Net Assimilation Rate Calculators

0 calculators tagged with “Net Assimilation Rate”

Net assimilation rate (NAR) is a plant growth analysis parameter that measures the rate of dry matter increase per unit leaf area per unit time. It reflects the net photosynthetic efficiency of leaves — the balance between photosynthetic carbon gain and respiratory carbon loss, integrated over the whole plant. NAR is typically expressed in g/m²/day or mg/cm²/day. Together with leaf area ratio (LAR), NAR determines relative growth rate: RGR = NAR × LAR. NAR is influenced by light availability, leaf nitrogen content, temperature, CO₂, and leaf age.

All Calculators

No calculators found for this topic.

NAR Formula

NAR = (W₂ − W₁) × (ln LA₂ − ln LA₁) / [(LA₂ − LA₁) × (t₂ − t₁)]

W₁, W₂ = plant dry mass at times t₁ and t₂; LA₁, LA₂ = total leaf area at same times. Units: g/m²/day or mg/cm²/day. This formula assumes continuous, proportional growth between harvests.

Simplified Approximation

For small time intervals: NAR ≈ ΔW / (LA_mean × Δt), where LA_mean = (LA₁ + LA₂)/2 and ΔW = W₂ − W₁.

Relationship to RGR and LAR

RGR = NAR × LAR

LAR (leaf area ratio) = total leaf area / total plant dry mass (m²/g). A plant can achieve high RGR through high NAR (efficient photosynthesis per unit leaf area), high LAR (lots of leaf area per gram of plant), or both. Most interspecific variation in RGR is explained by LAR rather than NAR.

Factors Affecting NAR

  • Light: High irradiance increases NAR up to light saturation
  • Leaf N: Higher N per unit area → more Rubisco → higher photosynthetic capacity
  • Temperature: Optimal for most crops: 20–30°C
  • CO₂: Elevated CO₂ increases NAR by reducing photorespiration
  • Leaf age: Old leaves have declining NAR as Rubisco declines and respiration increases

Glossary

Net Assimilation Rate (NAR)
Dry mass increase per unit leaf area per day (g/m²/day); reflects whole-plant net photosynthetic efficiency; one of two components of RGR = NAR × LAR.
Leaf Area Ratio (LAR)
Total leaf area per unit total plant dry mass (m²/g); reflects the quantity of photosynthetic surface deployed per unit biomass investment; the other component of RGR = NAR × LAR.
Relative Growth Rate (RGR)
Fractional rate of biomass increase per unit biomass per day: RGR = (ln W₂ − ln W₁)/(t₂ − t₁); decomposes as RGR = NAR × LAR in classical plant growth analysis.

Frequently Asked Questions

NAR measures the increase in plant dry mass per unit of leaf area per day (g/m²/day). It reflects photosynthetic efficiency — how much new biomass a plant produces per unit of photosynthetic surface. NAR = ΔW / (mean leaf area × Δt). High NAR means the leaf area is highly productive; low NAR means photosynthesis barely exceeds respiration. NAR is one of two components of relative growth rate: RGR = NAR × LAR.

RGR (relative growth rate) = NAR × LAR. NAR (net assimilation rate) measures photosynthetic efficiency per unit leaf area (g/m²/day). LAR (leaf area ratio) measures leaf area deployed per unit plant mass (m²/g). A fast-growing plant can achieve high RGR by having efficient leaves (high NAR), lots of leaf area per gram of plant (high LAR), or both. Studies show most interspecific differences in RGR are driven by LAR variation rather than NAR — thin, high-SLA leaves give high LAR.

Leaf nitrogen concentration strongly correlates with NAR because the majority of leaf nitrogen is invested in photosynthetic enzymes, especially Rubisco. Higher N per unit leaf area → more Rubisco and chlorophyll → higher maximum photosynthetic rate → higher NAR. Nitrogen fertilization increases NAR in N-limited crops, and the correlation between leaf N and photosynthetic capacity holds globally across species. This is the mechanistic basis for the global leaf economics spectrum link between leaf N and leaf productivity.

Gross photosynthesis (Pgross) measures total CO₂ fixation by leaves. NAR is a whole-plant measure that accounts for: (1) respiratory losses at the whole-plant level (leaves, stems, roots all respire); (2) allocation of carbon to non-photosynthetic tissues; and (3) it uses dry mass change, not gas exchange. NAR = net carbon accumulation per unit leaf area after all respiratory losses. Gross leaf-level photosynthesis measured by gas exchange is always higher than NAR, which is integrated over longer periods and whole plants.