© 2004 Heron Publishing—Victoria, Canada
Drought tolerance and transplanting performance of holm oak (Quercus ilex) seedlings after drought hardening in the nursery
Pedro Villar-Salvador (1, 2), Rosa Planelles (3), Juan Oliet (4), Juan L. Peñuelas-Rubira (1), Douglass F. Jacobs (5) and Magdalena González (3)
1. Centro Nacional de Mejora Forestal “El Serranillo,” DGCONA, Ministerio de Medio Ambiente, Apdo. 249, 19004 Guadalajara, Spain / 2. Corresponding author (pvsalvador@mma.es) / 3. Instituto Nacional de Investigaciones Agrarias (INIA), Departamento de Medio Ambiente, Apdo. 8111, 28080 Madrid, Spain / 4. Universidad de Córdoba, Departamento de Ingeniería Forestal, Apdo. 3048, 14080 Córdoba, Spain / 5. Hardwood Tree Improvement and Regeneration Center, Department of Forestry and Natural Resources, Purdue University, West Lafayette,
IN 47907, USA / Received March 21, 2003; accepted February 8, 2004; published online August 2, 2004
Summary
Drought stress is the main cause of mortality of holm oak (Quercus ilex L.) seedlings in forest plantations. We therefore assessed if drought hardening, applied in the nursery at the end of the
growing season, enhanced the drought tolerance and transplanting performance of holm oak seedlings. Seedlings were subjected
to three drought hardening intensities (low, moderate and severe) for 2.5 and 3.5 months, and compared with control seedlings.
At the end of the hardening period, water relations, gas exchange and morphological attributes were determined, and survival
and growth under mesic and xeric transplanting conditions were assessed. Drought hardening increased drought tolerance primarily
by affecting physiological traits, with no effect on shoot/root ratio or specific leaf mass. Drought hardening reduced osmotic
potential at saturation and at the turgor loss point, stomatal conductance, residual transpiration (RT) and new root growth
capacity (RGC), but enhanced cell membrane stability. Among treated seedlings, the largest response occurred in seedlings
subjected to moderate hardening. Severe hardening reduced shoot soluble sugar concentration and increased shoot starch concentration.
Increasing the duration of hardening had no effect on water relations but reduced shoot mineral and starch concentrations.
Variation in cell membrane stability, RT and RGC were negatively related to osmotic adjustment. Despite differences in drought
tolerance, no differences in mortality and relative growth rate were observed between hardening treatments when the seedlings
were transplanted under either mesic or xeric conditions.
Keywords:
Mediterranean, nonstructural carbohydrate, osmotic adjustment, plasmalemma stability, residual transpiration, root growth
capacity, stomatal conductance, water stress.
