American Mountain Ash


Like all green plants, the American Mountain Ash tree is a phototroph, which means it produces its own food through photosynthesis.  There are two steps in this process that contribute to the overall process.  The first step is the light dependent reaction.  This begins when sunlight strikes a leaf.  The energy from the sun is then used to make adenosine triphosphate (ATP).  This ATP, along with other molecules made during the light dependent reactions, is then used in the light independent reaction.  The light independent reaction produces carbohydrates that the American Mountain Ash tree can then use for food.  The general equation that represents photosynthesis is:


CO2 + H2O + light energy ----> carbohydrates + O2 + H2O


Once the tree produces nutrients in the leaves through light dependent and light independent reactions, you may be wondering how do the nutrients get to the other parts of the tree?  The answer to this is phloem!  The phloem, which are tubes located to the outer edge of the tree, lie just underneath the bark.  It is made up of cells called sieve tube members, companion cells and parenchyma cells.  The nutrients travel through the sieve tube members, which do not contain a nucleus, since their function is to move the nutrients.  A nucleus would just get in the way and hinder the traveling of the nutrients.  The function of the companion cells is to take over metabolism for the sieve tube members.  Parenchyma cells, however, are used for storage and support. 

As you can see, light energy isn’t the only necessity for photosynthesis.  Carbon dioxide and water are also required.  How does carbon dioxide enter the leaves?  It enters through little openings called stomata.  Water is absorbed by the roots, but how does it travel to the rest of the plant?  Water is pulled up the tree by tubes called xylem.  When the water evaporated from leaves, water from the xylem flows in through diffusion.  The cohesion of the molecules and the adhesion to the sides of the tube prevent the column of water from being broken, and therefore allows water to be drawn up the xylem to the rest of the tree.

Spectral reflectance and chlorophyll fluorescence are rapid methods that are used to measure how much stress a plant is undergoing. A study was performed on four species of trees, that included Abies balsamea, Betula papyrifera, Picea rubens and Sorbus americana.  The researchers wanted to determine how quickly reflectance and fluorescence change following branch cutting from these trees.  (Richardson, 2002.)  They basically wanted to see how quickly each one recovered after branch cutting.  For one experiment, no treatments were used to maintain the freshness of the cut branches.  After the first 12 hours, the changes in reflectance and fluorescence barely changed for any of the trees.  However, there were visible signs of desiccation in the Betula papyrifera and Sorbus americana.  The Sorbus americana leaves were dry and crisp after 24 hours.  Two to three days after the branches were cut, Betula papyrifera and Sorbus americana showed rapid declines in their ratio of variable to maximum fluorescence and the two conifers still showed minimal changes in the ratio of variable to maximum fluorescence.  Obviously from this experiment, it can be seen that conifers are better able to manage their water loss than the other two broadleaf species, Betula papyrifera and Sorbus americana.  The second experiment tested to see if Betula papyrifera was kept cool and moist after cutting, would it also show slight changes in the fluorescence and reflectance.  After three days, it was determined if it is kept cool and moist between time of collection and measurement of reflectance and fluorescence, many broadleaf specimens could be extended for a longer period of time.  There is clearly a difference in response to branch cutting between conifers and broadleaf species.  It is concluded, due to the fact that coniferous foliage is able to minimize water loss compared to the broadleaf species, that coniferous foliage is also better at managing stress.  (Richardson, 2002.)             

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