photosynthesis

What goes on inside cacti “guts”

Saguaro1-189x300The Arizona Daily Independent recently ran an article entitled “Cacti “Guts” Can Purify Contaminated Water” which reported on new research about using cactus mucilage to remove contaminants in water. That lead me to review the basics of plant metabolism and how that metabolism has evolved to deal with the environment.

All plants carry out two basic and opposite chemical reactions: respiration and photosynthesis. During respiration, carbohydrates are oxidized to water, carbon dioxide and energy. Written as a formula, this would be Carbohydrate + O2 >> H20 + CO2 + Energy.

During photosynthesis, water, carbon dioxide, and energy become carbohydrates plus oxygen: H20 + CO2 + Energy >> Carbohydrate + O2. The energy for this reaction comes from sunlight which is captured by chlorophyll and stored as stable chemical energy.

Photosynthesis in about 90% of plants produces a 3-carbon sugar as the first stable product. In botanical terminology, these are called C3 plants. About 3% of plants produce a 4-carbon sugar and hence are called C4 plants. C4 plants use carbon dioxide more efficiently than C3 plants and lose less water through evapotranspiration. C4 plants can grow faster than C3 plants at high temperatures. C4 plants include most summer grasses in warm climates, many weeds, and some important crops such as corn, sorghum, and sugar cane.

Cacti and other succulents use a more extreme metabolic process called Crassulacean Acid Metabolism (CAM) to supplement the respiration and photosynthesis process. About 5% of plants use CAM.

In succulents, water is stored in leaves, stems, or roots and in the plant “flesh.” Water is bound into extracellular mucilages that hold onto water very tightly.

CAM plants keep their stomates (pores) closed during the hot day to prevent water loss via evapotranspiration. Their stomates open at night to take in carbon dioxide. But since there is no sunlight to provided the energy for photosynthesis, the carbon dioxide is stored as an organic acid for use the next day. CAM plants lose about one-tenth the water through evapotranspiration compared to C3 plants. CAM works best when the diurnal temperature variation is large such as in the Sonoran desert where it is common to have a 25- to 30 degree (F) difference between the high temperature of the afternoon and low temperature every night. Under extreme drought conditions, CAM plants keep their stomates closed both day and night. Their metabolism slows to using moisture within plant tissues and carbon dioxide released by respiration is recycled into the photosynthesis process.

Some plants, such as agavi deserti, can switch between CAM and C3 metabolism depending upon the abundance of water. Idling CAM plants can resume full growth within 24- to 48 hours of a rain. Agaves can sprout new roots within 5 hours after a rain according to the Arizona-Sonora Desert Museum.

Desert-adapted plants use other strategies as well. For instance, mesquite trees have very deep roots that can tap groundwater sources making them less dependent on rain. Have you noticed that many leafy desert plants have relatively tiny leaves? Many tiny leaves have a higher surface area to volume ratio compared to a large, broad leaf. That allows the plant to be a better radiator of heat. The Foothills Palo Verde is the extreme example of this. Even oak trees in our region have small leaves compared to those of more temperate climates. Another adaptation is that leaves may be grayish rather than deep green. This reflects light better and keeps the plant cooler. Some plants, such as Prickly Pear cactus and Jojoba have a vertical orientation of their pads or leaves. This orientation allows them to get more direct sunlight in the cooler early mornings or late afternoon rather than in the hottest part of the day. Photosynthesis is more efficient during these cooler times.

Reference: A Natural History of the Sonoran Desert, Arizona-Sonoran Desert Museum.

Can You Get Potable Water From a Cactus?

Generally NO. Moisture within the pulp of a cactus is very acidic and many cacti contain toxic alkaloids. You can, however, eat the fruit.

The moisture is acidic because of the way many succulents, including cacti, carry on photosynthesis, the process by which carbon dioxide and water are turned into carbohydrates.

Most plants have their pores (stomates) open during the day to take in carbon dioxide, and use sunlight as a catalyst for the reaction: Carbon dioxide + water becomes sugar + oxygen. But in the desert, plants with pores open during the hot days, lose much water through evapotranspiration.

So, succulents use a modified version of photosynthesis called CAM (Crassulacean Acid Metabolism). CAM plants open their stomates only at night when it is cooler so there is less evapotranspiration. Because there is no sunlight to act as a catalyst, carbon dioxide is stored as an organic acid, principally Malic Acid (C4H6O5). Carbon dioxide is gradually released from the acid during the next day. CAM plants use about one-tenth the water to produce each unit of carbohydrate compared to standard photosynthesis. The price: a much slower growth rate.

Many plants contain malic acid, but usually in lesser quantities than found in cacti. Also cooking generally destroys the acid.

Besides malic acid, succulents produce Oxalic Acid (C2H2O4), which is toxic, as another product of photosynthesis. “Its chief function seems to be sequestering metals, principally calcium. Calcium oxalates often occur as crystalline minerals within the cactus pulp. Their function seems to be aiding structural integrity and enzymatic processes. In fact two crystalline calcium oxalate minerals have been identified in all cacti tested: CaC2O4.2H2O (weddellite) and CaC2O4.H2O (whewellite).” [Source: Plant Physiology, February 2002, Vol. 128, pp. 707-713.] Oxalates are also formed with heavy metals such as copper, perhaps to reduce toxicity to the plant.

Oxalic acid is toxic to humans because it combines with calcium in our bodies to produce calcium oxalates which clog up our kidneys.

So, what about the barrel cactus. Can’t we get water from those? Did you bring along a machete and solar still?

The Seri Indians sometimes used the Fishhook barrel (Ferocactus wislizeni) for emergency water. However, drinking the juice on an empty stomach often caused diarrhea, and some Seri report pain in their bones if they walk a long distance after drinking the juice. The Seri called the Coville barrel (Ferocactus emoryi), “barrel that kills” because eating the flesh of the cactus causes nausea, diarrhea, and temporary paralysis. Think you can tell the two apart? (See: Edible Desert Plants – Barrel Cactus Fruit).

What about Prickly Pear pads we sometimes see in grocery stories or on the menu of Mexican restaurants? What you see are generally young spring pads which naturally contain less oxalic acid. Cooking leaches out the acid. In an emergency you can eat the young pads raw. And there are some spineless cultivars that naturally contain little oxalic acid which can also be eaten raw. These were developed mainly as cattle feed.

The bottom line is you really cannot get a drink from a cactus in spite of what you may have seen in old cowboy movies.