There is no comparison to the joy of that first sip of coffee in the morning. A few sips of it make you motivated to tackle the day. As Alfred Renyi once said: “A mathematician is a device for turning coffee into theorems.” Personally, I have a fresh cup of coffee as soon as I open my eyes in the morning. That bitter taste of coffee is a perfect beginning to a productive day. However, the drink we all love is under threat!!! Global warming threatens its quality and quantity.
Increasing levels of CO2 have a direct link to global warming (The Causes of Climate Change). Global warming results in an increase in the average global temperature (Indian Journal of Science and Technology), which causes events, such as droughts, that have a negative impact on coffee species.
It is fascinating to note that the coffee crop is actually the second most valuable commodity exported by developing nations (The Climate Institute, 2016). Out of around 90 species of coffee, Robusta and Arabica coffee are dominant in the coffee trade (International Scholars Journals, 2016, 2(8), 160-163). In fact, 99% of world bean production is due to these 2 coffee species, and 70% of the supply is due to Arabica coffee
Robusta coffee seems to be more vigorous, robust, and productive, but Robusta’s quality is lower than that of Arabica coffee. It is also less susceptible to climate change than Arabica (Natural Resources Institute, Working Paper Series, 2012). Robusta coffee is native to a dense, equatorial forest (Congo basin) as a midstory tree, in a region of the temperature variation of 23-26 °C (with the optimal temperature for growth being 20-30 °C ). Arabica coffee is native to East African forests. It has an optimal temperature for growth of 14-28 °C. (Thus, its coffee plantations are found to become marginal in regions with an average air temperature of 24-25 °C). When the temperature is higher than 23 °C, the ripening and development of Arabica’s cherries are accelerated, but when the temperature is above 30oC, the heat causes a reduction in growth.
Altered rainfall patterns and rising temperatures already impact coffee quality, the prevalence of pests and diseases, and yields. For example, in 2014, a third of crops were destroyed by drought in Brazil. Predictions for the end of the century indicate that the world could warm by 2.6-4.8 °C. This number may sound small, but such a temperature increase will have remarkable consequences for global agriculture. According to regional studies, an increase in temperatures could result in critically low yields of Tanzanian Arabica by 2060. The majority of the 25 million coffee farmers in the world are smallholders, and for the majority, coffee is the primary source of income. They are under a significant threat due to global warming. The issue is also very well explained in the video below:
2) Coffee photosynthesis is very sensitive to high temperatures, with almost no photosynthesis happening at 34 °C, according to (A review. Braz. J. Plant Physiol. 2006, 18, 55– 81). As temperature rises beyond optimal temperatures, the enzymes responsible for carrying out photosynthesis become denatured, so the photosynthetic rate declines rapidly and then stops (Units Used to Measure Chlorophyll).
Also, when the temperature rises above optimal for the coffee plant, this can cause a reduction in the accumulation of sucrose in the bean and modify the levels of various compounds (which make a contribution to taste, aroma, and flavour after roasting).
In addition, increased temperature causes a decrease in the density of bean, yield, and mass (Frontiers in Plant Science, 2018, 9, Can Elevated Air [CO2] Conditions Mitigate the Predicted Warming Impact on the Quality of Coffee Bean.) When temperatures become too high, the content of caffeic acid decreases, but the content of total CQAs (chlorogenic acids), trigonelline, p-coumaric-acid and caffeine increases (their structures are shown in figure l below).
Chlorogenic acids are transformed into chlorogenic acid lactones when roasting coffee beans (Why is Coffee bitter? – The Chemistry of Coffee). Chlorogenic acid lactones are significant contributors to taste (Tweaking Coffee’s Flavor Chemistry); therefore, the bitterness of coffee increases if their concentration increases. This is because the breakdown products of chlorogenic acid lactones in roasted coffees influence the bitterness. These products are phenylindanes (their bitterness taste is more intense than that of chlorogenic acid lactones). Also, these phenolic acids can be broken down to form tri and di-hydroxybenzenes (e.g. hydroxyhydroquinone). Scheme 1 shows these reactions that contribute to the bitterness of coffee. There is also a fascinating blog post that talks about heat effects on the taste of different foods, such as coffee.
