How Plants Transpire Water

How Plants Transpire Water

Plants require certain things in order to aid their growth and ability to photosynthesize. When light energy is converted into chemical energy, plants will naturally produce a carbohydrate and use carbon dioxide. Water is just as important as light or carbon dioxide, so below BioTabs explains how plants use water, how growing conditions affect plants and what to consider when adding water into the garden.

Transpiration Explained

A simple way to think about transpiration is to compare how we sweat and drink water to replace what has leaked out of our pores. Once re-hydrated, our bodies will retain the relevant water levels in accordance with the salt count, which is why cramp can often be experienced when a person is extremely dehydrated and has no more water or salt left to power their muscles.

When a professional fighter cuts weight, not only are they cutting out the amount of water their muscles can retain they are also restricting the amount of salt uptake. Plants react the same way when adapting to the conditions of the grow room and depending on if the humidity is too dry or too wet, then the plants will either transpire water or retain it. The way that this is done is both through the root zone and the leaf tissue.

High Transpiration Produces Leaf Curling
High Transpiration Produces Leaf Curling
Heat Stress & Light Burn Sause Bleaching & Spots on Cannabis Leaves
Heat Stress & Light Burn Sause Bleaching & Spots on Leaves

Leaf Tissue

Once a leaf has been removed from a plant, it will not take long to break down, lose moisture and as a result lose structure. Ready to throw to the compost pile and be broken down by whatever microbes are present, this gives you an idea of how delicate plant tissue is. The foliage of a plant is responsible to absorb as much light energy as possible and produce Chlorophyll.

The more chlorophyll that is produced then the more light energy is converted, meaning that the greater the amount of stomata present on the surface of the leaf tissue. An optimally healthy plant will be a lush, waxy green that is fresh and smooth to touch, displaying full integrity in terms of performance.

When the growing environment is too dry, plants will immediately adapt and be forced to work with the low humidity so in an attempt to not transpire will often curl the sides of the leaf inwards and sometimes droop.

Stomata will begin to close, meaning that the rate of photosynthesis is dramatically reduced until conditions improve. Oscillating fans can cause tip burn and de-saturate leaf tissue so much, the leaf falls off with the tap of a finger. When transpiration such as this occurs above the growing medium, then you must think about how it is affecting the root zone.

The Root Zone

Root hairs come into contact with the thin film of water that surrounds the growing medium. As they do this they will absorb nutrients out of the water and also from the surrounding substrate. A well aerated growing medium will allow for air pockets to build up and improve wicking and capillary action, as roots will have a tendency to grow towards air pockets

The reason why over-watering a plant can have negative effects is not because the plant cannot take the amount of water, however the fact that the air placement after the feeding will be minimal, especially if the growing medium is over saturated and has poor drainage to begin with. Coco and perlite is an excellent growing medium and works extremely well when mixed with an organic compost, worm castings and other organic supplements.

Plants have the ability to send water to the xylem using what is called root pressure. How plants choose to transpire water is not always down to their natural function, however more how they adapt to their growing environment. A very dry grow room with high temps can cause the growing medium to transpire water or nutrient solution into the atmosphere and become exhausted out of the grow room. The best way of understanding how plants use the water accordingly is through vapour pressure deficit.

Vapour Pressure Deficit

This describes the amount of water pressure that is between the leaf tissue and the surrounding air. By measuring the relative humidity and using a thermal camera to measure the surface area, a calculation is made that gives the number of kilopascals, which is a derived unit of pressure. V.P.D charts are created as a rough guide to what temperature and humidity your plants should be, however the leaf temperature can differ when taking different reading at various levels of the canopy. Also you must bear in mind that plants require specific levels of humidity during certain stages, meaning that finding the optimum environment for the strain you are growing is key.

Stomata which convert available CO2 into oxygen will function best when the humidity is high and the temperatures are warmer. Outdoors the levels of carbon dioxide are approximately 400 PPM, which is why growers who supplement their plants using CO2 tanks can achieve levels of 1200-1500 PPM.

Doing this means that plants require much hotter temperatures than most are used to and temperatures of 30-35 degrees Celsius are necessary for plants to intake that amount of carbon dioxide. Low V.P.D means that the humidity is high and stomata are most active. And high V.P.D means low humidity and growers should pay extra attention to the amount of nutrients they give their plants. If your V.P.D is off then you can balance things out by reducing the amount of P.P.M and reducing the electrical conductivity of your nutrient solution.

Feeding Times

For many growers across the world, preserving water and being as careful as possible is a way of life. It can be that their natural water source is not ideal for plants, or that they use reverse osmosis filtered water. Some organic growers, such as myself, enjoy a water only diet for my plants to allow the microbiology to take over, so a few tips regarding watering times will allow you to stretch your water supply as far as possible, whilst growing the best possible plants at the same time.

This principal applies to growing outdoors however the same can be applied indoors, especially if you are pulling fresh air from outside and suffering from hot temps. The best times to feed your plants will be a few hours after sunrise and a few hours before sunset. The outside temperatures will dictate the amount of water that your plants transpire and at what rate, so by feeding constantly twice a day will give your plants plenty of time to absorb the water and overall cool down after a long day of outdoor or indoor lighting.

Feeding your plants during the hottest parts of the day may seem like the plants enjoy it at the time, yet due to the ground temperature, the temperature of the air and the humidity and water levels present in the growing medium, the rate of transpiration may be too aggressive for your crop to handle and when water is stripped from a plant, along with that comes many other issues with deficiency such as the transportation of Calcium with many other primary and trace elements.

Feeding your plants too late before lights out will cause water to build up and sit in the medium, at least until the plants have a reason to transpire through the roots or tissue. Finding the optimum times to feed is a learning curve but once you discover it, then you will really make the most out of your water supply and provide a consistent feeding regime for the root zone.

Top Tips On Plant Transpiration

  • Reducing the E.C. is one way to stay in accordance with your V.P.D chart without needing to fluctuate the temperature and humidity constantly. The rate of transpiration should take a higher priority than the amount of food administered.

  • Feeding during the hottest parts of the day is the fastest way for a growing medium to transpire water into the air. The same applies if foliar feeding or misting which is often done in large outdoor gardens where the climate can be extremely hot.

  • A growing medium should have both water retention and the ability to dry out quickly. A great additive to use is perlite which is rich in air pockets and can hold a lot of water, dry out fast and is very cheap.

  • When growing indoors, using oscillating fans to cause a cool fresh breeze and to keep carbon dioxide rich air around the plants leaves, avoid blowing fans directly at plants. This can cause increased transpiration and will lead to leaf curling and drying out.

Indirect Fans in Greenhouse Gently Blowing Cannabis Leaves Around
Indirect Fans Gently Blowing Leaves Around

High humidity can introduce airborne pathogens and plant disease into gardens, so make sure that ventilation is permanent and there is a fresh breeze gently blowing the leaves around. Stagnant air is the number one cause of mildew and mould so be extra vigilant if working with high humidity especially when flowering.