Factors Affecting Photosynthesis

The rate at which a leaf actually photosynthesises at any given moment, in any real field or greenhouse, is not set by any single variable in isolation but is jointly governed by several external environmental factors together -- principally light, the concentration of carbon dioxide, temperature, and the availability of water. Understanding how these factors interact, rather than treating each in isolation, was the central achievement of the English plant physiologist F.F. Blackman, whose 1905 Law of Limiting Factors remains the standard conceptual framework for this whole topic: when a process depends on several separate factors, the overall rate of that process at any given moment is determined by whichever single factor is currently closest to its minimum, or most limiting, value -- and improving any of the other, already-adequate factors further will have essentially no effect on the rate until that one limiting factor is itself relieved.

Light affects photosynthesis in two distinct respects: its intensity and its wavelength (quality). As light intensity increases from very low levels, the rate of photosynthesis rises correspondingly, since more photons are available to drive the photochemical reactions of the light phase -- but only up to a certain intensity, beyond which the rate plateaus and further increases in light intensity produce no additional gain, because some other factor (very often CO2 availability, or the maximum turnover rate of the Calvin cycle's own enzymes) has now become the limiting one instead. In terms of wavelength, as established in the section on pigments and spectra, blue and red light are considerably more effective at driving photosynthesis than green light, since these are the wavelengths chlorophyll absorbs most strongly.

Carbon dioxide concentration is, for most plants under most everyday conditions, the single most commonly limiting factor of all, precisely because its concentration in the ordinary atmosphere is quite low (roughly 0.04% by volume). Raising the CO2 concentration available to a leaf, experimentally or in a controlled greenhouse, correspondingly raises the rate of photosynthesis, again up to a saturation point beyond which further CO2 provides no additional benefit -- a relationship that is directly and commercially exploited in greenhouse cultivation through deliberate CO2 enrichment of the growing atmosphere to boost crop yields.

Temperature primarily affects the biosynthetic (dark) phase of photosynthesis, since the enzyme-catalysed reactions of the Calvin cycle, like all enzyme reactions, are strongly temperature-dependent, typically showing an optimum somewhere in the range of roughly 25-35°C for most plant species, with the rate falling off again at both lower and distinctly higher temperatures (the latter through progressive enzyme denaturation). The purely photochemical events of the light reaction itself, by contrast, are comparatively insensitive to temperature, which is precisely the kind of differential response that first led Blackman to conclude that photosynthesis must consist of at least two mechanistically distinct phases, as discussed in the section on early experiments. …