Differences in morphological and physiological responses to water-logging between two sympatric oak species (Quercus petraea [Matt.] Liebl., Quercus robur L.)

Pedunculate (Quercus robur L.) and sessile (Q. petraea [Matt.] Liebl.) oaks are known to display different ecological requirements, particularly relative to root hypoxia induced by water-logging. Q. robur is more tolerant to hypoxia than Q. petraea. We designed an experiment aiming at identifying morphological and physiological responses to root hypoxia that might differ between the two species. Potted seedlings were submitted during seven weeks to a water-logging treatment with 02 concentrations below 3 mg L-1 in the vicinity of roots. The treatment induced growth cessation in both species. Q. petraea displayed a lower tolerance to hypoxia as demonstrated by the higher number of seedlings suffering shoot dieback and leaf chlorosis as compared to Q. robur. This difference should be related to the high number of adventitious roots and hypertrophied lenticels that were formed in Q. robur, compared to Q. petraea. In the fine roots of the two species, the activity of pyruvate decarboxylase (PDC), the key enzyme of the fermentative pathway, was stimulated after 24 It of water-logging. Transcripts of PDC increased after 48 It of water-logging in Q. robur and not in Q. petraea. Interestingly, transcripts of haemoglobin (Hb) (possibly involved in the putative nitric oxide cycle) followed the same pattern of response than those of PDC. Enzymes of the sucrose degradation pathway displayed decreased activities after 3 weeks of water-logging, probably due to decreased carbohydrate availability. Alcohol dehydrogenase (ADH), sucrose synthase (Susy), and pyruvate kinase (PK) activities were higher in Q. robur after 3 weeks of water-logging. This study provided a set of markers characterizing the differences of tolerance to hypoxia between the two species for further studies on intra and inter-specific diversity.