# Photosynthetic Rates

• Author
Posts
• #12951
duyh
Participant

Hi everyone, I’m pretty confused with something regarding the rate of photosynthesis.

From my understanding, the rate of photosynthesis can either increase, decrease or stay constant if they’re variables such as light intensity, temperature, etc affecting it.

There is also an optimum point where the rate will stop increasing and plateau if only one factor is affecting it such as light intensity.

So my question for everyone is, what happens to the photosynthetic rate over time if all other variables are held constant? I.e. the only thing changing is the amount of time. So for example measuring the amount of oxygen released at 5mins then measuring it again at 20mins.

I’m doing a biology lab and can’t seem to grasp this concept. Would the rate just stay constant? Or does it actually decrease? I’ve been researching and can’t seem to find any information on the photosynthetic rate over time

Links to any references such as peer reviewed journals would be appreciated thanks.

• #98306
mith
Participant

Assume you’re looking at a simple model where photo synthetic rate is ONLY dependent on light intensity. If light intensity doesn’t change, how can you explain a change in photosynthetic rate?? Obviously, you cannot and therefore in this model, it is time invariant i.e. the photosynthetic rate does not change over time.

Expand the model to multiple input parameters–if none of the inputs change, your output doesn’t change as well (same argument as above).

HOWEVER, a model is not the same as what is actually happening(unless you have a really complete model, in that case your model will probably be too complicated to work with). For example, the simple model I just described assumes the photosynthetic machinery does not change over time. Obviously that’s not true. Chloroplasts can get damaged and growth occurs to name a few.

• #98326
JackBean
Participant

do you keep also the level of CO2 and O2 the same? Like some small plant in big greenhouse?

• #98338
jonmoulton
Participant

If all the relevant environmental parameters are held constant and assuming the environment is healthy for the plant, a plant moved from a somewhat different environment into that constant environment would be expected to adjust itself to the new environment and settle into a stable condition of steady-state photosynthesis. The change in the rate of photosynthesis over time would resemble a curve approaching an asymptote.

Plants optimize for light conditions by adjusting their biosynthetic investment into several systems. For instance, in low light conditions most (all, Jack?) plants will invest more new synthesis into light harvesting apparatus (photosystems, thylakoids, electron carriers) and less into carbon fixation apparatus (Calvin cycle enzymes). In higher light conditions there is not so much need to harvest a large fraction of the photon flux; instead, plants need to handle the high output of reduced NADPH from their photosystems so most plants will invest most new biosynthesis into their carbon fixation apparatus.

The tradeoff between light harvesting apparatus and carbon fixation apparatus is reflected in the changing photosynthesis versus irradiance curves (P vs. I) as plants adapt to new light conditions: in low light, plants adapt toward curves with steeper initial slopes and lower saturated photosynthetic rates while in high light, plants adapt toward curves with shallower initial slopes but high saturated photosynthetic rates (though photoinhibition in high light flux may prevent the plants from achieving those higher carbon fixation rates).

There are some big simplifications in this discussion and the sort of model I present is somewhat controversial. Modeling of P vs I response is a long-running and ongoing area of research.

• #98367
jwalin
Participant

yes very true…
opne very important thing to keep in mind is the time you have to give the plant to adjust. for everything to come into equilibrium.

everything seems to be covered…