Physiological and morphological responses to water stress in seedlings of the Mediterranean species Azara integrifolia grown under different soil fertilization regimes
Progress in successful restoration in Mediterranean-type ecosystems (MTE) is hampered by technical difficulties for plant survival in the field during the extended dry-summer period. In this context, a major challenge for future restoration efforts is to increase water use efficiency (WUE) of nursery-produced seedlings. Functional attributes, such as morphological and physiological traits, determine survival probabilities and growth rates of planted seedlings in restoration projects. Nursery practices, such as fertilization, can have strong effects on plant functional traits. Specifically, a reduction in nutrient supply could be beneficial because it reduces plant size and leaf area, while increasing biomass allocation to below ground structures, with consequently enhanced WUE. The aim of this work was to quantify the responses to drought of tree seedlings grown under four different fertilization treatments, and to determine which treatment is best suited to drought conditions in the field. We produced seedlings of Azara integrifolia, a Mediterreanean tree species semi-tolerant of shade, under contrasted fertilization regimes. Seedlings received weekly applications of complete nutrient solution (N-P-K and micronutrients), or the same nutrient solutions but without N, without P, and without N and P. Twenty plants of each fertilization regime were assigned to two levels of drought stress in a greenhouse: well watered (control) and moderate water stress (MWS), where water content was maintained at 30% of field capacity. We measured seedling survival, growth, and ecophysiological variables (leaf gas exchange, chlorophyll fluorescence, water use efficciency, specific root length, shoot-root ratio, specific leaf area) which are relevant for stress resistance during establishment in the field.
Plants grown without N, without P, and without N and P increased their specific root length 2 to 3 fold compared to fully fertilized plants, presenting deeper roots than seedlings grown under the complete nutrient solution. We expect that these morphological changes should result in improved survival of plants under water stress conditions charateristic of the summer dry period. By anticipating which traits will optimize seedling establishment in the field, and using well-designed nursery practices, the success of future restoration projects in terms of the number of established trees can be increased in MTEs.