The response of nodulated alfalfa to water supply, temperature and elevated CO₂: photosynthetic downregulation

Abstract

Plants grown in an environment of elevated CO2 and temperature often show reduced CO2 assimilation capacity, providing evidence of photosynthetic downregulation. The aim of this study was to analyse the downregulation of photosynthesis in elevated CO2 (700 µmol mol−1) in nodulated alfalfa plants grown at different temperatures (ambient and ambient + 4°C) and water availability regimes in temperature gradient tunnels. When the measurements were taken in growth conditions, a combination of elevated CO2 and temperature enhanced the photosynthetic rate; however, when they were carried out at the same CO2 concentration (350 and 700 µmol mol−1), elevated CO2 induced photosynthetic downregulation, regardless of temperature and drought. Intercellular CO2 concentration measurements revealed that photosynthetic acclimation could not be accounted for by stomatal limitations. Downregulation of plants grown in elevated CO2 was a consequence of decreased carboxylation efficiency as a result of reduced rubisco activity and protein content; in plants grown at ambient temperature, downregulation was also induced by decreased quantum efficiency. The decrease in rubisco activity was associated with carbohydrate accumulation and depleted nitrogen availability. The root nodules were not sufficiently effective to balance the source–sink relation in elevated CO2 treatments and to provide the required nitrogen to counteract photosynthetic acclimation.

Carbon balance, partitioning and photosynthetic acclimation in fruit-bearing grapevine (Vitis vinifera L. cv. Tempranillo) grown under simulated climate change (elevated CO2, elevated temperature and moderate drought) scenarios in temperature gradient greenhouses

“…2) and decreased photosynthesis measured either at 375 or 700 ppm 7 CO 2 (Fig. 3), in line with previous reports (Aranjuelo et al, 2005b; Del Pozo et al, (Fig. 4), supporting the hypothesis of impaired…”

Elevated CO2 and water-availability effect on gas exchange and nodule development in N2-fixing alfalfa plants

“…Absence of statistical differences in the photosynthetic, leaf conductance and intercellular CO 2 concentration rates (Table 2) confirmed that WD plants adapted their growth to available soil water content without suffering stressful growth conditions. Similar responses have been described in alfalfa (Aranjuelo et al, 2005b;Erice et al, 2006) and in wheat (Cabrera-Bosquet et al, 2007) exposed to long-term water limiting regimes.…”

“…TSS data (Table 3) confirmed that plants exposed to elevated CO 2 , especially in WD plants, had a massive production and build-up of carbohydrates. Carbon sink/source imbalance induced photosynthetic acclimation caused by depletions in the expression of Calvin cycle enzymes (Moore et al, 1999;Stitt and Krapp, 1999;Aranjuelo et al, 2005b;Reich et al, 2006;Rogers et al, 2006). However, it should also be noted that all the enzymes involved in the photosynthetic processes may not be equally affected by enhanced CO 2 (Moore et al, 1999).…”

“…Although the initial stimulation of net photosynthesis associated with elevated CO 2 is sometimes retained during long-term exposure (Gunderson and Wullschleger, 1994), it is often partially reversed in an acclimation process, often referred as "downregulation" (Long et al, 2004;Aranjuelo et al, 2005b;Ainsworth and Rogers, 2007). Down-regulation is usually accompanied by alterations in the gas-exchange characteristics that are indicative of a decreased carboxylation capacity (Ainsworth and Long, 2005;Ainsworth and Rogers, 2007).…”

“…Because photosynthesis and biomass production of plants with C 3 photosynthetic metabolism are currently limited by C a , both CO 2 fixation rates and plant growth increase as C a rises (Daepp et al, 2000;Aranjuelo et al, 2005b). An increase in CO 2 concentration from 300-350 to 680 mol mol −1 has been described as enhancing plant growth by 7% to 25% (Hadley et al, 1995;Daepp et al, 2000;Aranjuelo et al, 2006;Erice et al, 2006).…”