Plant Biology: Sunlight, Water and the Food We Eat

A single redwood in northern California can stand about 116 metres tall. Its topmost leaves are a football pitch's length above the soil, and each one is making sugar from air and light while a column of water, pulled from the roots with no pump anywhere in the tree, climbs to reach them. A cactus beside a desert road does something different: it keeps its pores shut all day and opens them only in the cool of the night. A field of maize in Mexico keeps its pores open in heat that makes a wheat plant in the next field struggle.

These are all answers to the same trade-off, and a plant biologist sees it everywhere: to take in carbon dioxide you have to open a hole, and every open hole lets water out. A graduate of this course still just sees plants as organisms solving that problem with light, pipes, chemistry and hormones, and still sees every grain on the table as the result of a long human argument with the same plants.

Photosynthesis and its light curve

Photosynthesis turns carbon dioxide and water into sugar and oxygen, using light. Light first knocks electrons loose in the thylakoid membranes of the chloroplast and charges up two carriers (ATP and NADPH); then an enzyme called Rubisco in the Calvin cycle uses that energy to attach CO₂ to a five-carbon sugar. The two stages limit each other, and that gives the signature shape every course draws: the light-response curve.

In dim light, every extra photon adds a little more fixed carbon, so the curve rises steeply and almost straight. In strong light the machinery downstream cannot keep up and the curve flattens at a ceiling, P_max. In the dark the plant still respires, so net uptake is negative. The light level where uptake exactly cancels respiration is the compensation point. A common textbook formula for the curve is a "non-rectangular hyperbola", a smooth bend between the steep start and the flat top:

The two curves teach opposite lessons. The sun leaf has a high ceiling, but pays for its machinery with higher respiration, so it needs more light just to break even (about 30 here). The shade leaf is cheap to run, breaks even in dim light (about 9), but tops out near a quarter of the sun leaf. At 600 the sun leaf nets about 17.3 and the shade leaf about 5.3; double the light to 1,200 and the sun leaf gains to about 20.9 while the shade leaf barely moves (5.4). That is why a forest-floor plant scorches in a clearing, and why a house plant that "likes bright indirect light" is not lying.

C3, C4 and CAM: three ways around Rubisco's flaw

Rubisco is the most abundant enzyme on Earth and has a flaw: it also grabs oxygen. Oxygen is about 500 times more plentiful than CO₂ in air, so in the model below Rubisco picks oxygen instead of CO₂ roughly one time in four at 25 °C, producing a toxic by-product that the cell must expensively recycle (photorespiration, which releases some of the carbon again). The share lost grows with temperature, because the enzyme's preference shifts and because warm leaves close pores and starve the inside of CO₂. Most plants (rice, wheat, potatoes, trees) simply pay this cost; they are C3 plants, named for the three-carbon molecule that is their first product.

Two other strategies evolved, in many separate lineages, to get around it. C4 plants (maize, sugarcane, sorghum, millet) first fix CO₂ in the outer mesophyll cells into a four-carbon acid using an enzyme that ignores oxygen, then pump that acid into inner bundle-sheath cells (the ring of cells called Kranz anatomy), where it releases CO₂ right next to Rubisco at high concentration. It costs extra energy but nearly stops photorespiration. CAM plants (cacti, agaves, pineapple, many orchids) do the same pumping in time rather than in space: pores open at night, CO₂ is stored as malic acid, and by day the pores are shut while the stored acid supplies Rubisco, which saves a lot of water. C4 species are about 3% of plant species but account for roughly a quarter of the carbon fixed on land.

Which wins depends on temperature and CO₂. Below, C3 is modelled with the standard Farquhar expression for photon efficiency, using a temperature-dependent photorespiratory compensation point; C4 is held at a flat efficiency, as measured in the lab, because its CO₂ concentrating pump masks photorespiration.

At today's roughly 420 ppm the lines cross near 29 °C: cooler, C3 is more efficient; hotter, C4 wins. At 180 ppm, like the ice ages, the crossover drops to about 13 °C, so C4 wins in almost any warm place; at 800 ppm it rises to about 42 °C. That is one reason researchers link the spread of C4 grasslands to low-CO₂ ages, and why some predict shifts as CO₂ rises (though water and nitrogen matter too). It is also why teams try to engineer C4 traits into rice.

