RGR (Relative Growth Rate) Calculators

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Relative growth rate (RGR) is the rate of biomass increase per unit of existing biomass per unit time, expressed as g/g/day or mg/g/day. It is the primary whole-plant growth performance metric in comparative plant physiology and ecology. RGR = (ln W₂ − ln W₁) / (t₂ − t₁), where W is dry biomass measured at two time points. RGR decomposes as RGR = NAR × LAR, where NAR is net assimilation rate (photosynthetic efficiency per leaf area) and LAR is leaf area ratio (leaf area per unit plant mass). High RGR is associated with high SLA and high leaf nitrogen, characteristic of fast-growing, resource-acquisitive species.

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RGR Formula

RGR = (ln W₂ − ln W₁) / (t₂ − t₁)

W = dry biomass (g); t = time (days). Units: g/g/day or mg/g/day. Example: plant grows from 2.0 g to 3.6 g in 14 days: RGR = (ln 3.6 − ln 2.0)/14 = (1.281 − 0.693)/14 = 0.042 g/g/day = 42 mg/g/day.

RGR Decomposition

RGR = NAR × LAR

NAR (net assimilation rate, g/m²/day) = dry mass increase per unit leaf area per day. LAR (leaf area ratio, m²/g) = total leaf area / total plant dry mass. LAR = SLA × LMF (specific leaf area × leaf mass fraction). Research consistently shows most interspecific RGR variation is explained by LAR (and SLA) rather than NAR.

Typical RGR Values

  • Fast-growing crops and weeds (sunflower, lambsquarters): 150–350 mg/g/day
  • Moderate (wheat, soybean): 100–200 mg/g/day
  • Slow-growing woodland herbs: 50–100 mg/g/day
  • Stress-tolerant shrubs and evergreens: 10–50 mg/g/day

RGR in Agriculture

Crop growth rate (CGR) = RGR × plant biomass density (g/m²/day at field scale). Harvest index (HI) = grain yield / total dry matter — the fraction of total biomass in the harvested organ. Green Revolution semi-dwarf wheat and rice varieties dramatically increased HI (~0.30 → ~0.50), explaining much of the yield improvement.

Glossary

Relative Growth Rate (RGR)
Fractional rate of biomass increase per unit biomass per day: (ln W₂ − ln W₁)/(t₂ − t₁); units g/g/day; decomposes as RGR = NAR × LAR; key whole-plant performance metric.
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 RGR components (RGR = NAR × LAR).
Leaf Area Ratio (LAR)
Total leaf area per unit plant dry mass (m²/g); LAR = SLA × LMF; explains most interspecific RGR variation; higher LAR = more photosynthetic surface per gram of plant.

Frequently Asked Questions

RGR = (ln W₂ − ln W₁) / (t₂ − t₁), where W₁ and W₂ are plant dry biomass at two time points (g) and t is time (days). Always use dry mass (oven-dried at 70°C), not fresh mass. Example: 0.5 g to 1.2 g in 10 days: RGR = (ln 1.2 − ln 0.5)/10 = (0.182 − (−0.693))/10 = 0.875/10 = 0.0875 g/g/day = 87.5 mg/g/day. Use data from the exponential growth phase for most accurate results.

RGR = NAR × LAR. NAR (net assimilation rate, g/m²/day) measures how much new dry mass is produced per unit leaf area per day — reflects photosynthetic efficiency minus whole-plant respiration. LAR (leaf area ratio, m²/g) measures total leaf area per unit plant mass — reflects how much photosynthetic surface is deployed per gram of investment. LAR = SLA × LMF (leaf mass fraction). High-SLA leaves give high LAR → high RGR without requiring higher NAR. Studies show most interspecific RGR variation is explained by LAR (primarily SLA) rather than NAR.

High specific leaf area (SLA = leaf area/leaf mass) means thin, low-density leaves that maximize LAR per unit biomass investment. High LAR gives high RGR (= NAR × LAR) without requiring fundamentally better photosynthetic biochemistry. Fast-growing plants (weeds, crop plants, early successional species) invest in quantity of leaf area (high SLA, high LAR) rather than long-lived leaves. This is the fundamental growth strategy trade-off: fast species sacrifice leaf longevity and defense for rapid surface area deployment, while slow species invest in durable leaves that last for years.

RGR decreases with: nutrient limitation (especially N and P reduce leaf N, photosynthetic capacity, NAR, and SLA); shade (reduces light capture, decreases NAR); drought (stomatal closure reduces CO₂ and NAR); cold temperatures (reduce enzyme activity and nutrient uptake); and genetic strategy (stress-tolerant species have inherently lower maximum RGR). Plants can partly compensate for low NAR by increasing LAR (thinner leaves), but there are structural limits on how thin viable leaves can be in stressful environments.