| Description |
Two stable isotopes of carbon exist in nature, 12c and 13C. Their physical and chemical properties differ due to slight mass differences. Because of these slight differences, the heavier 13C isotope is discriminated against in the photosynthetic pathway. Stable carbon isotope compositions obtained from plant tissue material are an effective tool in differentiating among plants from the three photosynthetic types (C3, C4, and CAM) since each type discriminates to a different degree. Isotope ratios also vary within each of these ranges according to the photosynthetic efficiency. Plants under greater stress have a decreased photosynthetic efficiency and generally show more positive carbon isotope ratios. However, a paradox has been discovered, in that the general trend is reversed among plants utilizing C4 photosynthesis. It has been proposed this reversal of the trend is due to leakage of CO2 from the bundle sheath cells back into the mesophyll cells of C4 plants. This leakiness of the bundle sheath allows for greater; discrimination against 13C thereby accounting for the more negative carbon isotope ratios. Leakiness of the bundle sheath increases with increasing stress, such as salinity or drought. It is proposed that salts in the soil can hinder the plants ability to uptake water or can destroy the physical integrity of the; photosynthetic mechanism if in contact with photosynthetic cells. Halophytic, C4 plant species, such as Atriplex canescens, are better adapted to surviving in saline environments because they accumulate salts in their leaves. This study attempts to show whether salts in the soil hinder uptake of water or cause damage to the photosynthetic mechanism. Thirty Atriplex canescens were grown along two different; experimental conditions: a salinity and a drought gradient. Pre- and post - experimental leaf samples under each experimental condition were tested; for their carbon isotope composition. A mathematical relationship has been developed between the carbon isotope composition and the leakiness of CO2 from the bundle sheath cells allowing for the calculation of leakiness. We found a significant difference in leakiness values among the plants along the salt gradient, with plants under the greatest stress treatment being the leakiest. There was no significant difference in leakiness among; plants along the drought gradient. These results suggest that leakiness of the bundle sheath of plants grown in saline soils is caused by a degradation of the cellular mechanism involved in photosynthesis and not in the plants decreased ability to uptake water. |
