Photosynthesis, transpiration, nutrient flow, and light response — in plain English
Anatomy gives you the names. Physiology tells you why every reading on the dashboard matters. This page is the "why" behind your tent: why VPD changes growth speed, why removing too many fan leaves stunts the plant, why a wet reservoir kills roots, why blue light keeps plants short, and why darkness matters as much as light during flower.
You don't need to memorize biochemistry. You just need to understand five mechanisms — and the rest of the dashboard, the alerts, and the Jarvis advice will start to make sense as a system instead of a checklist.
On this page
☀️ Photosynthesis — the food factory
A fan leaf is a chemical factory: it pulls CO₂ from the air, water from the roots, and energy from light to assemble sugar molecules — releasing O₂ and water vapor as byproducts.
Inside every fan leaf are millions of chloroplasts — tiny green factories holding chlorophyll. Chlorophyll absorbs blue and red light (it reflects green, which is why leaves look green). The captured energy splits water and rearranges CO₂ from the air into glucose, the sugar that builds everything else: stems, leaves, roots, trichomes, and ultimately the bud weight you harvest.
The simplified equation:
6 CO₂ + 6 H₂O + light → C₆H₁₂O₆ (sugar) + 6 O₂
That sugar is then shipped through the phloem — the outer layer of every petiole and stem — to wherever the plant is currently building tissue: roots in early veg, side branches mid-veg, buds in flower.
💧 Transpiration & VPD — the water pump
Plants don't have hearts. There's no pump. So how does water travel from a root tip 12 inches below the soil all the way up to a fan leaf at the top of the canopy?
Evaporation pulls it. Each leaf has thousands of microscopic pores called stomata (mostly on the underside). When stomata open to let CO₂ in, water vapor escapes — pulled out by the difference between the wet leaf interior and the drier tent air. As that water leaves, the column of water in the xylem (inner tube of the stem) is sucked upward like soda through a straw. New water enters the roots to replace it. Nutrients ride along for free.
That "suck" is what your dashboard calls VPD — vapor pressure deficit. It's a single number that captures how thirsty the air is for water:
VPD is computed from temperature and humidity together — that's why neither number alone is enough. 75°F at 70% RH and 75°F at 45% RH are completely different environments for the plant, even though the temperature reads the same.
If VPD drops too low (humid + cool), stomata close because there's nowhere for water to evaporate to. The plant stops drinking, stops eating (no transpiration = no nutrient flow), and over a long stretch shows symptoms that look exactly like a calcium or magnesium deficiency — even when the soil is full of both.
🧪 Nutrient uptake & mobility
Roots can only absorb dissolved ions — not solid nutrients. That's why pH matters: at the wrong pH, ions latch onto soil particles or precipitate out, and the plant can't pick them up even though they're right there. For soil, target pH 6.0–7.0. Outside that range you get a deficiency that looks like missing nutrients but is really a lockout.
Once inside the plant, nutrients fall into two camps:
| Group | Nutrients | Symptom appears on... | Why |
|---|---|---|---|
| Mobile | N, P, K, Mg | OLD (lower) leaves first | Plant strips them from older leaves to feed new growth. Lower leaves yellow first. |
| Immobile | Ca, S, Fe, B, Mn, Cu, Zn | NEW (top) leaves first | Once placed, they stay. New growth shows the deficiency because the plant can't reallocate. |
This rule alone diagnoses 80% of deficiencies. A yellow lower fan leaf with green veins? Mobile-N issue. New top leaves curling and pale at the tips? Immobile calcium. The plant tells you exactly where the problem is by where it appears.
