The effect of rhizosphere dissolved inorganic carbon on gas exchange characteristics and growth rates of tomato seedlings

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Abstract
The possibility that an enhanced supply of dissolved inorganic carbon (DIC = CO2 + HCO−3) to the root solution could increase the growth of Lycopersicon esculentum (L.) Mill. cv. F144 was investigated under both saline and non-saline root medium conditions. Tomato seedlings were grown in hydroponic culture with and without NaCl and the root solution was aerated with CO2 concentrations in the range between 0 and 5000 µmol mol−1. The biomass of both control and salinity-stressed plants grown at high temperatures (daily maximum of 37 °C) and an irradiance of 1500 µmol m−2 s−1 was increased by up to 200% by enriched rhizosphere DIC. The growth rates of plants grown with irradiances of less than 1000 µmol m−2s−1 were increased by elevated rhizosphere DIC concentrations only when grown at high shoot temperatures (35 °C) or with salinity (28 °C). At high light intensities, the photosynthetic rate, the CO2 and light-saturated photosynthetic rate (Jmax) and the stomatal conductance of plants grown at high light intensity were lower in plants supplied with enriched compared to ambient DIC. This was interpreted as ‘down-regulation’ of the photosynthetic system in plants supplied with elevated DIC. Labelled organic carbon in the xylem sap derived from root DI14C incorporation was found to be sufficient to deliver carbon to the shoot at rates equivalent to 1% and 10% of the photosynthetic rate of the plants supplied with ambient- and enriched-DIC, respectively. It was concluded that organic carbon derived from DIC incorporation and translocated in the xylem from the root to the shoot may provide a source of carbon for the shoots, especially under conditions where low stomatal conductance may be advantageous, such as salinity stress, high shoot temperatures and high light intensities.

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