Résumé
1. A core tenet of functional ecology is that the vast phenotypic
diversity observed in the plant kingdom could be partly generated by a
trade- off between the ability of plants to grow quickly and acquire
resources in rich environments vs. the ability to conserve resources and
avoid mortality under stress. However, experimental demonstrations remain
scarce and potentially blurred by phylogenetic constraints in
cross-species analyses. Here we experimentally decoupled growth potential
and stress survival by applying an off-season stress on contrasting
populations of the perennial grass Dactylis glomerata exhibiting a range
of seasonal dormancy. 2. Seventeen populations of D. glomerata,
originating from a latitudinal gradient from Norway to Morocco, were
subjected to three types of dehydration stress: winter frost in Norway,
summer drought, and early spring (off-season) drought stress in the south
of France. Growth rate, and two leaf traits (leaf width and leaf dry
matter content) suspected to be involved in the adaptation to dehydration
stress, were monitored under optimal conditions. We quantified plant
dehydration survival as the amount of plant recovery after a severe
stress. 3. Nordic populations were found to be winter dormant. Winter and
summer dormant populations better survived frost and summer drought,
respectively. However, no trade-off between growth potential and
dehydration survival was detected in non-dormant plants in early spring
when dehydration occurred unseasonably for all populations. Furthermore,
Mediterranean populations better survived an early spring drought. 4. Our
results highlight the importance of assessing plant growth potential as a
response to seasonal environmental cues. They suggest that growth
potential and stress survival trade-off when plants exhibit seasonal
dormancy but can be functionally independent at other seasons.
Consequently, the growth-stress survival relationship could be better
described as a dynamic linkage rather than a constant and general
trade-off. Moreover, leaf trait values, such as thinner and more lignified
leaves reflecting drought adaptation, may have contributed to the improved
drought-stress survival without resulting in a cost to growth. 5. Further
exploration of the growth-stress survival relationship should permit
deciphering the suite of plant traits and trait covariations involved in
plants’ responses to increasing stress.