the temperature responses of C3, C4, and CAM photosynthesis, and then discuss the underlying physiological and biochemical mechanisms for temperature acclimation of photosynthesis in each photosynthetic type. Finally, we use the published data to evaluate the extent of photosynthetic temperature acclimation in higher plants, and analyze which plant groups (i.e., photosynthetic types and functional types) have a greater inherent ability for photosynthetic acclimation to temperature than others, since there have been reported interspecific variations in this ability. We found that the inherent ability for temperature acclimation of photosynthesis was different: (1) among C3, C4, and CAM species; and (2) among functional types within C3 plants. C3 plants generally had a greater ability for temperature acclimation of photosynthesis across a broad temperature range, CAM plants acclimated day and night photosynthetic process differentially to temperature, and C4 plants was adapted to warm environments. Moreover, within C3 species, evergreen woody plants and perennial herbaceous plants showed greater temperature homeostasis of photosynthesis (i.e., the photosynthetic rate at high-growth temperature divided by that at low-growth temperature was close to 1.0) than deciduous woody plants and annual herbaceous plants, indicating that photosynthetic acclimation would be particularly important in perennial, long-lived species that would experience a rise in growing season temperatures over their lifespan. Interestingly, across growth temperatures, the extent of temperature homeostasis of photosynthesis was maintained irrespective of the extent of the change in the optimum temperature for photosynthesis (Topt), indicating that some plants achieve greater photosynthesis at the growth temperature by shifting Topt, whereas others can also achieve greater photosynthesis at the growth temperature by changing the shape of the photosynthesis–temperature curve without shifting Topt. It is considered that these differences in the inherent stability of temperature acclimation of photosynthesis would be reflected by differences in the limiting steps of photosynthetic rate.

/pdf/temperature-response-of-photosynthesis-in-c3-c4-and-cam-faakppw7ix.pdf

Temperature response of photosynthesis in C3, C4, and CAM plants: temperature acclimation and temperature adaptation.

C 4 herbaceous rhizomatous perennials have many positive attributes as potential and actual biomass energy crops on agricultural land. Two species from this group have become widely planted. Miscanthus × giganteus has been extensively studied and planted in Europe, and Panicum virgatum in N. America. To date, side-by-side comparisons of mature stands of these crops have not been reported in the peer-reviewed literature. We examined all peer-reviewed articles describing productivity of these species and extracted dry matter yields, nitrogen fertilization (N), temperature (growing degree days) and precipitation/irrigation. Only yields reported 3 or more years after planting were included. Despite being on different continents, trials spanned similar temperature, N and water ranges. Miscanthus × giganteus (97 observations) yielded an average 22 Mg ha −1 compared to 10 Mg ha −1 for P. virgatum (77 observations). Both crops showed a significant positive response to water and N, but not to temperature. Miscanthus × giganteus yielded significantly more biomass than P. virgatum across the range of all three variables. There were differences between the species in their apparent responses to these variables. Miscanthus × giganteus showed a stronger response to water, while P. virgatum showed a significantly stronger response to nitrogen. Since energetic viability, and profitability, of biomass crops hinges critically on high outputs of biomass energy for low inputs of money and fossil fuels, these results suggest that M . × giganteus holds greater promise for biomass energy cropping than does P. virgatum across the range of trial conditions undertaken to date.

A quantitative review comparing the yields of two candidate C4 perennial biomass crops in relation to nitrogen, temperature and water

We estimate the global bioenergy potential from dedicated biomass plantations in the 21st century under a range of sustainability requirements to safeguard food production, biodiversity and terrestrial carbon storage. We use a process-based model of the land biosphere to simulate rainfed and irrigated biomass yields driven by data from different climate models and combine these simulations with a scenario-based assessment of future land availability for energy crops. The resulting spatial patterns of large-scale lignocellulosic energy crop cultivation are then investigated with regard to their impacts on land and water resources. Calculated bioenergy potentials are in the lower range of previous assessments but the combination of all biomass sources may still provide between 130 and 270 EJ yr−1 in 2050, equivalent to 15–25% of the World's future energy demand. Energy crops account for 20–60% of the total potential depending on land availability and share of irrigated area. However, a full exploitation of these potentials will further increase the pressure on natural ecosystems with a doubling of current land use change and irrigation water demand. Despite the consideration of sustainability constraints on future agricultural expansion the large-scale cultivation of energy crops is a threat to many areas that have already been fragmented and degraded, are rich in biodiversity and provide habitat for many endangered and endemic species.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1757-1707.2010.01088.x

Bioenergy production potential of global biomass plantations under environmental and agricultural constraints

Field-grown Miscanthus × giganteus maintains high photosynthetic quantum yields and biomass productivity in cool temperate climates. It is related to maize ( Zea mays ) and uses the same NADP-malic enzyme C 4 pathway. This study tests the hypothesis that M. × giganteus , in contrast to maize, forms photosynthetically competent leaves at low temperatures with altered amounts of pyruvate orthophosphate dikinase (PPDK) and Rubisco or altered properties of PPDK. Both species were grown at 25°C/20°C or 14°C/11°C (day/night), and leaf photosynthesis was measured from 5°C to 38°C. Protein and steady-state transcript levels for Rubisco, PPDK, and phospho enol pyruvate carboxylase were assessed and the sequence of C 4 -PPDK from M. × giganteus was compared with other C 4 species. Low temperature growth had no effect on photosynthesis in M. × giganteus , but decreased rates by 80% at all measurement temperatures in maize. Amounts and expression of phospho enol pyruvate carboxylase were affected little by growth temperature in either species. However, PPDK and Rubisco large subunit decreased >50% and >30%, respectively, in cold-grown maize, whereas these levels remained unaffected by temperature in M. × giganteus . Differences in protein content in maize were not explained by differences in steady-state transcript levels. Several different M. × giganteus C 4 -PPDK cDNA sequences were found, but putative translated protein sequences did not show conservation of amino acids contributing to cold stability in Flaveria brownii C 4 -PPDK. The maintenance of PPDK and Rubisco large subunit amounts in M. × giganteus is consistent with the hypothesis that these proteins are critical to maintaining high rates of C 4 photosynthesis at low temperature.

http://www.plantphysiol.org/content/plantphysiol/132/3/1688.full.pdf

Abdul K. AL-Shoaibi

Papers

Cold tolerance of C4 photosynthesis in Miscanthus x giganteus: adaptation in amounts and sequence of C4 photosynthetic enzymes.