Disregarded Truth: Plants Transpire

Plants Transpire Most of the Water They Use!

Editor’s note: California Ag Today interviewed Allan Fulton, an Irrigation and Water Resources Advisor, UC Cooperative Extension Tehama County, in Redbluff CA, to comment on the debate about the agricultural industry’s use of water and to focus on a critical but disregarded process—that all plants transpire, even plants cultivated for the crops we eat.

Allan Fulton, Irrigation and Water Resources Advisor
Allan Fulton, Irrigation and Water Resources Advisor

CalAgToday: We hear in the media that our crops are using too much water. And while all plants need water to grow food, we also know that a high percentage of water taken up by all plants actually transpires back into the atmosphere, to form clouds and precipitation, right?

Fulton: Yes, when plants transpire, the water just returns to the local hydrologic cycle, leaving the harvested crop that we distribute elsewhere in the US or in the world actually very low in water content.

CalAgToday: When we think about transpiration, are the plants actually “borrowing” the water?

Fulton: Yes. We get a lot of questions about why we irrigate our crops so much, and it comes from the general public not being as close to farming everyday. The truth is, plant transpiration is a necessary biological process. The water cools the tree so it stays healthy and exits the leaves through special cells called stomata. While the stomates are open to allow water to transpire, carbon dioxide enters and is used in photosynthesis, making sugars and carbohydrates for the plant to create the fruits and nuts that we eat. So, an inadequately watered plant cannot take in enough carbon dioxide during transpiration, resulting in defective fruits and nuts that are smaller, shriveled, cracked—all the things the typical consumer does not want to buy.

Plants cannot gain carbon dioxide without simultaneously losing water vapor.[1]

CalAgToday: Can we say 95 or 99% of the water that is taken up by the plant gets transpired and definitely not wasted?

Fulton: Definitely. We converted to pressurized irrigation systems, micro sprinklers and mini sprinklers, so we have a lot more control over how much water we apply at any one time. We do not put water out in acre-feet or depths of 4-6 inches at a time anymore. So, much like when rainfall occurs, we can measure it in tenths, or 1 or 2 inches at most. As a result, the water doesn’t penetrate the soil very deeply, maybe only 1 or 2 feet each irrigation.

We are very efficient with the water, but because we deliver it in small doses, we have to irrigate very frequently. That is why we see irrigation systems running a lot, but they are systems that efficiently stretch our water supply and do not waste it.

CalAgToday: But again, the vast majority of the water that the tree is taking up is being transpired, right?

Fulton: Yes, most of the time, at least 90% of the water that we apply taken up through the tree and transpired so that photosynthesis can happen.UCCE Tehama County

CalAgToday: And transpiration increases on a hot day?

Fulton: Yes, we do get a little bit of loss from surface evaporation from wet soil, but we try to control that with smaller wetting patterns—drip-confined wetting patterns. When you think about it, the heat of the day is in the afternoon when many irrigation systems don’t run because of higher energy costs. There are incentives not to pump in the middle of the afternoon, but those who do try to confine the wetted area to limit evaporation. And the hot hours of the day make up about 4 hours of a 24-hour cycle, so we irrigate mostly during the nighttime and early morning hours to lesson evaporative loss.

CalAgToday: Growers are doing everything they can to conserve water. If the trees and vines are all transpiring most of their irrigated water, why is using water to grow food a problm?

Fulton: I think the emphasis throughout the United States has always been to provide a secure food supply. That security has many benefits, economically and politically; and in the end, we are trying to provide the general public with good quality, safe food at the best price possible.

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[1]  Debbie Swarthout and C.Michael Hogan. 2010. Stomata. Encyclopedia of Earth. National Council for Science and the Environment, Washington DC.

CIMIS

 

The California Irrigation Management Information System (CIMIS) is a program unit in the Water Use and Efficiency Branch, Division of Statewide Integrated Water Management, California Department of Water Resources (DWR) that manages a network of over 145 automated weather stations in California. CIMIS was developed in 1982 by DWR and the University of California, Davis (UC Davis). It was designed to assist irrigators in managing their water resources more efficiently. Efficient use of water resources benefits Californians by saving water, energy, and money.

The CIMIS user base has expanded over the years. Currently, there are over 40,000 registered CIMIS data users, including landscapers, local water agencies, fire fighters, air control board, pest control managers, university researchers, school teachers, students, construction engineers, consultants, hydrologists, government agencies, utilities, lawyers, weather agencies, and many more.

Bountiful Plant Transpiration

Guy Kyser on Plant Transpiration, Phytoremediation

By Patrick Cavanaugh, Deputy Editor

When plants get irrigated, not only do they provide food, but all plants transpire. Guy Kyser, a specialist at the UC Davis Plant Sciences Department, explained how plant transpiration transforms carbon dioxide into “food” for plants to grow.

