What do plants really need to grow? Is that what they're getting?

Think about what a plant needs.

Light, water, nutrients, atmosphere.

This list seems simple enough.  Understand these subjects and you can successfully grow crop plants, bedding plants, forests or fields.   

A few heads of lettuce. Drawing by Claire Collie

A few heads of lettuce. Drawing by Claire Collie

Think about the plant again.  

It’s a lettuce plant, in a field of other lettuce plants.  Sunlight streams down during the day.  Roots are anchored in the loamy soil.  Sometimes the soil is saturated with rainwater, sometimes it isn’t.  Nutrients, supplied from decomposing manure and plant litter, are sucked up by the roots.  Leaves respire and use carbon dioxide and oxygen from the gentle circulating air. 

This sounds like a happy plant.  But is it?

Plants absorb light in the blue and red parts of the visible light spectrum, between 400 - 450 nm and 600 - 700 nm.  Drawing by Claire Collie.

Plants absorb light in the blue and red parts of the visible light spectrum, between 400 - 450 nm and 600 - 700 nm.  Drawing by Claire Collie.

Does our planet provide the optimum environment for a plant? What if we could control a plant surroundings providing the exact environment it needs?

To some extent this is happening with crops like lettuce.  Global warming, growing human populations, limited affordable cropland, the local food movement, and the VEGGIE program to grow food in space have all sparked innovation to grow food crops in non-traditional ways; investigating what plants really need to grow and how to optimize that.

Plants need light to grow. 

Think back to Biology 101 - light initiates many plant functions including photosynthesis, circadian rhythms, seed germination, and flower induction. Crop plants need lots of bright, direct sunlight to be productive and grow.  Here in the northeast there isn’t enough light for adequate crop growth from about November to March.  So, growers either plan ahead for a period of low productivity (time for vacation!) or invest in lights to provide the additional light their plants aren't getting.

LED lights can emit a very narrow range of wavelength.  Using only red and blue LEDs means plants are given only the spectra they can absorb. Plants grown with LED lights can be very close to the light source because LEDs do not emit a lot of heat. Drawing by Claire Collie.

LED lights can emit a very narrow range of wavelength.  Using only red and blue LEDs means plants are given only the spectra they can absorb. Plants grown with LED lights can be very close to the light source because LEDs do not emit a lot of heat. Drawing by Claire Collie.

What if you just skipped using daylight all together?  Plenty of people are doing just that using LED lights.  Plants best absorb light in the blue and red wavelengths in the visible spectra.  LED lights can be blue or red so plants are given just the light spectrum they need to grow.  Both red and blue light are needed for healthy plant growth.  Adding white light to the spectrum can also add to the growth and nutrient content of lettuce plants [5].  

For Growing Underground, a salad green farm 33 meters underground in London, supplying lights is the only way their crops will grow.  Growing greens in abandoned underground tunnels means salad greens can go from farm to table in just a few hours. Vertical farms, where plants are stacked up on racks, have lights hovering right over the canopy level.  This is only possible because LEDs don’t emit a lot of heat like traditional lamps and can be placed directly over crops without burning them.  

Water and nutrient delivery is altered for densely grown crops like lettuce grown in vertical farms.  Gone is unpredictable rain that could saturate the soil or leave it dry.  Gone is heavy soil substrate that contains the nutrients plants need to grow.  Instead plants are grown in shallow ponds or gutters with their roots dangling into a moving solution of water and nutrients.  The nutrient solution is recirculated so water isn’t wasted through evaporation, run-off or is used by other plants.

Lettuce plants grown in an NTF system.  Water and nutrients flow down gently sloping gutters. Water is collected and recirculated through the system.  Drawing by Claire Collie.

Lettuce plants grown in an NTF system.  Water and nutrients flow down gently sloping gutters. Water is collected and recirculated through the system.  Drawing by Claire Collie.

What about the atmosphere around a plant? 

Pumping carbon dioxide into greenhouses is a common practice for plant growers.  Why? Because photosynthesis in more effective when carbon dioxide concentrations are increased.  Increased photosynthesis results in faster plant grown, higher yielding plants, early flowering and increased flower size. 

Why is this? 

