The abundance of transcripts of cab‐7 and cab‐3C, which code for the chlorophyll a/b binding proteins of the light‐harvesting complexes I and II, respectively, and the abundance of transcripts of Rca, which encodes Rubisco activase, were reduced in tomato plants exposed to high CO2 for up to 9d, whereas the abundance of mRNA from psa A–psa B and psb A, which encode the proteins of the core complex of PSI and the D1 protein of PSII, respectively, and the abundance of glycolate oxidase, which is involved in photorespiration, were not affected. However, the abundance of the transcript for the B subunit of ADP‐glucose pyrophosphorylase was increased after 1 d at elevated CO2. The chlorophyll a/b ratio decreased significantly over 9 d of exposure to elevated CO2. The responses of the nuclear genes to high CO2 were enhanced when leaves were detached so as to deprive them of any major sink. The responses of these transcripts to high CO2 were mimicked when sucrose or glucose was supplied to the leaf tissue, whereas acetate or sorbitol had no effect. Carbohydrate analyses of leaves grown in high CO2 or supplied with sucrose revealed that major increases occurred in the amount of glucose and fructose. Based on these and other published data, a molecular model involving the repression or activation of the transcription of nuclear genes coding for chloroplast proteins by photosynthetic end‐products is proposed to account for photosynthetic acclimation to high CO2 in tomato plants and other species.

Tomato plants were exposed to four concentrations of CO2 (350, 700, 1050 or 1400 μmol CO2 mol−1) for 31 d. The light‐saturated rate of photosynthesis (A) of the unshaded fifth leaf was measured at either an ambient CO2 concentration of 350 μmol CO2 mol−1 [A (350)] or at the level of CO2 at which the plants were grown. The chloroplast protein composition and the level of transcripts of nuclear or plastid photosynthesis‐associated genes (PAGs), as well as the main carbohydrate content of the fifth leaf maintained horizontal and unshaded, were also measured during leaf development. At 60 and 95% leaf expansion, the A of high‐CO2‐grown plants measured at growth CO2 was higher than the A (350) of the plants grown at ambient CO2. However, in the fully mature leaves, A (growth CO2) declined linearly as growth CO2 concentration increased. The A (350) of plants exposed to elevated CO2 up to 60% leaf expanion had not acclimated to high CO2. At 95% leaf expansion, A (350) was lower in plants grown at high CO2. A versus CO2 (Ci) for mature leaves showed that A of the plants grown at high CO2 was lower over the entire range than that for plants grown at present ambient CO2 concentration. Lines fitted to the linear part of the A/Ci curves were concurrent at a Ci of 49μmol CO2 mol−1 and A=−1.21μmol CO2 m−2s−1. This Ct value is close to Λ* (46 μmol CO2 mol−1), the compensation point at 27°C calculated from the equation described in Brooks & Farquhar (1985, Planta 165, 397–406). This A is an estimate of respiration in the light (R1) and was not affected by acclimation to elevated CO2. Thylakoid proteins (photo‐system I core protein, D1 and D2 of the photosystem II core complex, cytochrome f) were all reduced by elevated CO2 only in the fully mature leaves (31d exposure), whereas the large and small subunits of Rubisco and Rubisco activase proteins had already declined after 22 d exposure. Transcript levels of the plastid‐encoded PAG (rbcL, psbA, psaA‐B) were reduced in the mature leaves by elevated CO2 when expressed on a total RNA basis, but they were not sensitive to elevated CO2 when expressed on a chloroplast 16S rRNA basis. However, rbcS, rca and cab mRNA transcripts were lower in the plants grown at high CO2 than in control plants after 22 d exposure when expressed on a nuclear rRNA basis. The loss of these nuclear PAGs was correlated with an accumulation of soluble sugars and starch.

To predict the future carbon sequestering capacity of trees, we need information about the possible acclimatory mechanisms of plant growth and photosynthesis in rising atmospheric CO(2) under a variety of environmental conditions. We have, therefore, studied the growth response of a tree species (Prunus avium L. Stella (wild cherry)) to elevated CO(2) and characterized the associated changes in photosynthetic machinery of the leaf tissue. Self-pollinated seedlings and mature cuttings (clones) from the same parent plant of P. avium were grown for two consecutive growing seasons (about 60 days each) in ambient CO(2) (350 micro mol mol(-1) CO(2)) or elevated CO(2) (700 micro mol mol(-1) CO(2)) with a high or low nutrient supply. The degree of acclimation of leaf biochemistry and growth response to elevated CO(2) was dependent on the plant material (seedling or mature cutting) and nutrient supply. There was little or no growth response to elevated CO(2) in seedlings or cuttings in the low nutrient supply treatments, whereas, in both seasons, there was a strongly positive growth response to elevated CO(2) in seedlings and cuttings in the high nutrient supply regimes, resulting in increases in the root/shoot ratio and in carbon allocation to the roots. In contrast, the protein content and activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco, EC 4.1.1.39) were down regulated in elevated CO(2). The loss of Rubisco on an area basis in plants in the elevated CO(2) treatments was compensated for at the canopy level by increased leaf area. The loss of Rubisco protein was accompanied by decreases in the contents of chlorophyll and the thylakoid membrane proteins D(1), D(2) and cytochrome f, which are involved in light harvesting and photo-electron transport. We conclude that, in the medium- to long-term, the initial stimulation of biomass production by elevated CO(2) may be increasingly offset by a lower photosynthetic capacity per unit leaf area in perennial plants.

Tomato plants were transferred to diflerent CO,^ mole fraction.s (3.S(), 700, lCSO and 1400//mol CO.^ mol ') 31 d after sowing (2 "" of full expansion) and the light saturated rate of photosynthesis (P",^^) of the unshaded .Sth leaf was measured at either an ambient CO^ mole fraction, C\ of 350/miol CO^ mol " [P^^^^ (350)] or at the mole fraction of CO,^ at which the plants were grown. At 60 "" and 95 "o leaf expansion, P^^^ of high CO^ grown plants measured at growth CO^, was greater than the P^^,, (350) of the ambient CO^ grown plants. However, by leaf maturity, P,,,,,, (growth CO^) declined linearly as growth CO^ concentration increased. P",^,^ (350) of plants exposed to elevated C().^ up to 60'\, leaf expansion had not acclimated to high CO.^. At 95 "" leaf expansion, f^,,,,. (350) was smaller in the high COj grown plants. Z^^^,, (350) was predicted from Rubisco in vitro carboxylation capacity using tomato Rubisco kinetic constants. By 95% leaf expansion, high CO.^ grown plants showed similarities to the response of plants to low nitrogen supply, in terms of Rubisco and chlorophyll content. The observed and theoretical relationships between the initial slopes of the P,,,^^/C^ responses and Rubisco activity were statistically equivalent. Both short-term and long-term effects of elevated CO^ on dark respiration (/?") were also investigated at two stages of leaf development (50 and 100 "o expansion). /?" (growth CO.^) was smaller for the high CO^ grown plants compared with the control plants, whereas /«" (350) was either equal or greater for the plants grown in high CO^.

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