The factors that limit photosynthesis

A plant does not always photosynthesise at full rate. Three factors control its pace, and at any moment one of them limits all the others.

The law of the limiting factor

The rate of a reaction depending on several factors is set by the least favourable one.

Increasing a factor already in abundance achieves nothing: the missing factor is in charge.

   rate
      |          ______________  <- plateau: ANOTHER factor limits
      |        /
      |      /
      |    /    <- region where THIS factor is limiting
      |  /
      +------------------------> factor intensity

The three factors

Light. At low illumination the rate grows proportionally. Then it plateaus: the pigments are saturated, and every extra photon is wasted.

Carbon dioxide. Air contains about 0.04 % of it, which is very little. It is the most frequent limiting factor in full light — hence the CO₂ enrichment practised in greenhouses, which raises yields by 20 to 30 %.

Temperature. The reactions are catalysed by enzymes:

   rate
      |        __
      |      _/  \        optimum around 25-30 °C for most plants
      |    _/     \
      |  _/        \_     beyond that: the enzymes DENATURE
      +----------------> temperature
        5   25   45 °C

The fall beyond the optimum is not the mirror image of the rise: it reflects irreversible destruction of the enzymes, not a mere slowdown.

The fatal trade-off: stomata

CO₂ enters through tiny openings on the underside of leaves, the stomata. But water escapes through those same openings.

     stoma OPEN                  stoma CLOSED
   CO₂ ↓        ↑ H₂O          CO₂ ✗        ✗ H₂O
   -----[  ]-----              -----[][]-----
   photosynthesis possible     photosynthesis blocked
   but water is lost           but water is saved

A plant under drought must therefore choose: feed itself or avoid drying out. This dilemma explains the shape of desert plants — leaves reduced to spines, stomata opening only at night, fleshy stems.

The net balance: photosynthesis versus respiration

A plant respires constantly, day and night, like every living thing. It photosynthesises only in the light. What is measured is therefore always a net balance:

   night       : respiration alone         ->  the plant RELEASES CO₂
   dawn        : the two balance out       ->  COMPENSATION point
   full day    : photosynthesis >> respiration -> the plant ABSORBS CO₂

Hence a widespread myth: "don't keep plants in a bedroom, they use up the oxygen at night". True, but in derisory amounts — a green plant consumes about as much at night as a blown-out candle. Over twenty-four hours its balance remains strongly oxygen-producing.

Summary

• The law of the limiting factor: the least favourable factor sets the rate.
• Light: proportional, then a plateau (pigment saturation).
• CO₂: a frequent limiting factor, since air holds only 0.04 %.
• Temperature: an enzymatic optimum, then irreversible denaturation.
• Stomata impose a trade-off between CO₂ uptake and water loss.
• The plant respires day and night; what we observe is always a net balance.