Vigorous vegetative growth of CO2 enriched plants were observed but the photosynthetic rate didn't show high photosynthetic rates. The plant was given a high CO2 level for more than 40 months. Kindly please advise.
Long-term CO2 enrichment leads to photosynthetic acclimation where you will generally notice an increase in crop performance but no further increase in the rate of photosynthesis that could be ascribed to the down-regulation of Rubisco activity. Usually elevated CO2 improves nitrogen use efficiency, decreases water use at both the leaf and canopy scale and stimulates dark respiration via a transcriptional reprogramming of metabolism. However CO2 enrichment does not directly stimulate C4 photosynthesis, but can indirectly stimulate growth at times and places of drought.
True in lab experiments a down regulation of photosynthesis is observed but in the field this phenomena is often not seen at all. e measured photosynthesis on larch trees, which were exposed to elevated CO2 (ca. 550 ppm) at 2140 m a.s.l. and found no response to elevated CO2. We are in the process of publishing this data. A similar result was found in Bader, Siegwolf & Körner in Planta, 232: 1115-1125; 2010, for several deciduous tree species. There could be species specific differences, but the contrasting in responses between lab and field is hard to explain.
Accumulation of photosynthetic products with prolonged CO2 supply may reduce photosynthesis by metabolite dependent inhibition.
This is one explanation. Another could be the use and gradual shortage of essential nutrients like N or P. Still it is not fully clear, why we can find the down regulation in well fertilized lab plants, whereas in the field often no response can be observed at any time. Yet there are numerous valid assumptions.
But so much can be said that in cold environments the transformation from carbohydrates to biomass is drastically slowed down, which could reduce/deactivate any regulation on the CO2 uptake.
From our experiments we could observe that 1. Among different crop plants pulse crops are more responsive as nitrogen availability is not a limiting.2. Among the C4 cereals like sorghum it was observed that genotypes with tillers and and bigger cob size or crop under moisture stress showed higher response to elevated CO2. 3. The response to enhanced CO2 is crop specific and within a crop again it is genotype specific. Indeterminate nature is promoted by elevated CO2 in many crops due to availability of more photosynthates to form extra pods/seeds. 4. We also observed enhanced photosynthetic rates under elevated CO2 levels with decrease in leaf protein content. It can be concluded that as with any other treatment, when we have to assess the response of elevated CO2, one should take enough care that the plant or crop is not encountering any other stress like light, nutrition, moisture etc.
Photosynthetic acclimation to high CO2 levels is a complex phenomenon indeed. There are many factors to consider: plant type, nutrition level, ontogeny, accumulation of carbohydrates, etc. We tried to unravel this from a phytohormone perspective. You may wish to consider this hormonal factor affecting oil palm photosynthesis? Do read on: https://www.researchgate.net/publication/12296099_Effects_of_elevated_CO%282%29_and_nitrogen_nutrition_on_cytokinins_in_the_xylem_sap_and_leaves_of_cotton?ev=prf_pub
Article Effects of Elevated [CO2] and Nitrogen Nutrition on Cytokini...
For C3 plants, there can be acclimation. In seedling studies with pots it was often due to pot binding and/or nutrient limitations. In FACE studies some acclimation is observed and might be due to progressive nutrient limitations or feedback inhibition due to increased foliar starch concnetrations.. Thus when Pn is measured under ambient CO2 conditions Pn can be lower for elevated CO2 plants than ambient CO2 plants. However, it is very rare that when Pn is measured under elevated conditions that Pn is actually lower for elevated CO2 grown plants. If the latter happened, I have to think it is likely due to some major confounding factor in the experiment such as taking plants grown under ambient CO2 in pots where root proliferation is restricted, subjecting them to a large step incrrease in CO2 growth concentration and not providing proper nutrition to maintain critical nutrient levels as growth rate increases.
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