Between the branches of the star-shaped fruit forest, land will be cultivated. At La Evidencia, we know that where healthy and wild plants grow in abundance, there are several species of plants that combine to create a community connected by their roots and mycorrhizal symbioses. [For more information on mycorrhizae, please read the article on soil microorganisms and their symbioses with plants here]. Each species brings its beneficial properties to the ecosystem, allowing for survival, cohabitation and maximized productivity. That is why, at La Evidencia, we want to use a technique used by Native Americans (Mexico) to grow three plants simultaneously on a piece of land. Traditional Milpa is an association between corn, beans and squash;
Corn seeds are sown at a distance of 50 cm of each other. When corn plants reach about 15 centimetres of height, beans are sown at their feet, also squash seeds are sown between each corn stalk. Benefits perceived by each members of this association are:
1. Corn acts as a stake for climbing beans. At the same time, corn plant produces a lot of organic matter. Corn plant incorporates a lot of CO2 during photosynthesis, which adds a lot of carbon to the ecosystem when it is consumed by predators and broken down by micro-organisms. Indeed, photosynthesis is a set of biochemical reactions that will result in the transformation of 6 molecules of CO2, associated with water (H2O), into a sugar molecule (C6H12O6).
Through photosynthesis, green plant cells transform CO2 and H2O into carbohydrate molecules (C6H12O6) thanks to the sunlight energy in organelles called chloroplasts.
Photosynthesis
The photosynthesis takes place inside the chloroplasts. First, inside the thylakoid membrane, light strikes a first photoelectric system (PSII on the photo) which is a complex containing several electron transferors. Excited by photons of sunlight, electrons of transferors will eventually reach the chlorophyll molecules (P680).
Once on P680, electrons will provide the energy needed to transform water (H2O) into ½ O2 and H+. Electrons on P680 are then transferred to a second photosystem (PSI) via an electron transport chain in a mannier that more H+ is transferred from the outside of the thylakoid (stroma) to the inside. The difference in H+ concentration between the inner and the outter areas of thylakoids allows the generation of ATP by ATP synthase when H+ naturally diffuses from its highest to the lowest concentration. ATP (Adenosine Tri-Phosphate) is the energy storage molecule in all living beings. ATP releases energy when the atomic bond between the last two phosphate atoms is broken.
On the second photosystem, electrons are transferred to another form of chlorophyll (P700) providing the energy needed to generate NADPH, the second source of biological energy (after ATP). The second stage of photosynthesis: once ATP and NADPH have been generated from the energy transmitted to chloroplasts by light, these biological energies will be used in chloroplasts for CO2 fixation by the Rubisco. Rubisco is an enzyme that will bind CO2 in a space located on the surface of leaves connecting to the outside air when the stomata are opened (see photo). During multiple chemical reactions, called the Calvin-Benson cycle, CO2 is incorporated into the glycerol-3-phosphate (G3P) which is then combined to form glucose molecules (C6H12O6).
Respiration
Plants don’t just do photosynthesis, they also breathe, just like animal cells. Breathing occurs day and night in all living cells. -) First, glucose (issued from photosynthesis) is split in the cytoplasm into pyruvic acid in a process called oxygen-free glycolysis, producing a small amount of ATP, NADH and pyruvate.
-) Second: Pyruvate is the first product of a series of chemical reactions, the Krebs cycle, that occur within mitochondria (other organelles present in cells). It releases molecules of NADPH and carbon dioxide.
Finally, the NAPH released during the Krebs cycle feeds an electron transport chain (with oxygen, O2, as the final acceptor of electrons for forming H2O ) within the mitochondria membrane. As with photosynthesis, a difference in H+ concentration is created between the internal and external space of the mitochondria that feeds the generation of ATP by passive diffusion.
The relative rates of photosynthesis, which produces food molecules (C6H12O6) , and respiration, which burns these food molecules to produce energy (ATP), influence the overall productivity of plants. When photosynthesis activity exceeds breathing (i.e., during the day), C6H12O6 is produced then plants grow. When respiration exceeds photosynthesis (i.e., at night), growth slows.
Photosynthesis and respiration both increase with increasing temperature, but above a certain level, photosynthesis stabilizes while rate of respiration continues to increase.
This can lead to a depletion of stored energy. In fact, it has been established that the optimal leaf temperature for photosynthesis is between 15 and 25°C. The reason for this is that when the temperature increases, the water evaporates faster. Thus, in order to not lose all its water supply, plants close their stomata. But by closing the stomata, CO2 does not enter air chamber on the surface of the leaves. The Rubisco will then fix oxygen instead of CO2 because oxygen concentration in air chambers increases due to the breathing that continues to occur. This process is called photorespiration and leads to the loss of carbon, nitrogen, NADPH and ATP.
Therefore, it may be preferable to protect plants from the sun when the temperature is too high (above 25°C). Especially since the photosynthesis rate no longer increases with the luminous intensity at a certain threshold of light. But, plants from warm countries, due to induced losses of nutrients; have adapted to reduce photorespiration when the temperature is high. There are two adaptations:
-) C4 photosynthesis (3% of all plants; ex: in common grasses, corn, sugar cane, millet, sorghum) In contrast to conventional photosynthesis, where the molecule resulting from the fixation of CO2 is composed of 3 carbon atoms (C3), in C4 photosynthesis, the molecules formed by the addition of CO2 are composed of 4 carbon atoms. CO2 is attached and stored in oxaloacetate in mesophyll cells that are in contact with air chambers and do not contain chloroplast. Mesophyll cells then transferred CO2 to photosynthetic cells under the form of malate or asparate.
