Download Rose Science Photosynthesis

April 2012
Bumblebees animate the garden, and they like roses. They’re big and easy to see, and they come in different color
patterns, all on a bold black background. They’re not aggressive. They appreciate the work we do to provide roses for
them to feed from, and we aren’t using the pollen they’re eating anyway. And we like photos of roses with bees in them!
All that on top of important pollination services to provide us with food like apples, tomatoes, cranberries, blueberries,
raspberries, sunflower seeds, et cetera. How could you not like bumblebees?
References: Griffin, Brian L. 1997. Humblebee Bumblebee. ISBN O-9635841-3-8.
Xerces Society for Invertebrate Conservation. www.xerces.org.
Cranshaw, Whitney. 2004. Garden Insects of North America. ISBN 978-0-691-09561-5
Rose Science
Photosynthesis – Part VI, How plant nutrition affects photosynthesis.
Gary Ritchie, Master Rosarian
In this installment we will briefly review the three main plant macro-nutrients – nitrogen (N), phosphorus (P) and
potassium (K) - and highlight the role that each plays in the process of photosynthesis.
Nitrogen is the most important plant macro-nutrient, making up about 2.8% of the above ground dry weight of a rose
plant. The effects of nitrogen nutrition have been carefully studied in hybrid tea roses. What we know from this work is
that roses allocate a larger amount of nitrogen to the upper leaves, which promotes a higher rate of photosynthesis in
these leaves. This is because these upper leaves, high in nitrogen, also exhibit a higher rate of electron transport in the
light reaction (see October, 2011 Clippings), and a higher rate of formation of the enzyme Rubisco in the Calvin-Benson
Cycle (see November, 2011 Clippings). As indicated earlier, both of these are rate-limiting steps in photosynthesis.
Nitrogen, of course, is an essential component of amino acids. Since amino acids are the building blocks of proteins, and
all enzymes are proteins, it stands to reason that a nitrogen deficiency in roses will lead to a reduction in enzyme formation and an overall reduction in the rates of biochemical reactions – including those involved in photosynthesis.
The average rose plant may contain roughly 0.25% phosphorous. Not very much, but this level is fairly typical of most
plants. A normal phosphorus concentration range would be from 0.2% to 1.0%, with levels greater than 1% being toxic
and below 0.2% indicating deficiency.
Phosphorus has many very important functions in the plant. When you think of phosphorus, think energy. After plants
“capture” light energy they convert it immediately into chemical energy during the light reaction of photosynthesis. As
we have learned, much of this chemical energy is stored in the form of “high energy” phosphate bonds in the chemicals
ATP and NADPH – the primary products of the light reaction. These, then, are used to drive the Calvin cycle, which
converts CO2 into glucose. So phosphorous is absolutely critical to photosynthesis.
Potassium ions (K+) lower the osmotic potential of a solution. This means that if a cell contains lots of K+ ions, it can
“suck” water in from its surroundings. Guard cells, the cells that surround and enclose stomata, open and close as K+
concentrations increase and decrease and turgor builds and ebbs. So the ability of a leaf to absorb CO2 through its open
stomata, and to feed it into the Calvin Cycle, is facilitated in large measure by the K+ concentration in the guard cells.
Potassium is also essential for protein synthesis, and enzyme formation and activation. So photosynthesis is affected by
potassium in several ways and a K deficiency greatly impedes photosynthesis.
Next time we’ll have a look at a few of the key plant micronutrients and outline the role they play in photosynthesis.
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