Water: pulled, not pushed

Where does the pull come from? Evaporation. Water leaves through pores (stomata) in the leaf, and the sticky cohesion of water molecules means the loss tugs on the whole column of water in the xylem, the dead hollow pipes of wood, down to the roots. This is the cohesion–tension idea, argued by Dixon and Joly in 1894. The water is under negative pressure: stretched, like a rope. That is odd, because a suction pump can only lift water about 10 metres, when the column breaks. Tree xylem is narrow and lined with water-loving walls, so the stretched water stays together, but it is fragile: air bubbles (embolisms) can snap a pipe.

Gravity alone costs 0.0098 MPa for every metre of height (about 0.1 atmosphere per metre, so roughly one atmosphere for every 10 m). Friction in the narrow pipes adds more. The water potential at the top is the soil's potential minus both:

At 100 m with the default sliders the top leaf sits at −1.78 MPa, a suction of about 17.6 atmospheres, far past what a pump manages. In a 2004 study of coast redwoods, George Koch and colleagues measured leaf water stress rising with height and estimated that gravity and path resistance could limit trees to about 122 to 130 m, though they said they were not predicting that trees would reach it; the tallest then known was 112.7 m. The stomata decide the water bill: a leaf typically loses several hundred water molecules for every CO₂ it takes in, which is why C4 and CAM, and every drought-adapted plant, are in the end water-saving stories.

Hormones and senses: how a plant that cannot move decides

A plant cannot run, so it grows toward what it needs. Charles Darwin and his son Francis showed in 1880 that grass seedlings bend toward light only if the tip is uncovered, so the tip must send a signal. In the 1920s Frits Went caught that signal in agar blocks: auxin, which moves to the shaded side of the stem and makes cells there elongate, so the stem curves toward the light. Auxin also drives gravity responses and organises roots. Others complement it: cytokinins promote cell division and shoots; gibberellins lengthen stems; ethylene, a gas, ripens fruit; abscisic acid closes stomata in drought. Plants also sense light colour through phytochromes (red versus far-red decides whether to stretch or branch, and when to flower), so a plant "sees" a neighbour's shadow before touching it.

The mix matters more than any one hormone. Skoog and Miller showed in 1957 that in tissue culture the ratio of auxin to cytokinin steers a lump of undifferentiated cells: lots of auxin gives roots, lots of cytokinin gives shoots, and a middle ratio gives unorganised callus.

The same system explains why pinching off the tip of a shrub makes it bushy: the tip's auxin was suppressing side shoots. Plants also react to touch and injury. The Venus flytrap closes only if two touches arrive within about twenty seconds, which acts as a short memory, so a raindrop does not trigger it.

Domestication: crops from every continent

About 10,000 years ago people in several regions, independently, began saving and sowing seed. The wild plants had traits that suit them and not farmers: seeds that shatter and scatter, dormancy, bitter chemicals, many branches. Selection for the opposite (seeds that stay on the plant, big grains, uniform sprouting) is domestication, evolution run by people. It happened in separate places:

Crop	Where	When (approx.)	What changed
Wheat	Fertile Crescent (Southwest Asia)	~11,500 years ago (first cultivation c. 9600 BC)	Tough rachis, free-threshing grain; bread wheat is a hexaploid with three genomes from hybridisation with goatgrass
Rice	Yangtze basin, China	~9,000 years ago (estimates span about 8,000 to 13,500)	Non-shattering, larger grain
Maize	Southwestern Mexico (Balsas River valley)	~9,000 years ago	From teosinte: one ear, naked kernels, one main stalk
Potato	Andes of southern Peru and Bolivia, near Lake Titicaca	7,000 to 10,000 years ago	Tubers, with toxins bred down; thousands of Andean varieties
Sorghum	Eastern Sudan, Africa	>5,000 years ago	Larger grain, free-threshing; drought tolerant, a Sahel staple

Look at maize. Teosinte has tiny, hard-cased seeds and many side branches; maize has a huge cob on a single stalk. Surprisingly, a few genes with large effect, such as teosinte branched 1 (tb1, which suppresses side branches), account for much of the difference, which is what makes rapid domestication plausible. Monoculture brings a risk that the Andes avoided by growing thousands of potato varieties: in the 1840s Europe, and especially Ireland, relied on very few potato types; the late-blight pathogen Phytophthora infestans struck in 1845 and, together with economic and political conditions under British rule, helped cause a famine that killed an estimated 775,000 to 1.5 million people. Genetic diversity is crop insurance, and the wild relatives in the Andes now supply blight-resistance genes to breeders.

