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Some effects of supplemental carbon dioxide on the physiology of plant growth and development Hicklenton, Peter R.


This research was concerned with some physiological effects of supra-normal CO₂ concentrations on cucumber, tomato and Japanese Morning Glory (Pharbitis nil), and with measurement of CO₂ levels in a commercial greenhouse. Measurements of CO₂ concentrations in a cucumber greenhouse showed that, in the early stages of crop development, early morning CO₂ levels reached 0.18% as a result of straw decomposition in the plant beds. Later in crop development, daytime levels were much lower and required gas combustion to restore high concentrations. Stomatal resistances in cucumber leaves were relatively insensitive to high greenhouse concentrations. Variation in stomatal resistance through the crop canopy was, however, detected. Generally, the two most recently developed leaves showed higher resistances than those of a slightly greater physiological age. Differences in leaf irradiance could not fully explain this effect, which may be related to the stage of leaf development. Subsequent experiments on greenhouse tomato crops showed that CO₂-enriched (0.09% CO₂) plants flowered earlier and produced 30% more fruit than those grown in normal air. Photosynthetic rates were inherently higher in apical and basal leaves developed under CO₂enrichment at irradiances above 50 m⁻²s⁻¹ .Behavioral indicies of photosynthetic efficiency indicated an enhanced capacity to utilize CO₂ in enriched plants. Measurements of CO₂ exchange in leaves of plants grown in chambers at 3 CO₂ concentrations (0.03, 0.1 and 0.5%) confirmed the enhancement of inherent photosynthetic rates in young leaves of 0.1% grown plants. Reduced rates of photorespiration, total O₂ inhibition of photosynthesis, glycolate oxidase (GaO) activity, and an increased rate of ribulose-biphosphate-carboxylase (RuBP-case) activity, contributed to this enhancement. Maximum photosynthetic rates in young leaves developed at 0.5% CO₂ were similar to those developed in 0.03% CO₂. Growth rates of the 0.1% CO₂-grown plants were higher than the similar rates of plants from the 0.03 and 0.5% regimes. Apparently maximum benefit from CO₂ enrichment is achieved by maintaining atmospheric CO₂ concentrations close to 0.1%. At a later stage of development, however, GaO and RuBP-case activities were similar in the 0.03 and 0.1% CO₂-grown plants and photosynthetic rates did not differ between growth regimes. Observations on the effects of 0.03, 0.1, 1.0 and 5.0% CO₂ on development in the Short-Day Plant Pharbitis nil revealed that 1.0 and 5.0% CO₂ modified normal flowering. These concentrations induced a weak flowering response in Long-Days and Short-Days and promoted stem elongation and leaf production under both photoperiods. These modifications were apparently unrelated to patterns of CO₂ exchange which showed a relatively small increase above 0.5% CO₂. These results are discussed in relation to possible mechanisms for the effects of supra-normal CO₂ concentrations on development. The diversity of physiological effects mediated by CO₂, and their relationship to one another are discussed.

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