“Water in the shallow soil is either going to evaporate, or it’s going to be transpired,” Kyser said, “which means it’s going to pass through a plant from the soil to the atmosphere. The transpiration process helps the plant fix CO2 and convert it into food or plant material. The plant is really a little solar powered machine that catches CO2 out of the atmosphere and turns it into something solid so it’s not a greenhouse gas anymore.”

Kyser also explained the phytoremediation* process that occurs in plants grown in less-than-ideal soil and water conditions, in which these plants take up polluted water from its roots while removing impurities from the soil, retain the impurities, and release pure water into the atmosphere. Kyser said, “So, with phytoremediation, you can use the plants to purify poor soil or water situations, then mow off the plant material when the process is done. All the toxins stay in the plant, and only the pure water goes through.” Despite being a slow process, phytoremediation is effective in toxin removal, environmentally friendly, and does not harm soil health.

*Phytoremediation: Ancient Greek for “plant” and Latin for “restoring balance”

CROPS ACTUALLY BORROW WATER VIA TRANSPIRATION

Almonds and All Other Crops Borrow Water Via Transpiration

By Patrick Cavanaugh, Associate Editor

Editor’s note: We interviewed Allan Fulton, UC Cooperative Extension Irrigation and Water Resources Farm Advisor, Tehama County, on an important, yet overlooked, process of a plant’s water use in agriculture. We zeroed in on a major process all plants have called transpiration in which crops borrow water.

Allan Fulton
Allan Fulton, UC Cooperative Extension Irrigation and Water Resources Advisor,
Tehama County

California Ag Today (CAT): We hear in the media that our crops are using too much water. And while all plants need water to grow food, we also know that a big percentage of the water taken up by all plants actually transpires out of the plant, back into the atmosphere, to form clouds and precipitation, right?

Fulton: Yes, the water just returns to the hydrologic cycle.  Most of the crops we are distributing, whether within the US or globally, are actually very low in water content.

CAT: When we think about transpiration, the plants are actually borrowing the water. Isn’t that true?

Fulton: We get a lot of questions from the general public about why we irrigate our crops so much.

The truth is, plant transpiration is a necessary biological process. The water cools the tree so it stays healthy. The water exits the leaves through special cells called stomata. While they are open, stomata allow water to transpire and carbon dioxide to enter the plant for photosynthesis, making sugars and carbohydrates, which, in turn, are used to make the fruits and the nuts that we eat.

So if we don’t have an adequately watered plant that is allowed to transpire, the plant won’t get enough carbon dioxide. The plant will grow defective fruits, and nuts—smaller, shriveled, cracked, all the things that the typical consumer does not want to buy.

CAT: Now, when we are talking about transpiration, can we say  most of the water taken up by the plant ends up being transpired back to the environment for reuse? So the water transpired by the plant is definitely not being wasted?

Fulton: Definitely. Since we converted to pressurized systems, micro-sprinklers and mini-sprinklers, we have a lot more control over how much water we apply at any one time. We no longer measure water output in depths of acre-feet, or even 4-6 inches at a time anymore. Much like rainfall, we measure it in tenths of an inch, or 1 or 2 inches at most.

As a result, we use less water, so it doesn’t penetrate the soil very deeply, maybe only 1 or 2 feet each irrigation. We are very efficient with the water; but, we have to irrigate very frequently because we apply it in small doses, and we run our irrigation systems a lot. So,   we are definitely using systems that stretch the water supply, not waste it.

CAT: But again, the vast majority of the water that the tree takes up is transpired, right?

Fulton: Yes. Most of the time, at least 90% of the water we apply is taken up through the tree and transpired for photosynthesis to occur.

CAT: Does transpiration increase on a hot day?

Fulton: Yes, plus the plant looses a bit from surface evaporation  from the wet soil, but we try to control that with smaller wetting patterns and drip-confined wetting patterns.

When you think about it, the heat of the day occurs in the afternoon, and many systems don’t run because of energy costs.  There are incentives not to pump in the middle of the afternoon; but for those who do, try to confine the wetted area to limit evaporation. The hot hours of a day comprise about 4 hours of a 24-hour cycle, so a lot of irrigation occurs at night-time or in the early morning hours to lesson evaporative loss.

CAT: Growers are doing everything they can to conserve water. If the trees and vines are all transpiring most of the applied water, what is the problem with using water to grow food?

Fulton: I think the concept throughout the United States has always been to secure our food supply. That security has many benefits—economically and politically. In the end, we are trying to provide the general public with great quality, safe food, at the best price possible.

CAT: And nutritious food. We are growing the food people need more of.