Winter is coming: salad green edition

A few weeks ago Alena wrote about trees and how they prepare for cold, winter weather.  Salad greens and other winter crops prepare for cold weather too, but they don’t use the same mechanisms.  

Salad greens in the garden. Photo credit: Claire Collie

Salad greens in the garden. Photo credit: Claire Collie

Salad greens are a year-round crop for many farmers.  Even in the winter, and even in New Hampshire.  Growing crops in greenhouses, high tunnels and using plasticulture are cultural practices used by farmers to do this, but plants prepare and adapt internally to colder temperatures as well.

Salad greens are tender, water-filled plants.  When water freezes, it expands, bursting the cell walls in the leaves and killing the leaf.  In scientific terms: the cause of salad green death is membrane damage caused by freezing injury [1].  Have you ever frozen lettuce in your fridge by accident and ended up with gross, mushy stuff?  That’s because the leaves froze and the cell walls burst.  The result was gross, mushy stuff when the lettuce thawed.

This baby kale has been frozen during the past two nights, but you'd never know by the afternoon.  Photo credit: Claire Collie

This baby kale has been frozen during the past two nights, but you'd never know by the afternoon.  Photo credit: Claire Collie

 

Most salad greens can survive growing in below freezing temperatures, at least for a little while.  Leaves will freeze, but they recover when warmed back up.  

How do they do it?  

By increasing the concentration of certain proteins, antioxidants, and sugars in plant cells.  

(Spinach is the hardiest leafy green crop and so it is used in the examples written here.  Other salad greens, like lettuce, arugula, and kale are not quite as hardy, but use the same mechanisms for surviving at cold temperatures.)  

Mechanisms for surviving at cold temperatures:

Synthesize proteins

Production of Cold Acclimation Proteins and Heat Shock Proteins (CAPs and HSPs) are induced in spinach plants by cold stress.  These proteins help keep cells functioning as usual by protecting membranes from rupture, keeping proteins properly folded and transporting proteins into organelles [2,3].  Proteins also reduce the solute potential in spinach cells so the freezing point is lower than that of water [4]. 

Synthesize antioxidants

Plants produce antioxidants to combat photoinhibition.  Photoinhibition is the inability of a plant cell to photosynthesis because light absorption exceeds the cells ability to dissipate energy safely.  Photoinhibition leads to the creation of reactive oxygen species (ROS).  These molecules damage plant cells by breaking down proteins and deactivating enzymes.  At cold temperatures photoinhibition is increased.  Plants defend against ROSs by synthesizing more antioxidants and pigments.  Antioxidants either intercept the excess energy before it causes damage and dissipate that energy as heat or deactivate the ROS that have already been created [5].

Increase sugar concentrations

Sugar concentrations increase in crop plants when temperatures decrease [6]. Farmers and gardeners often wait until after the first frost to harvest crops because the increased sugar concentrations improve taste.  For example, the amounts of sucrose, glucose and fructose all increase by 10 - 20 % in spinach subjected to fourteen days of cold exposure.  When returned to a warmer temperature sugar levels decreased [2].  The role sugar accumulation plays in plant response to cold stress is unclear, but it is thought to help stabilize proteins and membranes so they will continue to function properly [7].

 

 
This spinach is frozen now, but will recover once it thaws.  Photo credit: Claire Collie 

This spinach is frozen now, but will recover once it thaws.  Photo credit: Claire Collie 

Fall planted spinach in a high tunnel for winter harvest. Photo credit: Claire Collie

Fall planted spinach in a high tunnel for winter harvest. Photo credit: Claire Collie

Other things including pigments, nutrients and polyamines are at play inside salad greens (and other cold hardy crops) to help them survive cold temperatures.  Of course, all of these protective measures can only do so much to keep salad greens alive.  Eventually cold weather will kill plants, which is why farmers move their salad green production into some sort of sheltered environment for the winter months.    

Eat your greens,

support a local farmer,

and stay hungry!

 

Sources
  1. Krouse, G. H., Klosson, R. J., & Troster, U. (1982). On the mechanism of freezing injury and cold acclimation of spinach leaves. In P. H. Li (Ed.), Plant Cold Hardiness and Freezing Stress (Vol. 2): Academic Press Inc. 
  2. Guy, C., Huber, J., & Huber, S. (1992). Sucrose Phosphate Synthase and Sucrose Accumulation at Low Temperature. Plant Physiology, 100(1), 502-508.
  3. Sung, D.-Y., Kaplan, F., & Guy, C. L. (2001). Plant Hsp70 molecular chaperones: Protein structure, gene family, expression and function. Physiologia Plantarum, 113(4), 443-451. 
  4. Hincha, D. K., Heber, U., & Schmitt, J. M. (1990). Proteins from frost-hardy leaves protect thylakoids against mechanical freeze-thaw damage in vitro. Planta, 180(3), 416-419.
  5. Jenke, L. (2011). [Plant Stress Biology Course].
  6. Proietti, S., Moscatello, S., Famiani, F., & Battistelli, A. (2009). Increase of ascorbic acid content and nutritional quality in spinach leaves during physiological acclimation to low temperature. Plant Physiology and Biochemistry, 47, 717-723.
  7. Taiz, L., & Zeiger, E. (2006). Chilling and Freezing Plant Physiology (pp. 687-689). Sunderland, MA: Sinauer Associates, Inc