Let's look at some data.
I once saw a commercial that said: “forget everything you thought you knew about slip covers”.
So I did.
Admittedly I don’t know a lot about slip covers. I do know quite a bit about plant stress. Up until recently, our constant flow of data on soil moisture has been studied along with weekly spot measurements of leaf water potential via pressure bomb. This year we switched to FloraPulse® plant stress sensors and holy moly, having real time data on plant stress is pretty wild.
First of all, how much data are we talking about?
This graph compares our real time plant stress measurements with what would be weekly midday pressure bomb measurements. Look at how much data we were missing just by looking at only weekly snapshots of plant stress. There is a lot of stuff going on those other 6 days, not to mention at night.
Some vines recover at night. Others don’t.
For my pressure bombers out there, you may remember that there have historically been two important times to sample leaf water potential: midday and pre-dawn. This means if you are a passionate whipper snapper slogging it out in a graduate program, you don’t just have to sample at the hottest time of day. You get to sample at 3 am too!
Why? Well aside from weeding out those who are more suited for the cellar kind of life, these sampling times give you different information. Midday gives you the most stress (most negative leaf water potential) the vine will experience that day. At night, the stomata close i.e. photosynthesis and transpiration pauses, and the vine goes into relative equilibrium with the soil. This is when the vine recovers as much as it can depending on how moist the soil is.
Realtime data is giving us a more complete picture. Some vines swing wide between having close to 0 stress (water potential) at night and being very stressed during the day. That time of maximum stress (most negative water potential) is later in the day than we originally thought – usually around 5pm local time. Others have a much tighter amplitude, getting moderately stressed during the day but recovering very little at night. For the most part, the degree to which vines recover tracks with soil moisture levels. There seems to be more nuance though and that could depend on rootstock, soil texture, or a combination of both.
Note that the FloraPulse® sensor measures trunk or stem water potential, which will differ from leaf water potential, which is what we usually measure with the pressure chamber.
The following graph is from a site where the vine recovers fully every night (SWP>-2 Bar) even though midday SWP continues to decrease (become more negative) over the course of the season.
This graph (below) showcases a site where vines don’t recover fully at night.
Recovery after irrigation varies
For the most part, vines recover rapidly when the vines get irrigated. The degree to which they remain in that recovered state depends. Again, this could be related to soil texture or rootstock or even variety. We have always considered the point at which root uptake starts to level off as indicative of the vine having a hard time finding water to extract. Plant stress does seem to track with that. We were right. Cool.
The following graph illustrates how irrigations can offer temporarily relief from water stress. After each irrigation, the vine gets gradually more stressed.
The below graph showcases a site where irrigations only offer brief relief from stress. Within 1-2 days, vines are as stressed as they were prior to each irrigation.
Here’s a case with a less uniform irrigation schedule. Notice vine stress mirrors soil moisture levelling off, meaning the profile is tapping out of water. A big irrigation means a big recovery, which may not be what you want if you’re looking to grow high quality winegrapes.
How does weather affect vine stress?
Water potential is essentially the tension in the sap as it moves from the soil through the plant and out into the atmosphere via the leaves’ stomata. This last part of the path can be augmented by the ambient temperature and humidity. It’s really the dryness of the air that drives water out of the leaf and into the surrounding air. That may seem like a complicated way to say, when its hotter outside vines are more water stressed and vice versa. This year has been a little nutty, so we’ve seen just how sensitive vine stress is to changes in the weather.
In the following graph, vines never get very stressed. Stem Water Potential varies with weather data.
Conclusion
At the beginning of the season, we weren’t sure what to expect. Granted, we had tested the FloraPulse® sensors on a limited basis but not for all of the locations we monitor. We were used to looking at polaroids. Now we’re looking at a feature length film. This data will allow farmers to determine exactly where and when vines need irrigation. More than that though, having access to so much information has the capacity to teach us things we never knew about plants and how they interact with their environment.
It’s a brave new world.
Interested in learning more? Contact [email protected] for more info.
