It's been the driest year for over 20 years. But let's talk about what I like the most, Agronomics and Physiology of crops.
Crops like Corn (a grass) has a type of leaf that facilitates fast reactions to changes in Tº and %RH (Relative Humidity) in the air. I define the Leaf as the Heart of a plant (any plant).
The type of venation that the foliage has is defined as a "Paralell Venation" with a mid rib in the center of the leaf and a lanceolate form.
That allows the corn leaf to react fast to any change.
Let's always remember that a plant does NOT take its water from the leaves!!! so it doesn't matter how much dew there is in the early hours of the day in the whorl and leaves, if the soil is lacking moisture the plants will be under a stress.
Let's imagine a hot summer day, a real hot one.
Tº: 102F
%RH: 95%
Heat Index: 108F
Water: It hasn´t rained in a week and it's been hot every day.
What could we expect from the plants to do? Photosynthesis? Uptake water and nutrients? Elongate their inter-node sections? keep the same leaf axil angle as always?
Well under these conditions several processes are triggered.
- Plant synthesizes acids in the stomes to provoke a temporary closure of them. This provokes the plant to stop interchanging O2 and H2O with CO2 with the atmosphere.
- Photosynthesis ceases as no gaseous exchange is taking place in the leaves. Carbohydrate mobilization might shift as the main sink changes (a sink is a structure that at a specific growth stage is requesting or demanding the higher amount of energy and sugars from the leaves).
- Hormone balance gets affected. Plant ceases Citoquinine (used in cell division and plant Growth), Auxines (used in tissue elongation) and Giberelines (plant Development) synthesis and starts synthetizing Absicic Acid (defoliation) and Ethilen (plant stress and maturation).
There are very many more changes and processes that are being triggered during a stressful condition. From a practical stand point, what we can see in a field condition is a symptom (something that happens due to a disease, is the visual effect of it when the disease is already established). The most common symtom is Leaf Curling, wilting, leaf rolling, plant withering. All of these symptoms are noticeable at different stages of the stress.
Leaves curl in response to a hot and moisture deficient environment, why? so they can reduce their Active Photosynthetic Area or Active Surface Area so they don't lose too much water by transpiration.
If transpiration continues after curling, the plant will close stomes. How to notice if that's happened? by touching the leaves, they feel warm and hot. Why? Because the water that's inside the venation in the xylem conducts is not moving anywhere and starts "boiling" inside the tissue. Also the water in the vacuoles gets hotter and hotter.
If the stress continues. As I mentioned before, that boiling water will "Cook" the leaf and the 1st cell group that dies is the one on the very tip of the leaf. So the process of "Die-Back" begins.
There's no more water inside the vacuole organels and the cells begin to die. This process moves towars the stalk.
It will usually begin in the middle to upper leaves. What's lower than that is mostly nutrient deficiency under stressful conditions.
Reproductive structures that were forming stop their development and yield gets strongly reduced. If stressed during induction and key stages like "definition of number of kernels per row or number of rows of kernels" will reduce yields substantially.
This is pretty much a Vague description of SOME of the things that happen when a plant is under water and temperature stress.
Thanks!