On request, asparate is transferred to bundle shealth cell where the CO2 will be re-extracted from the asparate and occur the Benson-Calvin cycle. By physically separating the CO2 fixation from the air and the Benson-Calvin cycle, the C4 plants ensure (i) that the CO2 concentration is always sufficiently high around the Rubisco; (ii) prevents the Rubisco from fixing O2, thus preventing photorespiration even when the stomata are closed. Corn is a plant photosynthesis in C4.
-) Photosynthesis CAM (ex: Crassulaceae, agave, pineapple, cactus, epiphytes, orchids and bromeliads) Rather than a physical separation of the CO2 fixation from the air and the Benson-Calvin cycle, CAM photosynthesis separates these two events in time. Stomata are opened at night when the temperature is lower, which allows the CO2 to be fixed and stored as acids (malic acid).
During days when stomata are closed and light supplies energy to chloroplasts, CO2 is again extracted from storage acids and the Benson-Calvin cycle occurs.
2. Climbing beans belong to the Fabaceae family. Plants of Fabaceae family are very important for nitrogen cycle in an ecosystem. [If you would like to learn more about the importance of Fabaceae for nitrogen input in an ecosystem, please read the article on soil microorganisms and their symbioses with plants here]
3. Squash are creeping plants whose large leaves cover the entire surface of the growing area under Milpa. This plays the very important role of constantly covering the ground, the three main reasons for this importance are: -) Prevent unwanted plants to grow. -) Maintain moisture by preventing evaporation. -) Provide shelter from the sun for soil microorganisms. Soil microorganisms are crucial to soil fertility. At La Evidencia, we are going to copy the idea of some tribes in North America that have added a plant whose flowers attract bees and will promote the pollination of beans and squash (corn being anemogamous).
Since terrestrial plants are attached to the soil, for sexual reproduction to occur, the male gametes (i.e. pollen or microsporangium on the stamens of the flower) must be transported to the female gametes (i.e. eggs that are within the pistil composed of style and stigma).
The transport of pollen grains from stamens to pistils is called pollination which will lead to fertilization if there is compatibility between gametes. Pollination may be carried out by wind (anamogamy) or animals (zoogamy) but also,in rare case, by water (hydrogamy).
In the case of zoogamy, (i) plants report the location of their gametes by bright colours and/or fragrance of their flower; (ii) pollinators feed on the highly energetic nectar produced by plants in nectaries. In order to feed on nectar in this way, pollinators push the stamens, which cover their bodies with pollen grains; (iii) By going to feed on the nectar of the next flower with their bodies covered with nectar, pollinators deposit pollen on its pistil. If the deposited pollen and pistil are compatible, it will migrate down trough the style to the eggs and fertilize them. The fertilized eggs mature into seed. Then, again because of their immobility, plants have to resort to strategies allowing the transport of seeds to favorable places for their germination . One of the strategies is to attract animals that will carry seeds further or further away. This strategy is called zoochory and, in this case, some parts of the flower (depending on the species), after fertilization, turn into a nutritious fruit enveloping the seeds that attracts the animals to eat them. At the same time, animals will transport the seeds into their digestive system, dispersing them.
Therefore, in order to achieve high yields of fruit, it is important to maximize the chances of pollination. While wind is present in all open places on Earth for the pollination of anemogamous species, animal presence for the pollination of zoogamous species is less predictable. That’s why, at La Evidencia, we’re going to try to attract pollinators as much as possible by planting native species of plants and trees that produce a lot of nectar. Nectar-producing plants and trees such as Stachytarphera frantzii and Acnistus arborescens will be planted with the objective of attracting pollinating birds such as hummingbirds, guit-guit and other passerines. They will ensure a certain level of ornitophilia that is necessary for pollination of trees such as papaya, banana and nutmeg. Close to trees requiring bats for pollination (chiropterophilie) such as mango, guava and dragon fruit, we will plant trees and palms such as Ipomoea alba and Astrocaryum mexicanum that provide food and shelter to nectarivore bats. Other pollinators that are important for ecosystem development are insects. In addition to attracting birds and bats with different shapes, colors and fragrances, plants mainly attract insects by rewarding them with their nectar for the service rendered (pollination). Insects of all kinds will be attracted by the diversity of plants in La Evidencia; we will encourage them to stay by distributing natural assemblages inviting them to take up residence and nest there. In addition to these wild insects, knowing that La Evidencia will be a very densely bloomed place requiring insect pollination, we will add two honey bee hives to the field. We want to add only two hives to ensure a high pollination rate while allowing wild insect populations to thrive with the large amount of nectar and pollen available. Two important advantages of having honeybee hives for us are the honey yield and the contribution of beeswax that is usable for many small applications as well as the realization of cosmetic products.
So we will follow the example of our ancestors and even try to adapt and diversify it by having 4 different zones with 4 different cultures similar to the Milpa: (i) Corn + squash + climbing beans + beets (ii) Sunflower + Bean + Butternut + Peanut (iii) Cassava + Peas + Carrot + Strawberry (iv) Sugar Cane + Chile + Sweet Potato + Lupin
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