Net Photosynthesis Calculator
Enter gross (total) photosynthesis and respiration to compute net photosynthesis Pn = Pg − Rd, quantifying the actual carbon assimilation after subtracting respiration.
Input Data
Results
At a glance:Net photosynthesis rate (Pn) is the rate at which a plant actually accumulates carbon, equal to gross (total) photosynthetic rate Pg minus the simultaneously occurring respiration rate Rd: Pn = Pg − Rd. Photosynthesis builds organics and releases O₂ from CO₂ and water in light — this is 'gross photosynthesis'; but plant cells respire constantly, breaking down organics and releasing CO₂ for energy. What an infrared gas analyzer (IRGA) measures in light is the 'net' value after the two cancel — which is why net photosynthesis is what decides dry-matter growth. When light is strong and Pn is positive, the plant net-absorbs CO₂ and accumulates dry matter; when light is very weak and gross photosynthesis just equals respiration, Pn is zero — this light intensity is the 'light compensation point'; if darker so respiration exceeds photosynthesis, Pn is negative and the plant net-releases CO₂. Experimentally one often decomposes it like this: measure respiration Rd in the dark first, then measure net Pn in light; adding them gives gross Pg (Pg = Pn + Rd). This calculator performs this conversion by Pn = Pg − Rd and returns Pn + Rd for back-calculation and checking, clarifying the three quantities among instrument readings and textbook problems.
Formula
Net photosynthesis: Pn = Pg − Rd.
Back-calculate gross: Pg = Pn + Rd.
Light compensation point: Pn = 0, i.e. Pg = Rd.
$$P_n = P_g - R_d$$$$P_g = P_n + R_d$$How to Use
- Enter gross (total) photosynthesis Pg (carbon assimilation before subtracting respiration).
- Enter the simultaneously occurring respiration Rd, same unit as Pg.
- The tool instantly computes net Pn = Pg − Rd; a negative net value means respiration exceeds photosynthesis.
Light intensity vs the three rates (illustrative)
| Condition | Gross Pg | Respiration Rd | Net Pn |
|---|---|---|---|
| Dark | 0 | 5 | −5 (net CO₂ release) |
| Light compensation point | 5 | 5 | 0 |
| Moderate light | 15 | 5 | 10 |
| Strong light (near saturation) | 25 | 5 | 20 |
Values are illustrative magnitudes (µmol CO₂ m⁻² s⁻¹); respiration is assumed fixed here but actually varies with temperature etc.
Case Studies
Net from gross and respiration
A leaf in strong light: gross Pg = 25 µmol·m⁻²·s⁻¹, dark respiration Rd = 5 µmol·m⁻²·s⁻¹.
Net Pn = 25 − 5 = 20 µmol·m⁻²·s⁻¹.
That is 20 µmol CO₂ net-fixed per m² per second — the actual amount accumulated as dry matter.
Negative net photosynthesis in weak light
In shade, a leaf has gross Pg = 3, respiration Rd = 8 µmol·m⁻²·s⁻¹.
Net Pn = 3 − 8 = −5 µmol·m⁻²·s⁻¹, negative.
Respiration exceeds photosynthesis; the leaf net-releases CO₂ and consumes reserves — prolonged low light is unfavorable for growth.
FAQ
How to distinguish gross, net, and respiration?
Gross (total) photosynthesis Pg is the total CO₂-fixation rate of photosynthesis itself; respiration Rd is the rate cells release CO₂ by breaking down organics; net Pn = Pg − Rd is the rate the plant actually accumulates carbon after the two cancel. An IRGA in light measures net photosynthesis; to get gross you must add back respiration (usually measured in darkness).
Why does the instrument measure a net value?
Because photosynthesis and respiration happen simultaneously in the leaf, and the instrument can only measure the net change of CO₂ in the chamber. In light, the net CO₂ absorption = photosynthetic fixation − respiratory release = net photosynthesis. To separate gross, measure respiration in darkness first, then use Pg = Pn + Rd (assuming light and dark respiration are similar).
What does negative net photosynthesis mean?
It means respiration rate exceeds gross photosynthetic rate, so the leaf net-releases CO₂ and net-consumes stored organics. This usually occurs when light is very weak (below the compensation point) or at night. Prolonged negative net photosynthesis prevents dry-matter accumulation and even consumes the plant, harming growth.
What is the light compensation point?
The light intensity at which net photosynthesis is exactly zero, so gross photosynthesis equals respiration (Pg = Rd) and the plant neither net-absorbs nor net-releases CO₂. Below it net is negative, above it net is positive. Shade-tolerant plants have a lower compensation point and can keep positive growth in dimmer conditions.
What units are common?
Plant physiology most often uses µmol CO₂ m⁻² s⁻¹ (micromoles of CO₂ fixed per m² leaf area per second). mg CO₂ dm⁻² h⁻¹ is also seen. This calculator does not fix the unit; as long as gross and respiration share a unit, the net result is in that same unit; it defaults to µmol·m⁻²·s⁻¹ for demonstration.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.