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Phoenician juniper: best at adapting to conditions of extreme drought

24.05.2004

Of the four representative species of the Mediterranean climate - the holm oak (Quercus ilex), the kermes oak (Quercus coccifera), the Aleppo pine (Pinus halepensis), and the Phoenician juniper (Juniperus phoenicea) - the last is the one which best adapts to the adverse conditions of water stress. However, this does not mean the disappearance of the other three species that have been studied. This is the conclusion drawn from his PhD by Francisco Javier Baquedano, agricultural engineer at the Public University of Navarre.

The study was centred on the Mediterranean area, a transition climatic region where weather effects are more pronounced, involving a climate characterised by a long, hot, dry summer. The summer drought coincides with a high evaporative factor and this is why it is considered as the most important factor limiting the production, growth and distribution of plants in the Mediterranean ecosystems.

The survival of plants under these adverse conditions greatly depends on their ability to compensate for loss of water and to their carbon-fixing properties. The balance between these two processes could be the key to the competition between the species and consequently, to the relative presence of flora in the ecosystems. It has also been suggested that the winter, cold and with little precipitation, influences in an important manner the development and distribution of the Mediterranean perennial species.

The Mediterranean species that coexist often have different tolerances to drought, presenting two response strategies to water stress: the avoidance of drought and the tolerance to it. The drought-avoidance species predict loss of water, closing the stomas and showing little change in water potential, but inhibiting the assimilation of CO2. On the contrary, the drought-tolerance strategy is found in those species that have relatively high stomatic conductance, associated with a great loss of water and low water potentials, but also with high assimilation of CO2.

Four representative species

Another novel aspect of the research is that, compared to the majority of studies which concentrate on a single species, this PhD by the agricultural engineer studied four very representative species of the Mediterranean climate - the holm oak, the kermes oak, the Aleppo pine, and Phoenician juniper. Of the four, the most studied to date has been the holm oak followed by the pine. However, there has been hardly any research on the other two species.

In this way, the study has the twin aims of investigating the strategies adopted by four distinct perennial Mediterranean species to cope with the various periods of water stress in the Mediterranean area. Taking these different strategies into account, the aim of the investigation was to see if one was more efficacious than the others when that stress was augmented somewhat. Moreover, the most adverse periods for these species and the principal factors that give rise to this stress are identified.

To this end, the following factors were studied; water potential, water content, gaseous interchange, chlorophyll fluorescence, photosynthetic pigment content and the antioxidant capacity of the holm oak, kermes oak, Aleppo pine and Phoenician juniper. These are measurable parameters related to photosynthesis and, above all, that guide us in the measurement of the antioxidant capacity, given that we are trying to find out the destiny of the energy captured by plants – it is assumed that in the adverse climatic conditions of the Mediterranean in the summer, plants capture an excess of energy.

Experimental study in Teruel

Following the development of a theoretical plan, an experimental study in the region of Teruel was carried out. The results show that the four species studied present adaptation mechanisms that enable them to survive in the most unfavourable of conditions, although these mechanisms vary according to the species. Thus, under unfavourable conditions, all four species reduce photosynthesis and increase the canalising of energy to alternative routes, such as heat dissipation carried out by the carotenoids and increasing antioxidant capacity. At the same time, a loss of photosynthetic pigments - principally chlorophylls - takes place and, as an end result, there is less capture of excess energy, thus avoiding photo-oxidation.

The content of photosynthetic pigments is also related to the capacity for adaptation by these species to the most adverse of conditions, given that the conifers, presenting a lower pigment content, dominate the most arid zones. On the other hand, the greater antioxidant capacity of a species carries with it the need for greater protection in the face of photo-oxidation. Moreover, phenotype flexibility of the four species is very limited.

Of the four species, it is Phoenician juniper that best adapts to the most adverse conditions. As regards the Aleppo pine, it distinguishes itself as a species of rapid growth and avoider of both drought and photoinhibition at mid-day. At this time of day, the pine needs a greater dissipation of energy given that photosynthesis is limited by the closing up of the stomas while, at the same time, maintaining a high level of electron transport. The other two species are worst at adapting to climatic change.

Although, in one way or another, the four species manage to adapt, in our research, we have proved the dominance of one species over another without this implying the disappearance of the latter. Thus, if the situation is favourable the species that dominate is the holm oak while, if the situation is more unfavourable, it is Phoenician juniper that best adapts.