Evaluation of drought response of two poplar clones (Populus×canadensis Mönch ' I-214' and P. deltoides Marsh. ' Dvina' ) through high resolution analysis of stem growth

Different irrigation effects on stem radius variation (Delta R) and maximum daily shrinkage (MDS) in Populus deltoides 'Dvina' and Populusx canadensis 'l-214' were studied to assess differences in drought tolerance between clones. One-year-old trees growing in concrete tanks were submitted to two irrigation regimes (natural rainfall and irrigation) from 24 June to 10 August, and Delta R was monitored by automatic point dendrometers. Independently of the irrigation regime, 'Dvina' showed a higher stem radial increment than 'l-214'. In both clones, the first response to changed soil water content was a significant increase in MDS, whilst Delta R decreased about 20 d later when pre-dawn leaf water potential (psi pd) dropped below -0.4 MPa. However, they displayed different strategies to overcome drought. 'Dvina' maintained a positive Delta R for longer than 'l-214', which had lower leaf psi pd and greater leaf abscission at the end of the drought period. After irrigation resumed, 'Dvina' showed a higher capacity to restore stem growth. 'l-214' was probably unable to recover secondary growth because of higher leaf abscission during drought stress and the production of newly expanded leaves during recovery. It is concluded that the larger radial growth of 'Dvina' derived from a better water use (carbon uptake versus water loss) than 'l-214' under limited water availability.