Sunshine: “Let there be light, and there was light!”

Fast facts: solar energy

• The sun is shining since 13 billion years and will continue some more billion years.
• The light quanta that hit the earth every day is 6,000 times more than mankind is using in primary energy today.
• The sun was and is in many cultures and religions like a god for life.
• 2.5 billion years ago photosynthesis has appeared. Plants grow with sunlight: 6 x CO2 (carbondioxid) + 6 x H2O (water) =1 x C6H12O6 (glucose) + 6 x O2 (oxygen).

• C4 plants (e.g., maize, sugarcane) have a higher sunlight efficiency compared to C3 plants (e.g., wheat, rice).

The sun has been shining on the earth for 13 billion years. 45 per cent of the 15 trillion ki-lojoules of light quanta that hit the earth every day are not absorbed by the atmosphere but reach the ground. This is six thousand times more energy per day than the primary energy consumed by mankind today. The earth's deserts receive more energy from the sun in 6 hours from the sun than mankind consumes in a year. On average, 17,280 kilojoules per day are produced on every square meter of the earth's surface (land and water), more than 200 W/m2.

In many cultures, the sun is seen as a symbol of life, fertility and renewal, as it enables plants to grow and provides warmth. Therefore, this star plays a central role in many religions and myth-ological systems around the world. Its significance ranges from a divine embodiment to a sym-bol of life and enlightenment. Ra  was the sun god, one of the most important gods in ancient Egyptian religion and considered the creator of the world and the source of all life. During the Amarna period, Pharaoh Akhenaten introduced the monotheistic worship of the sun god Aton , represented by a sun disc with rays ending in hands. In Greek mythology, Helios  was the sun god who travelled across the sky every day in a chariot drawn by horses. Later, the role of the sun god was often transferred to Apollo , who was also responsible for light, healing and the arts. In the Roman religion, Sol  corresponded to the Greek Helios and was wor-shipped as the god of the sun. In Hinduism, Surya  is the sun god who plays a central role in many ancient texts and rituals. In the Mayan religion, Kinich Ahau  was the sun god and an important god of the day. For the Aztecs, Huitzilopochtli  was an important sun god and god of war who required daily sacrifices to keep the sun in the sky. In Zoroastrianism, Mithra  was often associated with the sun. Although Christianity does not directly worship the sun as a dei-ty, there are numerous symbolisms that refer to the sun. Christ is often depicted with light and the sun's rays, symbolizing his role as the ‘light of the world’. In the Old Testament  (Hebrew and Christian bible) is written in Genesis 1:3-5 : “3And God said, 'Let there be light,' and there was light. 4 God saw that the light was good, and he separated the light from the darkness. 5 God called the light 'day', and the darkness he called 'night.' And there was evening, and there was morning—the first day." In Shintoism, Amaterasu  is the sun goddess and one of the central deities.

Photo 1: Lichens  are a community of fungi and photosynthetic cyanobacteria. They were among the first organisms to conquer the land. They are the most common life form in arctic regions and high altitudes. Reindeer can use lichens as food. Cladonia portentosa and C. ran-giferina grow as pioneer organisms on the granite plateaus in Scandinavia.

Light is energy and has initiated life via photosynthesis. This chemical reaction emerged around 2.5 billion years ago and provided the basis for life on the barren planet Earth. Around 5 per cent of the radiant energy from the sun that reaches the earth's surface can be converted into biochemical energy in the chloroplasts of the green cells (an average of 864 kilojoules per square meter). In the light reaction, chlorophyll molecules absorb the sunlight. In the subsequent dark reaction, this energy is used to convert atmospheric carbon dioxide (6 x CO2) and water (6 x H2O) into dextrose (glucose: 1 x C6H12O6) to produce. Breathable oxygen (6 x O2) is released back into the atmosphere as a waste product.

Cyanobacteria (blue-green algae), which are found in the oceans, were the first cells to carry out photosynthesis and produce oxygen with the help of chlorophyll. Even today, algae in the world's oceans are the most important oxygen producers on earth. They produce more oxygen than all the forests on earth. Today, algae can be found in all oceans, lakes, rivers, on land and in symbiosis with animals and fungi (lichens). The most important divisions are the red algae (Rhodophyta), the diatoms or diatoms (Chrysophyta), the brown algae (Phaeophyta) and the green algae (Chlorophyta).

Various types of algae form the plankton. The largest algae are seaweed, which can grow to more than a hundred meters in length. As phytoplankton, they are not only the food source for baleen whales, but also for all other animals in the world's oceans - directly or indirectly. They represent the beginning of the food chain in the sea - without plankton there is no marine life. Algae are distinguished by their color. The pigments overlay the color of the chlorophyll and therefore do not necessarily look green. These algae contain all the nutrients (especially the es-sential amino acids, minerals, and vitamins) that we humans need to live. In dried form, they can be kept almost indefinitely and do not lose their nutritional value.

Plants and bacteria capable of photosynthesis have utilized this and are independent of other organisms. They are therefore also known as autotrophs. The sugar formed during photosyn-thesis is richer in energy than the starting materials and serves as an energy store and scaffold-ing for the plants. The energy is used to build and maintain the entire life processes of plants, animals and ultimately humans.
The oxygen released during photosynthesis is needed by animals - including humans -, fungi and bacteria that are not capable of photosynthesis to breathe. Unlike plants and some types of bacteria, these heterotrophic organisms cannot utilize solar radiation directly as a source of ener-gy themselves. They must absorb and digest energy from other organisms. Plants not only pro-vide energy but also building materials.

Agriculture needs sunshine to let plants grow, but not too much and the right spectra. The Daily Light Integral (DLI) describes the number of photosynthetically active Fotons (individual parti-cles of light in the 400-700 nm range) that are delivered to a specific area over a 24-hour period. This variable is particularly useful to describe the light environment of plants. Outdoors, DLI values vary depending on latitude, time of year, and cloud cover.

Plants growing at high light invest less of their biomass in leaves and stems, and more in roots. They grow faster, per unit leaf area (ULR) and per unit total plant mass (RGR), and therefore high-light grown plants generally have more biomass. They have shorter internodes, with more stem biomass per unit stem length, but plant height is often not strongly affected. High-light plants do show more branches or tillers.
Highlight grown plants generally have somewhat larger seeds, but produce many more flowers, and therefore there is a large increase in seed production per plant. Sturdy plants with short in-ternodes and many flowers are important for horticulture, and hence a minimum amount of DLI is required for marketable horticultural plants. Measuring DLI over a growing season and com-paring it to results can help determine which varieties of plants will thrive in a specific location.

Plant species show different adaptations to light intensity. For example, photosynthesis special-ists, such as maize, sugar cane or sorghum as C4 plants , differ from C3 plants  (e.g., wheat, rice, vegetables, all trees). C4 plants do not achieve absolute light saturation even at high light intensity. This is why C4 plants are superior to C3 plants in terms of radiation utilization in full sunlight. In the case of C3 plants, a distinction is made between plants or organs adapted to strong light (sun) and weak light (shade). Although only 3% of flowering plant species use C4 carbon fixation, they account for 23% of global primary biomass production of plants.