This series of posts will highlight the plants that help you celebrate the Yuletide season.
If you have made any New Year’s resolutions to be healthier in 2014, hopefully today’s topic will inspire you. Citrus fruits of all kinds are bountiful in winter and have worked themselves into our Christmas traditions. Many receive an orange in the bottom of their stocking for Christmas. There’s even a tear-jerker short story about The Christmas Orange. The Orange Bowl will be played tonight.* Thankfully, oranges and other citrus products are no longer extravagant seasonal luxuries. However, this time of year they are at their peak if you live where they can be locally grown. If you are looking for healthy snacks to atone for your holiday dietary indiscretions, citrus is a great place to start. Here’s more on the science behind the citrus.
First, let’s start with some numbers:
Citrus production is a $3 billion market in the United States. That’s more than 11 million tons of citrus. Not surprisingly, Florida is the leading citrus-producing state (63% of US production totals) followed by California (34%), Texas and Arizona (3% combined). Despite our demand for this healthy fruit and citrus juice products, the market is not on the rise. Florida’s yields in both production and value have been losing ground in recent years (down 9% between the 2011/12 and 2012/13 seasons). Keep reading for more on the reason why.
Of course, it’s not just Christmas Oranges. Citrus fruits include lemons, limes, grapefruit, tangerines, pomelos and a slew of other varieties and hybrids. We eat these healthy whole foods because they are a good source of Vitamin C, but they don’t have a monopoly on this nutrient. Check out this list.** Citrus derives its name from the high content of citric acid, the pucker-inducing chemical that gives the fleshy fruit their distinctive sour taste. Of course, the distinctive smell of citrus comes from volatile chemicals stored in special glands in the peel. The main ingredient is a chemical called limonene. And what about their signature color? All immature citrus fruit goes through an immature green phase. There are chloroplasts within the cells of the peel layer of the fruit that supply energy to the developing tissue. At some point in the developmental program of citrus fruit ripening (triggered by colder temperatures), these chloroplasts undergo a major metamorphosis. They completely rearrange their membrane structure, shut down their photosynthetic machinery and kick carotenoid pigment production into overdrive to give the ripening fruit its characteristic yellow, orange or pink peel.
I know you’re not supposed to, but let’s compare apples and oranges. Apples and other stone fruit contain a continuous mass of fleshy fruit surrounding the seeds. Oranges and other citrus fruit have segments, seemingly arranged for convenient consumption. Have you ever wondered where these segments come from? Let’s talk about hesperidium (plant biology word of the day meaning citrus fruit type) development from the outside in. The peel and white pith layers surround a chamber called the locule. This chamber is where the seeds will form and is initially filled with air. As the fruit develops, the hair cells grow from the tissue enclosing the locules toward the center of the fruit. These hair cells swell with water, sugar and other flavor molecules as the fruit matures to become the juice sacs within the segments. Each locule is a separate segment of the hesperidium. Check out this graphic and description from the Mildred E. Mathias Botanical Garden at UCLA.
One of my favorite citrus fruits is the Satsuma (Citrus unshiu), a seedless variety that is cold hardy enough to be grown in the Baton Rouge area. It’s very similar to mandarin oranges or clementines- small, sweet, seedless and easy to peel***. If you are interested in trying them, but have the misfortune of living in a USDA hardiness zone less than about 8a you can order them online from places like Simon Citrus Farm or LaLagniappe. For those of you in Louisiana that would like to grow your own, I recommend checking out the LSU Agcenter citrus portal for more info.
Now, if you were interested enough to click on the Satsuma-ordering links above, you would have noticed bold disclaimers about quarantines and shipping restrictions. Don’t worry, you don’t have to fear for your safety, but you do if you are a citrus tree. This is because citrus trees have some formidable pathogens that growers and researchers are working hard to combat. Diseases like Sweet Orange Scab and Citrus Canker cause cosmetic damage to the fruit and reduce yields. Sweet Orange Scab is caused by a fungus (Elsinoë australis), and Citrus Canker is a bacterial disease (Xanthomonas citri). Both of these pathogens infect many citrus varieties. Fungicide application protocols can be used to control Sweet Orange Scab, but quarantine and local disposal of infected tissues are also key strategies to prevent their spread.
The most devastating pathogen for citrus growers is the bacterium Candidatus Liberibacter spp, which causes Citrus greening disease. It leaves the fruit half green and turns it inedibly sour and bitter. Eventually, the entire tree is killed. The bacterial pathogen is transmitted by an insect vector and growers must spray increasingly more pesticides and destroy acres of infected trees to control the spread of this disease. These factors (reduced supply and increasing costs of maintaining the orchards) are wreaking havoc on the citrus industry, especially in Florida. Growers and scientists are working fervently to come up with some solutions to save the citrus industry. One controversial option is introducing transgenic oranges with resistance to Citrus greening disease. The struggle of associating this wholesome fruit with the GMO debate has been eloquently covered in an article by Amy Harmon in ‘A Race to Save the Orange by Altering Its DNA.’ Genetic engineering may be the only strategy able to combat the disease on a timescale that can save the industry, as evidenced from this quote in the article.
“An emerging scientific consensus held that genetic engineering would be required to defeat citrus greening. “People are either going to drink transgenic orange juice or they’re going to drink apple juice,” one University of Florida scientist told Mr. Kress.”
Let’s hope that growers and scientists can come up with a reasonable solution to this epidemic before the Christmas Orange tradition becomes nothing more than a footnote of Yuletide folklore.
*This year’s match-up is Ohio State and Clemson. While I have some University of Michigan connections, I may still have to root against Clemson because I am still bitter about their win over LSU in the 2012 Chick-Fil-A bowl.
** Broccoli and kiwi have nearly twice the Vitamin C content.
***Ease of peeling is a highly desirable trait for the fresh fruit market. Who wants to spend a lot of energy tearing into the fruit, ripping most of it to shreds while getting squirted in the eye with acidic juice in the process? Varieties like Satsumas and Clementines have very thin pith layers that make it easy to pull apart the peel from the segments. For other citrus fruits, the peeling process is not so easy and it frustrates fresh fruit eaters and processors alike. Don’t worry, scientists are on it. Check out this review on orange peeling technologies.
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