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

The possibility that an enhanced supply of dissolved inorganic carbon (DIC = CO2 + HCO3-) to the root solution could increase the growth of Lycopersic on esculentum (L,) Mill. cv, F144 was investigated under both saline and no n-saline root medium conditions, Tomato seedlings were grown in hydroponic culture with and without NaCl and the root solution was aerated with CO2 co ncentrations in the range between 0 and 5000 mu mol mol(-1). The biomass of both control and salinity-stressed plants grown at high temperatures (dail y maximum of 37 degrees C) and an irradiance of 1500 mu mol m(-2) s(-1) was increased by up to 200% by enriched rhizosphere DIG. The growth rates of p lants grown with irradiances of less than 1000 mu mol m(-2) s(-1) were incr eased by elevated rhizosphere DIC concentrations only when grown at high sh oot temperatures (35 degrees C) or with salinity (28 degrees C), At high li ght intensities, the photosynthetic rate, the CO2 and light-saturated photo synthetic rate (J(max)) and the stomatal conductance of plants grown at hig h light intensity were lower in plants supplied with enriched compared to a mbient DIG. This was interpreted as 'down-regulation' of the photosynthetic system in plants supplied with elevated DIG. Labelled organic carbon in th e xylem sap derived from root (DIC)-C-14 incorporation was found to be suff icient 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-D IC, respectively. It was concluded that organic carbon derived from DIC inc orporation and translocated in the xylem from the root to the shoot may pro vide a source of carbon for the shoots, especially under conditions where l ow stomatal conductance may be advantageous, such as salinity stress, high shoot temperatures and high light intensities.