CARBON ISOTOPE DISCRIMINATION ANALYSIS AND SALT TOLERANCE OF RICE GENOTYPES

Authors: Poss, James; Zeng, Linghe; Grieve, Catherine

Submitted to: Cereal Research Communications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/21/2004
Publication Date: 10/20/2004
Citation: Poss, J.A., Zeng, L., Grieve, C.M. 2004. Carbon isotope discrimination analysis and salt tolerance of rice genotypes. Cereal Research Communications. 32(3):339-346.

Interpretive Summary: Carbon dioxide is fixed by plants during the process of photosynthesis. Stable carbon isotopes are carbon atoms stable in time (12C and 13C). About 1% of all carbon atoms are the heavier isotope(13C) and this ratio of natural abundance is constant in atmospheric carbon dioxide. When plants that photosynthesize via a specific metabolic pathway (C3) are subjected to salinity stress the ratio (13C/12C) is altered in the plant tissue. This is because of a slower diffusion of heavy carbon dioxide through plant stomata and preferential fractionation for 12C over 13C during the enzymatic carboxylation reactions of photosynthesis. This is referred to as carbon isotope discrimination or delta. In this study, salinity tolerance of rice was evaluated by ranking the relative decrease in discrimination of 13C of dried seedling tissues at 25 and 33 days after planting. The hypothesis is that high delta values (e. g. maximum discrimination) under salinity stress are positively correlated with increased salt tolerance in rice based on rankings in independent yield evaluations. To examine the mechanism for discrimination effects attributed to salinity tolerance, the two rice genotypes that had the greatest differences in salt tolerance were also followed with gas exchange measurements and discrimination determinations for flag leaf tissues in addition to seedling tissues. Stomatal effects are primarily responsible for changes in delta, however, there was evidence to indicate some effects on enzymatic fractionation. Possible refixation effects due to increased respiration in rice being a significant part of the carbon budget in salt stressed rice caused higher delta than would be predicted with gas exchange. High yielding rice was correlated with high delta in leaves and rankings due to isotopes were similar generally with those of yield rankings.

Technical Abstract: Rice yields are limited by moderate salinity stress. Rice cultivars were evaluated for salt tolerance in a greenhouse based upon differences in stable carbon isotope analysis under salt stress (EC=8 dS/m) and non-stressed (1 dS/m) conditions. Ten seedlings from each replicate and genotype were randomly sampled at 25 or 33 days after planting (DAP). Values for carbon isotope discrimination (delta) were obtained from dried and homogenized seedling tissue of 12 genotypes. Rice genotypes were ranked from low to high salt tolerance based on whole plant (delta) at these two early growth stages. Fractionation effects of salinity stress on (delta) were significantly different among genotypes at 33 DAP, but not at 25 DAP. The (delta) ranking was comparable to rankings developed based upon grain yield of the remaining plants in the study despite poor linear correlation of (delta) with any individual yield component. Two of the genotypes, known to have the largest difference in salt tolerance based on established yield parameters, were also chosen to compare their salt tolerance with isotopic techniques coupled with gas-exchange information obtained from the same population of similarly treated plants. For the 2 rice cultivars evaluated, differences in (delta) due to salinity were smaller than expected when instantaneous gas exchange ratios of leaf internal CO2 (CO2int) and ambient CO2 (CO2amb) were applied to predict leaf (delta). General agreement with leaf tissue (delta) rankings and a final salt tolerance ranking based on grain yield were observed. High (delta) in leaves of salt stressed rice appears to be influenced by an increased contribution of respired carbon to the total carbon budget of rice leaves. This contribution makes the use of stable carbon isotopes for rice salt tolerance studies more complicated than for other C3 species.