Growing Points: Understanding how plants ‘work’

August 21, 2012
Written by Dr. Mohyuddin Mirza
I have often wondered whether growers truly understand the real business they are in.

They have good business plans, spreadsheets, income and expense cash flows, and probably good money management. Recently, two growers I talked to mentioned how the tiny thrips have become their enemies and caused significant damage to their crops.

One cucumber grower experienced economic losses and one gerbera grower is out of business because these thrips could not be brought under control. Managing insects and diseases is a significant part of crop management and one has to be constantly alert for these problems.

But the real business is what plants do, openly and secretly, during the day and the night and during a 24-hour cycle.

In any business, one must know and understand what it is that you are doing.

If you manufacture a product, you must understand the entire process, from raw material to finished product, including the quality, quantity, efficiencies, deficiencies, etc.

AN INTEGRATED ‘FACTORY’ REQUIRING LIGHT AND WATER
■  In the greenhouse business, one must know that plants are extremely complex and integrated factories that manufacture a series of ingredients from very basic raw materials with the help of a large number of chemical reactions. It would not be wrong to say that your livelihood depends on the fact that plants take carbon dioxide from the air through openings called stomata and fix it inside the leaves with the help of light and water.

Also fascinating is that it takes approximately 500 kg of water to move through from the roots to the leaves and out the stomata to make one kg of dry matter, without raising the water temperature. A tremendous amount of energy is required to carry out this transpiration process.

What a remarkable processing factory! Your job is to understand this process and ensure that plants have everything necessary to get the job done properly and profitably. Look at these pictures and consider what the plant is trying to accomplish so that you can feed your family, and put the kids through school and university.
 
growingpoints
 
   
What is common in these pictures?

 These pictures show a sample of products grown in greenhouses. Tomatoes, peppers and cucumbers are removed from the plant to be sold, while the plants stay in the greenhouse. Lettuce is sold as such. Bedding plants and flowers are grown and sold. They have to appear attractive to consumers. You as a grower must understand the cultural practices to grow them and get good yields per unit area of the greenhouse.
 
BUSINESS OF THE PLANTS
■  All of our plants have “CLSTP” in common.
  • C is for Carbon dioxide.
  • L for sunLight.
  • S is for Stomata.
  • T is for Transpiration.
  • P is for Photosynthesis.
In Part 1 of this series, I would like to talk about carbon dioxide and light, just a basic understanding in relation to plant growth.

Carbon dioxide: Simply stated, plants absorb carbon dioxide from the air, combine it with water primarily in their leaf cells containing green coloured pigment chlorophyll and use light in this reaction, and then fix the energy as simple molecules of sugar (glucose). You have seen this equation in textbooks and I think are familiar with it. This process is complex and we don’t need to know all the chemistry involved in this simple reaction.
   
chart
 

What you see in the equation is that carbon dioxide, which is coming from air, is combined with hydrogen split from water, and with the help of light energy, is converted to sugar glucose and oxygen is released. You job as a grower is to make sure:
  • There is enough light around the leaves.
  • There is enough carbon dioxide around the leaves. Our normal air contains about 390 ppm of carbon dioxide. If fresh air is not brought into the greenhouse, the level can drop below 200 ppm. At that point, growth can be significantly reduced.
  • There is an adequate supply of water from the roots to leaves.
  • The climatic conditions are at optimum levels, so that plants can carry out the previously mentioned manufacturing process. Most interestingly, the ability of plants to fix the light energy (photosynthesis) is pretty stable at temperature ranges from 18ºC to 28ºC. I think that is a blessing for those growers who don’t have precise temperature controls.
  • Light: If I say that availability of adequate quantities of light – and proper quality of light – is the most important need of the plant to manufacture food, it is the right statement.
However, plants are such integrated machines that even if light meets the criteria of quantity and quality, other factors in the equation can become the limiting factors to utilize light.

A cucumber grower once called me in a panic to say that all of his plants were showing a yellowing of the lower leaves. There was multiple fruiting and very poor vigour, in spite of the fact sunlight levels had been pretty good.

When I visited the greenhouse, the grower mentioned he recently installed carbon dioxide generators and he turns them on at night and off during daytime hours. Somehow he understood that by turning these generators “on” at night, he would gain the double benefit of carbon dioxide and additional heat in the greenhouse, and plants would love it.

You can easily guess what went wrong. When we talk about the use of additional carbon dioxide, we link it with light. The carbon dioxide is used by plants when there is light. Here are a few other facts about the use and effects of light:
  • The amount of light has a direct bearing on the amounts of water used for irrigation. The higher the light, the more frequent the irrigation should be. There are different strategies for irrigation in spring, summer, fall and winter. Do you know these strategies?
  • More light means lower electrical conductivity (EC) in the rootzone and less light means higher EC. A common approach in growing tomato seedlings in winter is to raise the EC in the rootzone to higher than 5.0 millimohs. Many growers have it even higher than that to keep the growth compact.
  • The amount of light has a direct effect on greenhouse temperatures.
  • Light is the most important factor in opening the stomata under the leaf surface.
I will discuss transpiration, stomata and respiration – along with more about photosynthesis – in the next issue. I just want to make sure you better understand the needs of the plants you are growing.

Editor’s note: There was an editing error with Dr. Mirza’s feature in July 2012 (“What’s in your water?,” page 20). The following are the corrected paragraphs, located towards the end of the article. The corrected version of the article has been posted to the July features archives.

The tough questions always are what can be done at this stage. In this case, I advised the grower to bring in water from a known source, install charcoal filters, cut this growth off, apply activated charcoal, and hope that plants recover and are saleable. The lab results came back with 10 ppb of 2,4-D and 1.1 ppb of Picloram. The grower switched to a different source of water and charcoal filters were installed.

Consider the third case where a researcher bought a commercially available soil blend to conduct an experiment. A few days later he called me to look at his plants because something was “weird.” 


Dr. Mohyuddin Mirza is a greenhouse consultant. • drmirzaconsultants.com

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