Why Do Cacti Taste Bitter in the Afternoon?

sedumWhile cycling to work one morning, I spotted a dandelion coexisting with a sedum in a rock garden. It wasn’t a coincidence. Each has evolved one of two known biochemical strategies to deal with high temperatures and low humidity.  The dandelion’s biochemistry is also one of the reasons why a lawn left to its own devices will soon cease to be a monoculture.

Unlike dandelions and sedum, the majority of land plants are like Kentucky bluegrass, which is the common grass found in lawns. They are C3 plants. During photosynthesis, an enzyme known as RuBisCO catalyzes the capture of CO2 to produce a pair of three-carbon molecules. One remains within the cycle, and the other is used as a building block to produce sugars.

Unfortunately, high temperatures make it easier for RuBisCo to catalyze the reaction between the five-carbon compound and oxygen, instead of CO2.  With one less carbon, in what’s known as photorespiration, a three and a two-carbon compound (2PG) are produced. The latter is not totally wasted because a metabolized version leaves the chloroplast, eventually goes to mitochondria and works its way back into photosynthesis in the form of carbon dioxide. But the process wastes time and energy. Worse, when there is little moisture available and flabby guard cells cause a plant’s pores to close, carbon dioxide does not enter, and the rate of wasteful photorespiration increases.

C4photo
C4 plants use specialization of cells to fix carbon. This is ecologically vital because the efficiency of such plants makes them more water-efficient.

So what is the strategy that makes weeds like dandelions so tough? They have two types of chloroplast-bearing cells: one type(mesophyll) near the leaf surface where the oxygen-avoiding PEP carboxylase enzyme produces a four-carbon compound(oxoloacetate), and a second type(bundle-sheath) found deeper and below the surface where oxygen levels are low. There, CO2 is released from the 4-carbon compound, and it’s taken care of by a now less-distracted RuBisCo. Having more specialization requires a larger investment on the part of the C4 plant, but it pays off because it avoids the losses of photorespiration. Also, if dandelions can photosynthesize more efficiently, their pores don’t have to be as fully open as often, and less water is lost to evaporation.

Water is needed in photosynthesis to return electrons to chlorophyll molecules after they are excited. When losing electrons, water splits into oxygen and into H+. The subsequent proton gradient across chloroplast membranes then provides the energy required for ATP synthesis. Without ATP a plant’s (or animal’s) metabolism shuts down.

camSource of image

The sedum’s strategy is known as CAM photosynthesis. It is similar to C4, but it fixes its carbon dioxide at night so that its pores(stomata) can remain closed during the day, saving even more water. CAM plants have the same two carbon-fixing steps as C4 plants, but without the differentiation of cell types. Instead, they have both carbon dioxide-fixing enzymes within the same cell.  CAM plants’ PEP carboxylase is only active at night when pores open up, letting in carbon dioxide and storing it. The Rubisco is only active diurnally when pores are closed and oxygen cannot get in, avoiding photorespiration.

Note that the production of oxoloacetate from the fixing of CO2 at night eventually leads to the production of malic acid in their vacuoles.

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A saguaro cactus from the McDowell Sonoran Desert Reserve in Scottsdale, Arizona. Like all cacti, it uses CAM photosynthesis.

CAM stands for Crassulacean Acid Metabolism:  Crassulaceae is the plant family to which sedums belong; and acid, because of the malic acid formed.

This metabolic pathway is also found among another drought-tolerant family, the cacti. After a night of accumulating malic acid, it explains why they taste more sour in the morning  and later after converting malic acid to malate, the rise in pH explains why they become bitter-tasting in the afternoon.

 

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