Drought is considered the major environmental stress factor that affects the production of coffee (Agric. Food Chem. 2018, 66, 21, 5264–5274) in the majority of coffee-growing areas. A decrease in yield could be as much as 80% in arid years. Drought stress stimulates earlier ageing of the plants, which may result in a more direct consequence of hydraulic failure (the result of the plant being unable to move water from roots to its leaves). Draught progression also results in a decrease in the net photosynthetic rate per unit area. Such decreases are associated strongly with stomatal factors (coffee stomata are sensitive to air evaporative demand and soil water availability). Stomata are very small pores usually located in plant leaves tissue, allowing for gas exchange and taking in CO2. (What is The Function of Plant Stomata). Because stomatal conductance (degree of stomatal opening) (Drought-affected trees die from hydraulic failure and carbon starvation) decreases curvilinearly, so put more simply, as the amount of water within the leaf decreases, stomatal closure is triggered. Stomatal closure results in a decrease in photosynthetic carbon assimilation
(Front. Plant Sci., 12, 27 September 2021). Regarding evaporative air demand, even under constant temperature and well-watered conditions, research has shown that the net photosynthetic rate per unit leaf area decreases on average by 40%, while stomatal conductance decreases by 60%.
Drought-induced stomatal closure is a big issue, as it causes a decrease in the availability of CO2 to ribulose-1,5-bisphosphate carboxylase/oxygenase (abbreviated to RuBisCO). RuBisCO is a crucial enzyme used in the catalysis of CO2 fixation in photosynthesis (details are given below ). This is vital for the Calvin cycle, which occurs in the coffee plant’s cells. The Calvin cycle is a series of chemical reactions which convert hydrogen-carrier compounds and CO2 into glucose. Such reactions occur in the stroma of the cell (Calvin Cycle). The cycle is shown in figure 2.
The cycle (What happens during the Calvin-Benson cycle, or dark reaction?) consists of 3 stages:
1.) Carbon fixation:
This step involves the initial incorporation of CO2 into organic material. This requires 3 CO2 molecules because glucose is the end product of the reaction. For glucose, at least 3 molecules of CO2 are required (bear with us on this one…). 3 CO2 molecules contain 3 C atoms, so they will form 1 triose (carbohydrate molecule). CO2 reacts with a very reactive phosphorylated 5 carbon-sugar ribulose bisphosphate (RuBP) at the beginning of the cycle, as shown in Scheme 2.
The rubisco (ribulose biphosphate carboxylate) enzyme catalyses this reaction (Units Used to Measure Chlorophyll). The 6-carbon intermediate immediately breaks into 2 molecules of 3-PGA (3-phosphoglycerate). This stage ends as carbon (which was originally part of CO2) becomes part of an organic compound (hence the name of the stage fixation, indicating that the fixation of this carbon occurred).
2.) Reduction:
Here, the previously fixed carbon gets reduced to energy rich G3P. Each 3-PGA molecule receives an additional phosphate from ATP (adenosine triphosphate) (which is in the process gets converted into ADP (adenosine diphosphate) as shown in Scheme 3), forming 1,3-phosphoglycerate. ADP is then converted back into ATP via complex light induced reactions. Later, NADPH (reduced nicotinamide adenine dinucleotide phosphate)donates a pair of electrons to it, reducing it to G3P (glyceraldehyde 3-phosphate) carbohydrate (NADPH in that process is converted into NADP+ (nicotinamide adenine dinucleotide phosphate). Out of 6 produced G3P, 5 get recycled in the regeneration stage, while plants use 1 G3P to make glucose. Just like with ADP and ATP, NADP+ is converted into NADPH via complex light induced reactions. Scheme 4 shows the redox reaction between NADPH and NADP+.
The overall reaction of 3-PGA conversion into G3Ps is shown in Scheme 5.
3.) Regeneration of CO2 acceptor, RuBp:
Through a complicated series of reactions, the carbon skeletons of 5 G3P get rearranged into 3 5-carbon RuP (ribulose phosphate). RuP molecules get phosphorylated to RuBP (this process requires 3 ATP molecules, which are produced from ADP due to reactions involving light). The cycle then repeats. The decrease of CO2 intake for this process will, as a result, decrease glucose production, and glucose is a food for all life!
