Gardening glossary
Respiration
Plants do not only photosynthesise. They also respire. Every living cell in a plant — roots, stems, leaves, fruit — is constantly burning the sugars made during photosynthesis to release the chemical energy stored in their bonds. The byproducts are carbon dioxide and water, the exact opposite of photosynthesis.
The simplified equation:
`C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (ATP)`
Three things follow from that equation:
1. **Plants need oxygen.** Roots in particular need oxygen dissolved in the soil water to respire. This is why soggy, waterlogged soil suffocates roots — there is no oxygen left in the pore spaces. Root rot is, fundamentally, a failure of root respiration. 2. **Respiration happens day and night.** Photosynthesis only runs when there is light. Respiration runs 24 hours a day. In low-light conditions, a plant can respire away more sugar than it can produce — net negative — and slowly decline. 3. **Warm temperatures speed respiration.** A hot night burns through stored sugars faster than a cool one. Tomatoes set fewer fruit in heatwaves partly because nighttime respiration outpaces daytime photosynthesis.
Practical implications:
- **Light vs heat balance for houseplants.** A plant in a dark winter room kept warm will respire away its sugar reserves and look more tired in March than a plant kept cool. Match temperature to light intensity wherever possible. - **Root rot risk.** Dense, peat-heavy, or compacted soils starve roots of oxygen. Add perlite, grit, or chunky bark to keep air space in the root zone. - **Post-harvest storage.** Apples and other fruit continue respiring after picking, slowly burning their stored sugars and softening. Cold storage slows respiration and extends shelf life — the same principle behind refrigeration. - **Sealed terrariums** work because a closed system reaches an equilibrium between daytime photosynthesis (which consumes CO₂ and produces O₂) and round-the-clock respiration (which does the opposite). Both processes have to balance over a 24-hour cycle for the system to be stable.
If you understand respiration alongside photosynthesis, almost every "why is my plant unhappy" question becomes easier to debug.