Most crop plants have a C3 metabolic pathway to turn carbon dioxide into sugars. The first step in this pathway is catalyzed by the enzyme Rubisco (ribulose-1,5-bisphosphate). This is one inefficient enzyme and under ambient atmosphere conditions a bottleneck can be formed.  If atmospheric carbon dioxide concentrations are increased this reaction speeds up, thereby increasing the net photosynthetic rate and faster plant growth.

For example, lets say our plant has a leaf temperature of 25 °C (a balmy 77 °F which sounds pretty nice right about now).  The current amount of carbon dioxide in the atmosphere is about 372 μmol/mol (or 390 ppm).  If carbon dioxide concentration around the plant is increased to 550 μmol/mol, then Rubisco limited photosynthesis would increase by 36% [2].  This is what greenhouse plant growers take advantage of by pumping carbon dioxide into their greenhouses.  For vegetable seedling and lettuce production increasing carbon dioxide to 800 − 1000 ppm increases growth, and is economically sustainable.  For other plants, like African violets and some Gerbera, less additional carbon dioxide is economically sustainable - too much can damage the crops [4].

The amount 550 μmol/mol is a significant because it’s the estimated atmospheric carbon dioxide concentration in 2050 [2].  

Radish and lettuce plants grown in the field (on the left) were smaller than those grown with additional carbon dioxide (on the right) [5]. Drawing by Claire Collie

Radish and lettuce plants grown in the field (on the left) were smaller than those grown with additional carbon dioxide (on the right) [5]. Drawing by Claire Collie

So, an increase in carbon dioxide concentrations in the atmosphere is good for crop plants right? It’s not that simple.  With time plants will acclimate to higher concentrations of carbon dioxide and won’t sustain the increased net photosynthesis [2].  In grasslands this takes about 6 years.  Of course climate change won’t just change the atmospheric carbon dioxide concentrations.  Crop plants are also affected by changing temperature and weather patterns. 

Another factor to consider is that changing photosynthesis and the C3 metabolic pathway with elevated carbon dioxide levels may change the nutrient content of plants.  Since we eat crop plants this could affect our diet.  For example, lettuce and radish crops grown in elevated carbon dioxide conditions had higher total nitrogen, protein nitrogen, and nitrate levels than when grown in field conditions.  They also had lower dietary fiber than field grown crops [4].  

Think back to our lettuce plant, but now picture it in a different environment.  Pink light from red and blue LED lights glows from overhead.  Water and nutrients are provided to roots through a recirculating nutrient solution.  The atmosphere has elevated carbon dioxide so plants grow faster.  Plants are stacked on racks many feet high.

‘Pinkhouses’ as such farms called are being built, but it’s still relatively expensive for growing non-specialty food crops.

Is this a good thing?

Does it matter where food comes from?

 

 

Cool projects to read about

Space farming / Milk crate farming in NYC / Vertical Pinkhouses / Gotham Greens rooftop farm / Growing Underground in London

Sources

1. Jonathan M. Frantz, 2011, Carbon Dioxide in a Commercial Greenhouse Reduced Overall Fuel Carbon Consumption and Production Cost When Used in Combination with Cool Temperatures for Lettuce Production. Horttech.

2. Stephen P. Long, Elizabeth A. Ainsworth, Alistair Rogers and Donald R. Ort.  2004. Rising atmospheric carbon dioxide: plants FACE the future. Annu. Rev. Plant Biol.

3. JD McKeehen, DJ Smart, CL Mackowiak, RM Wheeler, and SS Nielsen. 1996. Effect of CO2 levels on nutrient content of lettuce and radish. Adv Space Res.

4. T.J Blom, W.A. Straver, F.J. Ingratta, Shalin Khosla, and Wayne Brown. 2002. Carbon Dioxide In Greenhouses. Ontario Ministry of Food and Agriculture.

5. Kuan-Hung Lina, Meng-Yuan Huang, Wen-Dar Huang, Ming-Huang Hsu, Zhi-Wei Yang, Chi-Ming Yang. 2013 The effects of red, blue, and white light-emitting diodes on the growth, development, and edible quality of hydroponically grown lettuce (Lactuca sativa L. var. capitata). Scientia Horticulturae.