S21 - Session O2 - Using δ13C of phloem sap as an indicator for short-term changes in leaf carbon isotope discrimination

Authors: Mathilde Vantyghem *, Roel Merckx, Rebecca Hood-Nowotny, Rony Swennen, Maria Heiling, Christian Resch, Gerd Dercon

Water shortage is likely the most important abiotic limitation for banana. The development of climate-smart varieties and agronomic practices is ongoing, but the process is hindered by a lack of field-applicable drought stress evaluation methods. Our earlier research indicated that carbon isotope discrimination in bulk leaf material (δ 13 C bulk ) can serve as a proxy for drought stress in banana. However, most of the carbon in a leaf is structural and as such, δ 13 C bulk provides information on the stress conditions during leaf development. Information about current conditions could be advantageous for early stress detection and water management. The δ 13 C of phloem sap (δ 13 C phloem ), containing recently assimilated carbon, has successfully been used as a short-term indicator for drought stress in trees. We have developed a phloem sap sampling method for banana and evaluated its usefulness under greenhouse and field conditions. Phloem sap and leaf samples were collected in an open-ground greenhouse, both in the morning and at noon. δ 13 C phloem was found to be significantly higher at noon than in the morning, which was not the case for δ 13 C bulk . The experiment was repeated in the field where samples were taken from irrigated and rainfed plants during the dry season. In accordance with our findings in the greenhouse, δ 13 C phloem was higher at noon than in the morning. This was however only true for the irrigated treatment. Moreover, δ 13 C phloem values in both treatments were not significantly different from one another at noon, while they were in the morning. This indicates that diurnally changing environmental conditions affected the rainfed and irrigated plants differently and that such effects could be detected by changes in δ 13 C phloem . We can thus conclude that δ 13 C phloem is a useful proxy for short-term changes in leaf carbon isotope discrimination and therefore has the potential to serve as an indicator for instantaneous drought stress.