Tuesday, 11 July 2006

Effect of Heterosis on Rhizodeposition in Zea Mays L.

Tanja Mimmo, Luciano Cavani, and Maria Angela Cané. Department of Agroenvironmental Sciences and Technologies, Alma Mater Studiorum – University of Bologna, Viale Fanin 40, Bologna, 40127, Italy

The phenomenon of heterosis is the superiority of the heterozygote compared to the parental homozygotes. In other words, the performance of the hybrid is higher than that of both its parents. Even if the heterosis effect is more visible in the aerial parts of the plants it seems very likely that heterosis influences also the rhizosphere and thus the rhizodepositions. The objective of the present work was to investigate the quantity and quality of rhizodeposits of different pure lines and hybrids Zea mays L. The field trial was carried out at the experimental farm of the University of Bologna at Cadriano (near to Bologna, Northern-Italy). Two couples of pure lines (Lo1016 and Lo964, B73 and H99) and their related hybrids were sowed following a randomized block design. Plants were harvested after 40 days. Roots were first shaken mechanically and rhizosphere soil was sampled consequently by brushing. Chemical and physical characteristics of bulk and rhizosphere soil were determined (organic carbon, total nitrogen, acid phosphates, organic acids, bioavailable phosphate). In addition, root-derived rhizodeposits were discriminated against microbial-derived ones using a continuous flow isotopic ratio mass spectrometer (CF-IRMS). The method is based on the different natural abundance of the stable 13C isotope in C4 plants. The different isotopic signature permits to follow the carbon flux of a C4 plant quantifying and discriminating between root or microbial- derived carbon. Results showed that as expected both hybrids, Lo1016xLo964 and B73xH99, changed their root morphology increasing their root surface. Organic carbon and total nitrogen of rhizosphere soil showed no significant differences between hybrids and pure lines. However, rhizodeposition was much higher in the rhizosphere as the d13C was less negative in rhizosphere samples than in bulk soil samples. The determination of biological activities such as acid phosphatases pointed out some significant differences only between the two parental pure lines Lo1016 and Lo964 and their hybrid Lo1016xLo964: the enzymatic activity was much higher in Lo1016 than Lo964 and Lo1016xLo964. Alkaline phosphatases did not show any significant differences. Phosphate availability was measured as P-Olsen and indicated that the hybrid is significantly 'better' than both its parental pure lines or at least as good as the best parental pure line. In conclusion, the hybrid seems to be more efficient at the rhizospheric level. In other words, the quantity of rhizodeposits of the hybrid does not seem to differ very much from the related pure lines whereas the quality differs significantly. For instance, the hybrids Lo1016xLo964 and B73xH99 would induce a higher phosphorous availability increasing the nutrient uptake. Acknowledgement: This project was funded by the Department of Agroenvironmental Sciences and Technologies, Alma Mater Studiorum - University of Bologna, Italy (project °¥DISTA per i giovani').

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