The Green Revolution and its critics

By the 1950s the old wheat varieties had a problem. Feed them more nitrogen and they grew taller, and fell over (lodging) before the harvest. The fix was height: a Japanese dwarf cultivar, Norin 10 (bred by Gonjiro Inazuka), was crossed into wheat by Orville Vogel in the US and used by Norman Borlaug in Mexico from the 1940s, and in 1966 the International Rice Research Institute in the Philippines released the semi-dwarf rice IR8. Shorter, stiffer plants put more of their growth into grain: harvest index, the grain share of the whole plant, rose. The dwarfing genes turned out to be mutations in gibberellin synthesis or response, so the Green Revolution was a hormone story: a plant that sees less gibberellin makes a shorter stem. Borlaug won the Nobel Peace Prize in 1970.

The logic holds: the tall variety peaks at a modest dose and then falls, while the semi-dwarf keeps responding up to about 180 kg of nitrogen. In the real world the results were large. Annual rice production in the Philippines rose from 3.7 to 7.7 million tonnes in about two decades after IR8 was released, and Mexico moved from importing about half its wheat in 1943 to exporting by 1964. Supporters, such as the economists Evenson and Gollin, argue that modern varieties raised yields across Asia and Latin America, held down food prices, and spared land that would otherwise have been ploughed.

The critics are serious too. Because the new varieties needed fertiliser, irrigation and credit, benefits went first to farmers with all three, and widening inequality followed in some regions. Heavy use of nitrogen and pesticides polluted water; irrigation drew down groundwater in places such as India's Punjab; replacing many local varieties with a few risked diversity. The Indian physicist and activist Vandana Shiva, in The Violence of the Green Revolution (1991), argues it damaged soils, traditions and farmers' autonomy. Defenders reply that without the higher yields, famines would have been worse or more land would have been cleared. Neither side is wrong about the facts it cites; the fight is over what to count. Africa, where sorghum, millet and cassava dominate, saw far less of the first Green Revolution, and later initiatives have tried different approaches. Genetic engineering and gene editing are debated in the same terms.

What a graduate just sees

• Every open pore trades water for carbon. Nearly every plant adaptation is about that exchange.
• A leaf's light curve rises, bends and flattens: sun leaves have high ceilings and high running costs, shade leaves the reverse.
• Rubisco also binds oxygen; C4 and CAM are two solutions, one in space, one in time. C4 wins hot and low-CO₂.
• Trees pull water under tension; gravity costs about a tenth of an atmosphere per metre of height, and tall trees are near a hydraulic limit.
• Growth is steered by hormone balance: auxin to the shade side; the auxin-to-cytokinin ratio decides root versus shoot.
• Domesticated crops are products of selection on a few key genes, independently on every farming continent.
• Monocultures are brittle; diversity is insurance.
• The Green Revolution was a harvest-index story with real gains and real costs, both countable.

Where it shows up next

• Ecology (35/48): a leaf's carbon budget scales up to forest and ocean productivity and the carbon cycle.
• Evolution (35/47): domestication is the best-documented case of selection, and C4 shows convergent evolution.
• Cell Biology (35/44) and Biochemistry (35/43): the chloroplast, the thylakoid and Rubisco kinetics reappear in detail.
• In the world: crop breeding and food policy, forestry, drought and heat research, biofuel and carbon-capture debates, urban greening and gardening.

If you want to go deeper

• 📘 Textbook. Lincoln Taiz, Eduardo Zeiger, Ian Møller and Angus Murphy, Plant Physiology and Development (Sinauer / Oxford University Press; use the latest edition). The standard upper-level text on photosynthesis, water and hormones.
• 📘 Textbook. Ray Evert and Susan Eichhorn, Raven Biology of Plants (Freeman / Macmillan, 8th edition). Broader: structure, diversity and evolution.
• 📘 Gentler. Michael Pollan, The Botany of Desire (2001), a human-eye view of apples, tulips, cannabis and potatoes; or Zoë Schlanger, The Light Eaters (2022), on how plants sense and respond.
• 🌐 Free course. OpenStax Biology 2e, free online, with chapters on photosynthesis and on plant structure, transport and sensing; plus HHMI BioInteractive films.