2024-09-26 PL-INT-2024-008813 GP 2045 Draft Comments - A. Longo

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significant reduction in multiple cycles[71].These data do suggest that e[CO2]may enhance fruit flavour and fruit yield at the expense of nutritional value. Impact of elevated [CO2]on yield and quality of fruiting trees Sweet clonal cherry(Prunus avium L.)plants were grown for 19 months in climate-controlled greenhouses at ambient(1994-358 ppm;1995- 360 ppm)or e[CO2]('700 ppm).Elevated[CO2]treatment increased photosynthesis and dry matter production,leaf(550/0)and stem(61%),after two months at'7oo ppm,however,this initial stimulation is not sustained.Photosynthetic rates were less after 10 months of growth than after 2 months of growth,and only small increases in dry mass are still evident after 10-months,suggesting that sweet cherry acclimates to e[CO2]due to long- term exposure[72].Due to the young nature of plants studied compared with fully grown mature trees(deciduous tree 15-32 m in height and with a trunk up to 1.5 m in circumference[73,74],no information is available to determine the impacts of e[CO2]on fruit yield or quality.In Nashi pear,a CO2-enriched atmosphere of'700 ppm increased fruit weight,diameter and length along with a 22.5%increase in Brix,(a key measure of sweetness for marketable fruit[75]).However,this also resulted in a reduction in fruit firmness demonstrating that improvements in yield can be nullified by negative impacts on fruit quality(Table 3). While these studies are limited in,they do indicate the potential of CO2-enriched growth for improving photosynthesis,increasing yield and quality of tree crops.However,they also suggest that some crops,especially perennial crops,may become acclimated to higher[CO2]and any gains may be lost over time. Does increasing carbon assimilation increase environmental tolerances? The work presented above also suggest that increasing CO2 uptake could have other benefits.It is notable that growth of fruit crops in carbon enriched atmospheres has a similar effect of protecting against environmental stresses,such as drought and elevated temperature,that may become increasingly common due to climate change as plants genetically engineered to increase carbon assimilation.For example,in melon (Cucumis melo),growing plants in e[CO2]has been shown to mitigate yield losses from increased salinity[67],and in sweet pepper,e[CO2]of 80o ppm was sufficient to rescue any significant yield loss of total and marketable fruits from salinity stress(2o mmol L 1 NaCl)[42].It could be hypothesised that increasing CO2 assimilation increases sugar and chlorophyll content triggering salt tolerance.However,it should be noted that these results are not universally translatable.Gray et al.[76]demonstrated in soybean that e[CO2]was insufficient to protect yields from drought conditions triggered by higher temperatures demonstrating that benefits in some crops may not be translatable across all crops of agronomical importance.Furthermore,in tomato plant Zhou et al.[77]showed that plants grown in e[CO2]were more sensitive to combined drought and heat stress;e[CO2]drives gs and transpiration reducing net photosynthesis and therefore productivity,which is concerning given that greenhouses tend to have elevated temperatures compared to the external environment due to the nature of their construction,glass and metal,and therefore e[CO2] in an enclosed system may negatively impact on yields if water supplies are limiting.This demonstrates that irrigation within greenhouse environments is an essential element and adjusting water regimes to maintain productivity and optimise water-use efficiency. It is also important to note that it is the increase in atmospheric[CO2]that causes the increase in air temperature(along with associated stresses) by absorbing energy and preventing it from being radiated out into space(see[78,79]);as such one might view that the cause cannot mitigate its own effects,however,in some crops where both[CO2]and temperature increase simultaneously,yields were maintained compared with data where temperature is increased in the absence of e[CO2]leading to yield loss and these results cannot be ignored,but a better understanding of the impact of cause and effect climate change on crop yields needs to be researched,otherwise,the logic consequences would be further increase amounts of [CO2]in the atmosphere to increase crop tolerance against the effects of ever-increasing temperatures. Interestingly,some parallels do exist between photosynthetically genetically modified crops and increased tolerance to salinity.In Arabidopsis, over-expression of Sedoheptulose-1,7-bisphosphatase(SBPase),which enhances CO2 assimilation rates by increasing the regeneration of the Rubisco substrate RuBP[8o],enhances salt tolerance through increases in sucrose,starch and chlorophyll content were reported[81].This suggests that increasing photosynthetic rates,either through increasing the availability of[CO2]for photosynthesis or increasing the plants'ability to assimilate[CO2]under ambient conditions could have a similar protective effect.It would be interesting to explore if increased carbon assimilation rates,through atmospheric manipulation or genetic modification,can have a positive impact on crop resistance to high salt environments and other abiotic stresses in large field trials or commercial greenhouses.There is currently evidence that over-expressing the Calvin-Benson cycle (CBC)enzyme SBPase can increase tolerance to chilling stress in tomato[82]and the expression of the cyanobacterial CBC bifunctional fructose- 1,6-bisphosphatases/Sedoheptulose-1,7-bisphosphatase enzyme in soybean prevent yield loss under high temperature[83].Köhler et al.[83] concluded that the manipulation of CO2 uptake could mitigate against the effects of global increases in temperature under e[CO2].This may be deemed especially important given the expected impact of global climate change.This suggests that increasing carbon assimilation through manipulation of photosynthesis[84,85]can have similar outputs to improved photosynthesis through growth in an enriched carbon atmosphere and further demonstrates the viability of this approach for improvement of yield and quality in fruiting crops.This must be studied considering the recent work showing that improved carbon assimilation also results in improved nutrient uptake and an increase in NUE[86]. Future opportunities As[CO2]surpasses 55o ppm,Asat will be limited by the rate of RuBP regeneration rather than Rubisco activity suggesting there is scope to improve plant photosynthesis to increase yield in greenhouse environments where CO2 is routinely increased to 1000 ppm or more for short periods of time. These short time-periods are furthermore unpredictable and chaotic given that greenhouses must be vented,due to external environmental PDF conditions,to maintain,as close as possible,optimal growing conditions i.e temperature and humidity inside the growth facility.Furthermore,the Heip(" [CO2]dosing capacity must be economically beneficial,especially given the chaotic nature of CO2 loss to the environment during periods of venting. Oxford University Press uses cookies to enhance your experience on our website.By selecting`accept all'you are agreeing to our use of cookies.You can change your cookie setting More information can be found in our Cookie Policy.