This is the third in a series of posts on carbon fixation in Mohave desert plants. In this post we will focus on plants that use CAM carbon fixation which includes cactus, yucca and agave. The most important benefit of CAM to plants is the ability to leave most leaf stomata closed during the day. Plants employing CAM are most common in arid environments, where water comes at a premium. Being able to keep stomata closed during the hottest and driest part of the day reduces the loss of water through evaporation and transpiration, allowing such plants to grow in environments that would otherwise be far too dry. Plants using only C3 carbon fixation, for example, lose 97% of the water they take up through the roots to transpiration – a high cost avoided by plants able to employ CAM. The Mojave Desert is the northernmost “hot desert” in North America and essentially a transition land between the Great Basin and Sonoran. It’s the smallest of the Big Four, covering some 54,000 square miles of southeastern California, southern Nevada, and itty-bitty strips of southwestern Utah and northwestern Arizona. Roughly speaking, the Great Basin Desert yields to the Mojave at the northern range limit of creosote bush, the defining shrub of North America’s hot deserts; its distribution essentially outlines them. You can rightly think of it as the hot-desert equivalent of big sagebrush. But the trademark plant of the Mojave, the one whose geography basically maps out this desert, is the Joshua-Tree. This outsized yucca actually flourishes best on the Mojave margins, reaching peak development on middle slopes of foothills and bajadas. Interestingly, the Joshua-Tree uses C3 carbon fixation while most of the remaining yucca and agave use CAM carbon fixation, along with all of the cactus species.
This post is the second on my series on Mohave desert plants, this time focused on C4 plants, not really as complex as it might sound. The C4 photosynthetic pathway has evolved an estimated 45 times in terrestrial plants (Sage 2004), and is most prominent in grasses, which account for roughly 25% of global terrestrial primary production (Still et al. 2003) and include important crop and weed plants and potential biofuels such as maize, sugarcane, sorghum and switchgrass. The highest rate of photosynthesis is typically observed in C4 plants. The photosynthetic rate in such plants is known to be directly related with the variation of the solar rays in the daytime. Maximum rate of photosynthesis occurs in the red and blue regions of the visible light as seen in the absorption spectra of chlorophyll a and b. These are of economic importance as they have a comparatively higher photosynthetic efficiencies in comparison to other plants. C4 photosynthetic plants outperform C3 plants in hot and arid climates. By concentrating carbon dioxide around Rubisco C4 plants drastically reduce photorespiration. The frequency with which plants evolved C4 photosynthesis independently challenges researchers to unravel the genetic mechanisms underlying this convergent evolutionary switch. The conversion of C3 crops, such as rice, towards C4 photosynthesis is a long‐standing goal. Nevertheless, at the present time, in the age of synthetic biology, this still remains a monumental task, partially because the C4 carbon‐concentrating biochemical cycle spans two cell types and thus requires specialized anatomy.
Desert plants tend to look very different from plants native to other regions. They are often swollen, spiny, and have tiny leaves that are rarely bright green. A desert always has a limitation of water but the temperature may be hot or cold, high altitude and cloudy like parts of Costa Rica or low altitude and windy like the Cape Preserve in South Africa. The strange appearance of these plants is a result of their remarkable adaptations to the challenges of the desert climate. Desert plants have developed three main adaptive strategies with diverse implementations often in different species with convergent evolution to the same form: succulence, drought tolerance and drought avoidance in annual plants. Each of these is a different but effective suite of adaptations for prospering under conditions that would kill plants from other regions. These differences often extend to the cellular level with the development of special structures to store water in leaves and stems, the periodic shedding of leaves, and special adaptations to even the basic photosynthesis process. Chlorophyll (the green pigment in plants) is the only known substance in the universe that can capture volatile light energy and convert it into a stable form usable for biological processes (chemical energy) through the Calvin Cycle and the enzyme RuBisCO. Green plants use blue and red light energy to combine low-energy molecules (carbon dioxide and water) into high-energy molecules (carbohydrates or starch), which they accumulate and store as energy reserves. There are at least three variations of photosynthesis, all of which use the same basic mechanism, C3 carbon fixation used by most plants, C4 carbon fixation used in about 3% of plants and the CAM (crassulacean acid metabolism) carbon fixation pathway that evolved in plants like cactus as an adaptation to arid conditions.
Sometimes, looking for plants and flowers in winter can be interesting, particularly near a source of fresh water in the desert. In November, I visited Rogers and Blue Point Springs on the north shore of Lake Mead in the Lake Mead National Recreation Area. Rogers Spring and other springs in the “North Shore Complex” comprise one of the terminal discharge areas for the regional carbonate-rock aquifer system of eastern Nevada and western Utah. The source of the water to this spring and other regional carbonate-rock aquifer springs is uncertain. The prevailing theory suggests that much of the recharge water that enters the carbonate-rock aquifer occurs in the high mountain ranges around Ely, Nevada, located 250 miles north of Lake Mead. As this ground water flows south through the carbonate rocks, it encounters several faults along the way, including the Rogers Spring Fault, which has caused the older carbonate rocks (primarily limestone and dolomite) to be displaced against younger evaporite deposits of the Muddy Creek and Horse Spring formations. Here, the lower permeability of these evaporite deposits, along with high subsurface water pressure, forces the ground water in the carbonate rocks to flow upward along the fault and emerge at the surface as Rogers Spring.
The Mojave desert surrounds Las Vegas and extends almost all the way to Los Angeles. Even though most visitors to Las Vegas are interested in gambling, restaurants and shows, there is a beautiful natural world just outside the city limits. The Mojave desert is named after the Mojave Indians who met explorer Father Francis Garces in 1776 after he successfully crossed the Mojave desert. Mojave tribal peoples were concentrated along the Colorado River and the Mojave trail was their main trading route. Other intrepid explorers would follow Garces, including Jedediah Smith in 1826 and John Fremont in 1844.