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.