Not much progress to post about today, but I am bored so I made some word salad.
Making a PPFD map of the area under the lights is helpful for making sure the plants are receiving appropriate amounts of light.
This pineapple plant, for example, has gotten tall enough that the canopy is receiving around 960 umol/m^2 s PPFD. It's not burning or anything, but this is more light than the plant needs, and may actually be decreasing photosynthetic activity.
To figure out what an appropriate amount of light is, we can refer to something called a light response curve:
https://en.wikipedia.org/wiki/PI_curve
Here's an example of what a light response curve looks like:
Source:
https://www.researchgate.net/figure...pensation-point-is-the-minimum_fig2_328859471
The main things indicated on such a curve are the following:
- Light compensation point: When net photosynthesis is zero. This basically means the plant is receiving just enough light that its photosynthetic activity breaks even with its metabolic needs. This represents the absolute minimum amount of light the plant needs to survive. At any light intensity below the compensation point, the plant undergoes dark respiration, where carbohydrates are metabolized to produce energy.
- Increasing light intensity above the compensation point results in a net gain in CO2 assimilation, meaning that the plant is actually producing sugars. This part of the curve is light limited at first, then becomes CO2 limited as light increases.
- Light saturation point: The point at which more light no longer increases photosynthetic activity, as the reaction becomes rate limited by something else, eg. carbon dioxide availability, temperature, enzymatic activity etc. This represents the maximum amount of light the plant is able to make use of. Going above this point can do things like induce stress colouration, but is a waste of money if you're paying for the electricity to run lights.
- At excessively high light intensities you reach a point where radiation damage to the photosystem (photooxidative stress, 10.1111/j.1399-3054.1994.tb03042.x ) actually decreases photosynthetic activity. This is photoinhibition, and is generally to be avoided.
Back to the pineapple:
Looking at this light response curve for a pineapple plant (DOI 10.1007/s12042-010-9057-y) shows that peak electron transport rate (ETR, a measure of photosynthetic activity) occurs at a PPFD of ~700 umol/m^2 s. At the ~1000 PPFD I was giving it, the plant is at best at saturation point and at worst starting to undergo photoinhibition, so keeping the plant so close to the light would mostly be a waste of power.
Lowering the pineapple away from the light means it's making more efficient use of the light. It also means the plants to either side aren't as shaded, resulting in about a 15% increase in PPFD for them.
Unfortunately, light response curves aren't as easy to find for Nepenthes as they are for commercially important crops like grains, fruits and vegetables. You can sort of guesstimate what's best by looking at light response curves for common species, keeping in mind that light response curves change with things like temperature or water stress, as well as how much the plant is being fed.
Examples: N. miranda (DOI:
10.1007/s11120-022-00987-8) or N. alata (
10.5402/2012/263270), which show peak ETR around 200 and 400 umol/m^2 s PPFD, respectively:
Good thing that Nepenthes are adaptable and typically grow well under a range of light intensities.
Carnivero wrote an informative article a couple years ago describing how Nepenthes respond to light:
The physiological response of Nepenthes pitcher and leaf morphology due to environmental factors illustrates the interplay between light, metabolism and nutrient uptake. Unlike most other carnivorous genera, the trapping structure of Nepenthes is well segmented between photosynthetic (leaf) and...
www.carnivero.com
I see the same sort of changes in my plants.
For example, this N x bloody mary originally arrived with very large, thin and floppy shade-adapted leaves. They quickly turned red under my lights (at ~150 PPFD), and started going chlorotic. The new leaves were smaller and thicker in comparison, but continued to turn red and developed some spotting. Currently, the plant receives around 180 PPFD and has acclimated to the light intensity, returning to making green leaves.
Same sort of story with this BE-4099 receiving ~220 PPFD:
Green floppy leaves => red and chlorotic => thicker leaves with varying amounts of red pigmentation => thicker green leaves