In a bit more detail (sorry, we warned you that it is complicated…): The G3P molecule produced in the reduction stage is then converted into dihydroxyacetone phosphate by the Triosephosphate isomerase enzyme. Then, a combination of the resulting molecule with another G3P molecule by aldolase enzyme to form fructose-1,6-biphosphate occurs. (Glycolysis and Fermentation). The resulting molecule is then catalysed by phosphofructokinase enzyme reaction into fructose-6-phosphate. The resulting molecule is then converted into glucose-6-phosphate by a phosphoglucose isomerase enzyme. To get rid of the phosphate group, the Hexokinase enzyme converts glucose-6-phosphate into glucose, getting rid of the phosphate group, finally resulting in glucose! The purpose of Mg2+ is to stabilise the charge of the negatively charged phosphate group of ATP. Scheme 6 shows these reactions.
3.) Drought can also cause oxidative stress, leading to negative effects in several biological processes (The Roles of Environmental Factors in Regulation of Oxidative Stress in Plant).
The cause of oxidative stress is due to the accumulation and overproduction of reactive oxygen species (ROS). These are molecules that have 1 or more activated oxygen atoms ( including free radicals ) (Mechanisms of oxidative stress in plants: From classical chemistry to cell biology). They promote oxidative stress through the oxidation of cell compounds.
Initially, in this process triplet oxygen (3O2) accepts an electron. It, therefore, loses it’s spin restriction and becomes O2•–. (the electron source is due to the leak of electrons in the ETC (electron transport chain) process (Respiratory electron transfer pathways in plant mitochondria), which involves coupling electron transfer from organic substrates into molecular O2 with proton translocation across the inner mitochondrial membrane. As a result, O2•– forms, which is more reactive than O2. Due to O2•– molecules short half life of (1-1000 µs), it can only diffuse for a few micrometres from the generation site. O2•– is involved in a variety of reactions, with the reaction with H+ one of the most common examples, forming HO2•–. 2 HO2•–. molecules form H2O2. These interact with Fe2+ and Cu+ (that were reduced by O2•– from Fe3+ and Cu2+ respectively) to produce •OH (which is key to oxidative stress and modification of organic molecules).
Numerous studies have shown that elevated concentration of CO2 in the air results in a greater supply of CO2 to ribulose-1,5-biphosphate (RuBP) carboxylase/oxygenase (RuBisCO). This boosts the RuBisCO carboxylation over oxygenation processes. This could then enhance plant vigour, improving the plant’s ability to endure different environmental stresses as well as decreasing the severity of some diseases (Mitigation of the Negative Impact of Warming on the Coffee Crop: The Role of Increased Air [CO2] and Management strategies). Also, crop yields are improved.
On balance, global warming negatively impacts coffee species; Robusta coffee plants are more resistant to raised temperatures and cope better with drought stress than Arabica cultivars. By transferring genes from Robusta to Arabica, there is an opportunity to cultivate new elite cultivars. Other useful strategies include grafting of arabica scions onto chosen vigorous Robusta rootstocks could be a faster alternative when compared to a slower breeding one to increase resistance towards negative factors, such as drought. In fact, (Global Breeding Network) in 2022, World Coffee Research will launch a global breeding network, which aims to improve breeding populations for coffee breeding programs around the world, which should help design the coffee of the future, which will not only be more resistant to predicted climate changes but also better tasting.
Have you learned anything new and fascinating? We would be happy to hear your opinion in the comments below!
Contributors: Maxim Semchenkov (research, writing, illustrations), Tom Silcox (proof-reading), Natalie Fey (review).
Picture credits:
Main image: by Chevanon Photography, https://www.pexels.com/photo/close-up-of-coffee-cup-on-table-312418/
Coffee plant image: by Daniel Reche, https://www.pexels.com/photo/red-and-yellow-coffee-berries-on-branch-1556665/
All other figures drawn ourselves.
Picture it... 