Why do orange trees (and other citrus trees) die if they freeze during winter?

Question

I know that orange trees will die if they freeze through during the winter, but today I started to wonder why. What is actually different about their physiology from other kinds of trees that can handle being frozen during the winter? Orange trees go into a dormant state during cold weather---just like other deciduous trees---so why doesn't this protect them from cold damage like it does (for example) and oak tree.

I have a hunch that it has to do with moisture content and softness of the wood in the branches, but I'm not sure.

Answer 1

Citrus trees growing outside the Valley are at a distinct disadvantage with regard to climate, i.e., winter almost always will be accompanied by one or more freezes. Citrus trees are subtropical to tropical in nature; thus, they may suffer severe damage or even death because of freezing temperatures.

http://aggie-horticulture.tamu.edu/exten...

Citrus trees are not generally frost hardy. Mandarin Oranges (C. reticulata) tend to be the hardiest of the common Citrus species and can withstand short periods down to as cold as −10 °C, but realistically temperatures not falling below −2 °C are required for successful cultivation. Tangerines, tangors and yuzu can be grown outside even in regions with more marked sub-zero degrees in winter, although this may affect fruit quality. A few hardy hybrids can withstand temperatures well below freezing, but do not produce quality fruit. Lemons can be commercially grown in cooler-summer/moderate-winter coastal Southern California, because sweetness is neither attained nor expected in retail lemon fruit. The related Trifoliate Orange (Poncirus trifoliata) can survive below −20 °C; its fruit are astringent and inedible unless cooked but a few better-tasting cultivars and hybrids have been developed.

http://en.wikipedia.org/wiki/Citrus

                  Phsiology difference

A large part of a tree's body is found above ground (the branches and the trunk). These parts need to withstand the cold winter air. In winter water freezes to ice. The trees cannot absorb water, therefore, transpiration becomes impossible.Trees can be divided into two groups: deciduous trees, which lose all their leaves in autumn and evergreens, which maintain leaves on their branches throughout the year.

Evergreen trees, such as holly, usually have leaves which are covered in a thick layer of wax to stop them losing too much water. Conifers, such as pine trees, are evergreens which have long narrow leaves. This shape of leaf also helps to reduce water loss.

The sap of trees which live close to the Arctic is also special. In winter their sap becomes rich in sugars. The sugar stops the sap from freezing, in other words, it acts as an antifreeze. In the sugar maple the sap is particularly rich in sugar. In North America the sap of the sugar maple is collected for food.

Deciduous trees lose their leaves in autumn but the branches already have buds which contain next year's leaves. It is very important that the buds do not break (open) before winter is over or else the young leaves will be damaged. The leaves are needed for photosynthesis, so if the leaves are damaged the tree will starve.

Just like seeds and the root-storage organs, the buds of trees can "tell" when winter has passed and spring has arrived. Some trees, such as birch, will only open their buds when there is a certain number of hours of daylight. Other trees need to pass through a cold period before the buds open. For example, apple trees need to be kept below 7°C for 6 to 8 weeks before their buds will open.  

Answer 2

The ability of a tree or shrub to survive an extended cold period depends on the seasonal change in its metabolism to a quiescent or dormant state known as acclimation. The first stages of acclimation are induced in early autumn by exposure to short days and nonfreezing chilling temperatures, both of which combine to stop growth.

It is important to remember that plants are composed primarily of water, and that freezing of water inside living cells is fatal to individual cells and potentially deadly for the entire plant. Therefore, living tissues survive low temperatures by suppressing ice formation, or by allowing water to freeze, but only in areas of the plant that won't be injured by ice crystal formation.  Citrus lack the ability to suppress ice formation because they are not acclimated.

Many trees and shrubs commonly found in the hardwood forests of southeastern Canada and the eastern United States have the ability to suppress ice crystal formation in their cells, even at temperatures far below the freezing point. This "deep supercooling" is seen in species such as oak, elm, maple, beech, ash, walnut, hickory, rose, rhododendron, apple, pear, peach, and plum.

Plants appropriately adapted to the local climate, and those that have fully acclimated will usually survive even the coldest temperatures. But injury can occur when: (1) temperatures fall below a plant's maximum low temperature limit even after normal acclimation has occurred, (2) when premature freezing occurs before the plant has acclimated in the fall, (3) when unusually late freezes occur in the spring after the plant has deacclimated, and (4) when there are dramatic swings in temperature during the winter that cause a plant to deacclimate before the threat of severe freezing is over.

Hope that helps you out!