Abbreviations/initialisms are everywhere in agronomic publications and research in general.
OM (organic matter)
GEM (genetics x environment x management)
HI (harvest index)
NT (no-till)
Here's another one for your mental toolbox that you maybe haven't heard of before - PTQ.
PTQ, the photothermal quotient, has been tied to numerous plant growth and development responses since it was first coined by H.A. Nix in 1976.
But what IS the PTQ and why do we care? A great question - let's start by defining it.
Knowing who you're dealing with, we will (of course) be approaching this from the perspective of corn plants and ... tillering.
From a 30,000-foot perspective, the PTQ is a way for us to measure the energy available to a plant during a given period of time.
What do I mean by "period of time"? Well, we know that for corn, plant development is driven by temperature. We can predict this relationship with growing degree days (GDD), which is why we know that corn will emerge when 100-120 GDD have accumulated after planting. For more on how to calculate this and what "growing degree days" are, check out this blog post: https://rveenstra.wixsite.com/tillers/post/my-kind-of-math. If we pick a desired stage range (such as V4 to V7 - more on staging corn here: https://rveenstra.wixsite.com/tillers/post/corn-growth-development), we can predict how long it will take for a corn plant to get from stage A (V4) to stage B (V7) based on the temperatures that plant will experience.
What do I mean by "energy available"? As for all photosynthesizing plants, corn depends on the sun as its energy source. Every day, the corn plant is exposed to different levels of solar radiation. Solar radiation will be high on a clear day, but lower on a cloudy or perhaps rainy day. The key here is that more sun equates to more available energy!
Let's look at some practical examples of how we could see different PTQ!
The figure below was taken from our recent Extension summary article that is available here: https://bookstore.ksre.ksu.edu/pubs/MF3571.pdf.
On the top of this image, we can see three different temperature scenarios - high, average, and low (as indicated by the thermometers on the left). Notice that as temperature goes down, the timeline (left to right) to go from planting to stage V14 gets longer. More days are required to reach the same development stage because the temperatures are low and not pushing the plants as quickly through their development process.
At the same time, the plant stages in between these two endpoints (plant and V14) are also moving slower. We are particularly interested in the V4 to V7 window. Why? Because this is the period in a corn plant's lifetime when tillers would be appearing.
If all plants in these three temperature scenarios had access to the same intensity of sunlight, which would experience the highest energy availability during the V4 to V7 time period? The LOWER temperature, because it would take more time for the plant to push through the stages, experiencing sunlight each day as it progressed!
At the bottom of this image, we see a different scenario in which temperature is constant. GDD will be accumulating at the same rate for all plants in this scenario and it will take the same number of days to progress through development stages.
In this bottom scenario, we have three different levels of solar radiation - low (rain storms), average (some clouds), and high (clear skies). If all plants had the same number of days to experience solar radiation, which scenario would experience the highest energy availability during the V4 to V7 time period? The HIGHER solar radiation, because our time is fixed and the amount of solar radiation during that time frame would be higher in this case.
So why are we talking about PTQ?
In the Extension summary I linked earlier (https://bookstore.ksre.ksu.edu/pubs/MF3571.pdf), the PTQ was a big part of our findings looking at when tillers have potential to actually BOOST corn yields.
In our case, we found that fields experiencing higher PTQ (more solar radiation per time period - in our case before and during tiller appearance) were more likely to see positive yield contributions from tillers. This makes sense for several reasons!
Tillers are a way plants can adapt to their environment (called "plasticity"). This growth takes energy, and having a more abundant energy source allows the plant to grow tillers more easily. An increased PTQ provides this energy! Relationships between PTQ and tiller development have been documented in wheat and grain sorghum before, but not corn tillers (until now!).
Greater PTQ not only encourages tiller development, but it provides a greater energy source to the tillers after they appear! This "energy" is captured and stored through the photosynthesis process, which uses sunlight to make sugars. These sugars are saved in starch form for grain development later in the plant's lifetime. With more leaves to photosynthesize, the plant builds up a greater bank of these starch energy reserves. Research has shown that corn plants are able to move starches stored in tillers to developing grain, which means these plants have an extra "savings account" to put towards yields.
PTQ. Pretty neat, huh?
Want to learn more about the PTQ and what it means for other plant species? Here are some helpful links I used to write this blog post:
Veenstra, R.L., Ciampitti, I., Haag, L., Berning, D., & Carter, P. (2021). Tillering Impacts on Corn Yields. Kansas State University Agricultural Experiment Station and Cooperative Extension Service. https://bookstore.ksre.ksu.edu/pubs/MF3571.pdf
Veenstra, R.L., Messina, C.D., Berning, D., Haag, L.A., Carter, P., Hefley, T.J., Prasad, P.V.V., & Ciampitti, I.A. (2021). Effect of tillers on corn yield: Exploring trait plasticity potential in unpredictable environments. Crop Science, 61(5), 3660-3674. https://doi.org/10.1002/csc2.20576
Fischer, R. (1985). Number of kernels in wheat crops and the influence of solar radiation and temperature. The Journal of Agricultural Science,105(2), 447-461. doi:10.1017/S0021859600056495
Nix, H.A. (1976) Climate and crop productivity in Australia. In Climate and Rice. International Rice Research Institute, Philippines, 495-506.