This is the final post in my series on photosynthesis in Mohave desert plants. While green leaves are commonly considered as the primary sources of photosynthesis, higher plants can potentially use almost all vegetative and reproductive structures to perform carbon fixation. Chlorophyll-containing bark and wood tissue, most fruit, root and fertile flower organs typically perform an effective internal CO2 recycling using the respiratory released CO2. Photosynthetic stems have positive effects on the carbon economy of plants through two main mechanisms. Photosynthetic stems can either assimilate atmospheric CO2 and contribute to the net carbon gain of the plant through the process of stem net photosynthesis, or decrease respiratory losses by recycling CO2 previously respired by roots and stems through the process of stem recycling photosynthesis.Plants with green stems are categorized by one of three types of stem photosynthesis syndromes. Retamoids include leafless or almost leafless woody plants that have stomata in the stem epidermis allowing for gas exchange with the atmosphere (Schaedle 1975). Another type of photosynthetic stems are the succulent pachycauls or the sarcocaulescent group, which has large-sized stems with translucent exfoliating bark, a large amount of parenchymatous tissue that serves as a water reservoir, and non-succulent, drought-deciduous leaves (Franco-Vizcaino et al.1990). An example of this would be the Boojum Tree (Fouquieria columnaris) and the related Ocotillo (Fouquieria splendens) as both lack stomata and recycle respiratory released CO2 to survive during drought. Another type of photosynthetic stem is found in cactus, which take up carbon dioxide at night with stoma using CAM photosynthesis. The inadequacy of the current state of knowledge for describing or understanding the diversity of structure, function, and ecological significance of photosynthetic stems suggest areas for further research. The diversity of taxonomic types, and classes of photosynthetic stems, results in a large diversity of structural characteristics. In this post, I have simply chosen interesting examples without resolving these difficulties. I have also decided to throw caution to the wind and use lots of technical jargon.
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.