Downward Regulation of Photosynthesis and Growth at High CO₂ Levels

Abstract

Numerous photosynthesis and growth measurements of sour orange (Citrus aurantium L.) trees maintained in ambient air and air enriched with an extra 300 microliters per liter of CO2 have revealed the C02-enriched trees to have consistently sequestered approximately 2.8 times more carbon than the control trees over a period of three full years. Under field conditions in the natural environment, plants may not experience the downward regulation of photosynthetic capacity typically observed in long-term CO2 enrichment experiments with plants growing in pots.Atmospheric CO2 enrichment generally enhances photosynthesis and plant growth rates (14). In many instances, however, the initial stimulation gradually erodes with time (5, 6). This downward regulation is so ubiquitous that Mott (17) has concluded that sustained high growth and photosynthetic rates may not be possible at high CO2 concentrations, or that they are actually inhibited under such conditions, but "for quite unknown reasons." Hence, in an attempt to probe deeper into this subject, and perhaps to discover such reasons, we initiated a long-term study of CO2 enrichment effects on trees. This communication summarizes the first 3 years of our results. MATERIALS AND METHODSEight small sour orange tree (Citrus aurantium L.) seedlings were planted directly into the ground at Phoenix, AZ, in July of 1987 and surrounded in pairs by transparent walls of clear plastic film (10) treatment has been maintained without interruption to the present day.Since the experiment's inception, all trees have been manually flood-irrigated at intervals deemed appropriate to preclude the development of significant water stresses. In addition, they have been supplied with sufficient amounts of a commercial citrus tree fertilizer to preclude significant nutrient stresses.No physiological measurements were made on the trees throughout the first year of the experiment. On 12 clear days during the summers ofthe second and third years ofthe study, however, daylight measurements of leaf net photosynthesis were made at 1.5-h intervals by means of a Li-Cor LI-6200 portable photosynthesis system. And from the very start of the experiment, we have made one simple growth measurement at the midpoint of each month: trunk cross-sectional area at a height of 45 cm above the ground. This was accomplished by wrapping a flexible wire around the trees' trunks, measuring its length (the trunk's circumference), and converting that measurement to cross-sectional area. RESULTSThroughout the second summer of the study, the mean daylight net photosynthetic rate of the leaves of the CO2 enriched trees averaged 2.22 times that of the leaves of the ambient-treatment trees (1 1). Throughout the third summer of the study, this ratio was nearly identical: 2.23. Hence, between years 2 and 3, there was no sign of any downward regulation of photosynthetic capacity in the C02-enriched trees.Trunk cross-sectional area data for the active growth seasons of all 3 years of the experiment are presented in Figure ...

Modifications in Photosynthetic Pigments and Chlorophyll Fluorescence in 20-Year-Old Pine Trees after a Four-Year Exposure to Carbon Dioxide and Temperature Elevation

“…1). This gives some support to earlier finding that, even in a resource-limited environment, trees are capable of fixing more carbon in a doubled ambient [CO 2 ] environment, at least during a few years following the initiation of [CO 2 ] treatment (Idso and Kimball 1991, Marek et al 1997, Wang and Kellomäki 1997. The maintenance of high photosynthetic capacity in the Scots pine after four years of exposure may be related to the CO 2 -induced enhancement in the active sink such as leaf area and root absorption (Wang andKellomäki 1997, Kellomäki andWang 2001).…”

Estimation of Mesophyll Conductance to CO2 Flux by Three Different Methods

“…Because of the extremely low gm of C. aurantium (see also ref. 9), we would expect that this plant would exhibit more response to elevated C02, and this has been reported (8). However, this is probably not a good plant from which to generalize about CO2 responses (8) given that its gm can be so low.…”

The growth and gas exchange response of soil‐planted Norway spruce [Picea abies (L.) Karst.] and red oak (Quercus rubra L.) exposed to elevated CO2 and to naturally occurring drought

“…Wolfe (1994) has discussed how when plants are grown in the field with unrestricted rooting volume, the increased photosynthetic capacity is more likely to be maintained. Gunderson et al (1993) found no downward trend in photosynthesis throughout three field seasons in soil-planted white oak and yellow poplar, and Idso & Kimball (1991) working on soilrooted sour orange trees found no evidence of downward regulation of photosynthesis and growth after 3 yr exposure to elevated COg. In our experiment, the relative photosynthesis increase was gradually lost as drought developed (Fig.…”