Basic Plant Physiology Processes

Thought it would be helpful to create a thread to cover some basic plant physiology concepts. Seems like there is a lot of misinformation and odd techniques going around.


(For Dummies)

this topic could cover books

Photosynthesis- plant process responsible for creation of sugar ie (mass). near direct correlation between light level and photosynthesis. increasing photosynthesis will increase plant mass.



Respiration-plant process responsible for energy creation. turns sugars from photosynthesis into energy (ATP) used for cell growth. chiefly affected by temperature. increasing temps will increase respiration and leaf unfolding rate, decrease crop time.



Transpiration- plant process responsible which dissipates water from leaves. highly important as rate of transpiration directly affects water uptake. (this also affects mineral uptake of some elements which require bulk flow for uptake, (Ca, Mg) .) Transpiration is related to temp, (VPD) humidity, and light.


**massive oversimplifications here folks.

What is actually important is understanding how these different processes interact with one another, and how environment, ie temp, light, humidity, drive these.



First interaction to consider is photosynthesis v. respiration. (as these are the two main ones.) interestingly, from a chemical standpoint they are nearly the exact same process in reverse. photosynthesis uses h2o + co2 +light (energy) = CHO (sugar) +O2. respiration turns sugars (CHO ) +O2 = CO2 +H2O + energy (ATP) . anyways, the reason this relationship is so important is that one creates energy, the other uses it. you can almost think of this relationship as the "plant energy budget." the more sugars that are built, the more can be spent. (its also worth noting that final quality and yield is determined by LEFTOVER "sugars", sugars not used for ATP purposes, they can still be repurposed into something else.)



So, lets consider four examples.

1 high light high temp
high light low temp
low light high temp
low light low temp.


which one would produce the best quality ? the high light low temp. because it will have the highest leftover balance.

which one will finish first? the high temp high light.

Where does Transpiration play in? As stated earlier, water uptake is driven by transpiration, so in too cool and humid of climates plants will not be able to uptake enough water. On the flip side, if its too hot and dry, plants will need tons of water.

A very good example of the interplay between these processes is people having issues in general, and specifically with magnesium when changing from HID to LED lighting.
LED lights put out less heat than HID's so photosynthesis will (stay the same, or even increase) while respiration and transpiration both decrease. this means plants will develop more slowly, and will take up less water. less water uptake will result in less uptake of minerals dependent on bulk flow for uptake, (Mg, and Ca.) Mg, is also specifically needed for chlorophyll development so it is in high demand under led lighting. So, decreased temps result in less water uptake and therefore less nutrient uptake leading in a Mg def. . To combat this, one could A) just try and give the plant more Mg, but the issue isnt a lack of Mg in solution, its an inability for the plant to take up these nutrients. So, the real way to solve the issue is increase temps and air flow, or decrease humidity.