The big three (N-P-K) cycle by stage:
- Veg: high N (leaves), moderate P, moderate K
- Stretch / early flower: N tapers, P rises sharply (root + bud development)
- Mid–late flower: low N, moderate P, high K (bud bulking + ripening)
- Last 1–2 weeks: plain water flush so the plant uses up stored nutrients (cleaner smoke)
💡 Light spectrum & intensity
Plants don't see light the way humans do. They're sensitive to specific wavelengths that drive specific responses. Modern full-spectrum LEDs cover the whole productive range, but understanding the components helps you read the plant.
| Wavelength | What plants do with it | Stage where it dominates |
|---|---|---|
| Blue (400–500 nm) | Compact growth, tight internodes, thick stems, opens stomata | Veg / early life |
| Green (500–600 nm) | Penetrates deep into the canopy — reaches lower bud sites | Mid-flower |
| Red (620–700 nm) | Drives flowering, induces stretch, signals "sun is high" | Flower |
| Far-red (700–780 nm) | Reverses Pfr→Pr (resets the "is it night?" signal). Used for triggering early flower onset. | Transition |
PPFD (Photosynthetic Photon Flux Density, µmol/m²/s) is what the canopy actually receives at one moment. DLI (Daily Light Integral, mol/m²/day) is the total over 24 hours — like rainfall accumulation. Think of PPFD as now and DLI as the day's total.
🌙 The dark cycle & photoperiod
Cannabis is a short-day plant (technically a long-night plant). It uses a pigment called phytochrome to measure how long the dark period lasts. Phytochrome flips between two states: Pr (inactive, made in the dark) and Pfr (active, made in light).
Long uninterrupted darkness lets enough Pr accumulate to trigger the genetic program for flowering. Even a brief flash of light during the dark period — a porch light, a phone screen, an LED indicator — converts Pr back to Pfr and resets the count.
This is why 12/12 light/dark triggers flower in nature: it mimics autumn day length. And it's why a single light leak can cause:
- Delayed or aborted flowering
- "Re-vegging" (plant tries to grow leaves out of buds)
- Hermaphroditism — plants self-pollinate from stress, ruining sinsemilla buds with seeds
The dark period isn't just "not light" — it's when the plant does respiration: burning the day's sugars to actually grow tissue. Photosynthesis builds the materials; respiration assembles the plant. Both are needed. A plant kept on 24/7 light wastes energy and stresses out.
Most growers run 18/6 in veg (good growth, 6 hours respiration) and 12/12 in flower (triggers + maintains flowering).
🧬 Hormones & tropisms
Plants don't have brains. They coordinate their entire body — growing toward light, sending nutrients to flowers, waking up dormant buds — using hormones. You don't need to know all of them, but four are worth knowing because they explain why training works:
| Hormone | Job | Grower-relevant effect |
|---|---|---|
| Auxin | Made at the apex, flows down | Suppresses side branches (apical dominance). Cut the apex → suppression lifts → side buds wake up. This is why topping works. |
| Cytokinin | Made in roots, flows up | Promotes branching and leaf growth. Healthy roots = explosive top growth. |
| Gibberellin | Drives stem elongation | The stretch you see in the first 2 weeks of flower. Spikes in early flower, then tapers. |
| Abscisic acid | Stress + dormancy | Closes stomata when the plant is thirsty, ripens flowers late in life. Light stress triggers it — controlled stress can boost trichome production. |
Tropisms are how the plant orients itself:
- Phototropism — growth toward light. Auxin redistributes to the shaded side, making it grow faster, bending the stem toward the bright side.
- Gravitropism — roots grow down, shoots grow up. Why a snapped stem still tries to right itself.
- Thigmotropism — response to touch / wind. Stems thicken and woody up when wobbled. A small fan isn't just airflow — it's structural training.
📌 What this means for your grow
If you only remember six things from this page:
- Fan leaves are factories. Don't strip healthy ones — the plant needs that surface area to make sugars.
- VPD is the water pump. Too low = no transpiration = no nutrient uptake. Aim for 0.8–1.5 kPa range.
- pH gates everything. Wrong pH = lockout, not deficiency. Test your runoff.
- Where the symptom appears tells you what's missing. Bottom leaves = mobile nutrients (N/P/K/Mg). Top leaves = immobile (Ca/Fe/etc.).
- Light isn't a setting, it's a daily dose. DLI is the integral. PPFD is the instant reading. Both matter.
- Darkness is sacred in flower. Even small light leaks can hermie the plant. Tape every LED indicator.