Effect of elevated CO2 on Two Varieties of Guar (Cyamopsis tetragonoloba) Plants
By: Sonali Mehrotra1*, Ashish Praveen1, K. P. Tripathi2 and Nandita Singh1
According to the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2014), global warming has occurred in the recent past and will continue during the 21st century. Associated with the accumulation of greenhouse gases, climate change is expected to decisively affect agricultural production. Global atmospheric CO2 concentration is likely to surpass 550 ppm later in this century (IPCC, 2014), up from current concentrations of just above 400 ppm resulting in a greater rate of global climate change. Many climate change scenarios suggest global warming and altered precipitation patterns are anticipated to cause severe drought events in some regions. Global climate model predicts increased frequency and intensity of such drought episodes in a water-limited dry environment which will pose a significant challenge on crop production in future.
The world population is estimated to reach 8.3 billion by the year 2030, therefore, the demand for food grains is expected to increase by 50% globally. To feed such a population globally considerable stress will be imposed on increasing crop productivity due to combination of factors, like limited agricultural land, resource constraint of water and nutrients and rapid global environmental change. To ensure food security globally in the past and present times various assumptions were made and numerous experiments were designed to see the possible effects of increased carbon dioxide on the plant growth, photosynthetic rate and nitrate uptake and nitrogen metabolism.
Elevated CO2 (e[CO2]) by itself stimulates growth and yield of C3 crops through increased photosynthetic rate, which together with decreasing stomatal conductance (gs) leads to greater leaf-level water use efficiency (WUE). As e[CO2] improves plant WUE, the relative increase in grain yields under e[CO2] may be greater with decreasing water availability. Therefore, e[CO2] may provide a particular yield advantage in water-limited environments by reducing water use. Among C3 crops, grain legumes play a critical role in cropping systems because of their potential to improve soil fertility and cheap source of protein. Grain legume production in rainfed environments is mostly affected by the terminal drought.
Increased biomass and yield under e[CO2] are often associated with decreases in tissue [N] and grain protein concentration, leading to concerns about the nutritional quality of food crops. Several mechanisms may explain the decrease of [N] under e[CO2]. Carbon (C) and nitrogen (N) are the key structural elements for plant growth and constitute 45 and 5% of plant dry matter, respectively. Studies have reported drastic changes in nitrogen concentration in plants grown under elevated condition which indicate physiological and morphological changes occurring in plant. For example, limited nitrogen availability alters C:N ratio and it leads to nitrogen limitation as leaves acquire faster carbohydrates than nitrogen, that leads to decrease in nitrogen content in leaves. Studies have reported that under enhanced carbon dioxide concentration overall content of plant nitrogen decreases. This decline in the nitrogen in the plant is the indication that the restricted rate of nitrate translocation, its assimilation and photosynthesis occurs and differs greatly. The dilution hypothesis suggests that soil N supply or N uptake fail to keep up with the increased N demand of e[CO2]-stimulated biomass growth. It is found that effect of elevated carbon dioxide concentrations on photosynthetic activity and growth depends on total nitrogen available in plants which indirectly depends on soil nitrogen content. In legumes, the decrease in seed protein concentration under e[CO2], however, is less than in cereals or even absent, because e[CO2]-induced stimulation of N2 fixation may reduce the dependence of legumes on soil N re-sources. When N2 fixation and N uptake are constrained, seed N in legumes becomes dependent on the remobilization of previously assimilated N from vegetative tissues. However, remobilization of N to the grains has been shown to be modified by environmental factors, such as e[CO2].
India is the world largest producer of pulses. Highest being in Madhya Pradesh (23%) followed by Uttar Pradesh (18%) and Maharashtra (14%). Yet we have to meet increasing population demand. Pulses (grain legume) are part of daily intake for humans and animals, as they are rich in protein and fibers. Guar (Cyamopsis tetragonoloba L.) is a drought-tolerant legume characterised by a spring-summer life cycle, which has many uses such as human nutrition, animal fodder and industrial purposes. India produces approximately 80% of the world’s guar, followed by Pakistan (15%); the remaining 5% is produced in USA, Australia and South Africa. From guar seeds, two semi-transformed products are obtained: guar splits with a yield of 27-35%, obtained from endosperm, and guar meal, obtained from teguments and embryos which represent 65-73% of the processed seed. This latter is largely made up of proteins (more than 42%) and is used as a nutritional supplement in livestock feed, especially poultry. Guar splits are ground to produce guar gum, a high added value product, made up mainly of galactomannans, which are 75-85% of the endosperm. Worldwide, guar gum is used in the food industry as a thickener and stabiliser, as well as in many other industries, including the chemical, pharmaceutical, cosmetics, construction, textile, explosives, and paper industries. In recent years the price of guar seed has increased considerably, because of the exponential growth of demand for its use in hydraulic fracturing. Thus due to such unique features it is interesting to considered it as a model plant for studying the consequences of carbon dioxide elevation effect on plant growth, photosynthetic rate, nitrate uptake and nitrogen metabolism and predict its outcome on production (yield), seed weight, seed quality and biomass production.
Free Air Carbon dioxide Enrichment System (FACE) experiments have been conducted in several geographical locations around the world to estimate, under the most realistic agricultural conditions possible, the impact of the carbon dioxide levels projected for the middle of this century on crops species. The experiment was conducted in the FACE setup located in CSIR-National Botanical Research Institute, Lucknow, India. Seeds of Cyamopsis tetragonoloba two varieties RGC 1002 and RGC 1066 were taken from the belt of Rajasthan district of Jodhpur, Jaiselmer and Barmer semiarid areas. The two varieties RGC 1002 and RGC 1066 were grown under elevated carbon dioxide (490 ppm) and ambient carbon dioxide (300 ppm) concentration for 160 days and different growth parameters were determined. Plant yield and nutritional quality of seed is of utmost importance for leguminous crop plant. The present experimental study demonstrated that under CO2 elevation the overall biomass of the plant increased significantly, which suggest that fodder production increases 25–35% annually per unit area when compared with ambient grown plant varieties. Nitrogen reduction under elevation seems to affect the nutritional quality of leaf thus affecting seed quality and production. RGC 1002 variety was found to be more tolerant to elevated carbon dioxide concentration than RGC 1066 plant variety. Moreover, the C : N ratio increases showing a reduction in nitrogen content and increase in carbon content on mass basis in leaves which causes reduction in protein content leading to detoriation in the nutritional quality of seed.
1Plant physiology and Environmental Science Dvision, CSIR-National Botanical Research Institute, Rana Pratap Marg, Lucknow, 226001, U.P, India
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2Botany department, Dolphin (PG) Institute, Manduwala, Uttrakhand Technical University, Dehradun, India