Submitted to: Physiologia Plantarum
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 13, 2004
Publication Date: February 18, 2005
Citation: Sicher Jr, R.C. 2005. Interactive effects of inorganic phosphate nutrition and carbon dioxide enrichment on assimilate partitioning in barley roots. Physiologia Plantarum. 123:219-226. Interpretive Summary: Atmospheric carbon dioxide concentrations are rising and this may affect agricultural productivity and possibly the earth's climate in the near future. Most crop plants are bigger and attain greater total biomass when grown with enhanced carbon dioxide. However, the total amount of nitrogen in plants tends to be equivalent in ambient and elevated carbon dioxide treatments. The finding that total nitrogen and total biomass were not increased commensurately by carbon dioxide enrichment was the subject of the current study. Experiments were performed using barley seedlings grown in controlled environment chambers at ambient and elevated carbon dioxide. Rates of plant growth and photosynthesis were accelerated by carbon dioxide enrichment. This resulted in increased soluble carbohydrates in roots, although the inorganic nitrogen content of roots was unaffected by the carbon dioxide treatment. The organic nitrogen content of roots was increased by carbon dioxide enrichment and this was largely due to changes in the amino acid, glutamine, which is an important component of protein synthesis. The above results indicated that roots of barley seedlings have the ability to acquire and assimilate large quantities of nitrogen when the mineral nutrient supply was not limiting for growth. Differences of total nitrogen between treatments were likely the result of altered nitrogen metabolism in the shoot. This conclusion may be relevant to understanding the loss of photosynthetic capacity that occurs in carbon dioxide enriched plants. These results should be of interest to environmental scientists, government planners and crop modelers.
Technical Abstract: Metabolite levels and sucrose hydrolyzing enzyme activities were determined between 9- and 17-days after sowing using roots of barley seedlings (Hordeum vulgare L. cv. Brant) with ambient (36 Pa) and elevated (100 Pa) CO2 treatments in controlled environment chambers. Plant growth in elevated CO2 enhanced total dry matter on the final measurement date by 31% (P < 0.01). The shoot to root ratio also decreased from 0.36 to 0.30 (P < 0.05), respectively, when seedlings from the ambient and elevated CO2 treatments were compared. In contrast to starch, the principal root soluble carbohydrates, sucrose, glucose and fructose, were all 26 to 35% greater in the elevated than in the ambient CO2 treatment. Although soluble carbohydrate pools in barley roots varied diurnally, effects of growth in enhanced CO2 were consistent throughout the photoperiod. Glutamate, soluble protein, NO3-, and NH4+ levels in roots were all unaffected (P > 0.05) by plant growth in enriched CO2. However, glutamine concentrations were 11 to 28% greater (P < 0.01) in roots from the elevated compared to the ambient CO2 treatment. Moreover the glutamine/glutamate ratio rose from 3.8 to 4.5 (P < 0.01) in response to CO2 enrichment. Glutamate and to a lesser extent glutamine levels in barley roots increased up to 25% (P < 0.05) during the first three hours of the photoperiod. However, diurnal changes of NO3- and NH4+ concentrations were not evident. Soluble acid invertase was the principal sucrolytic enzyme in roots of barley seedlings. In spite of the increased carbohydrate levels in roots between 9 and 17 days after sowing, the activity of this enzyme was 12% less (P < 0.01) in the elevated compared to the ambient CO2 treatment. Insoluble acid invertase and sucrose synthase activities were comparatively low in roots of barley seedlings and these enzymes were unaffected by the CO2 treatment (P > 0.05). Neutral invertase activity was not detected. The above results showed that growth in elevated CO2 increased the flux of carbohydrates to roots and that under N sufficient conditions this stimulated the synthesis of specific amino acids. The activities of enzymes involved in sucrose hydrolysis were not increased commensurate with enhanced carbohydrate levels. This finding implied that the control of C flux into glycolysis occurred downstream from sucrose cleavage.