Loretta Gratani

Plant phenotypic plasticity in response to environmental factors

Plants are exposed to heterogeneity in the environment where new stress factors (i.e., climate change, land use change, and invasiveness) are introduced, and where inter- and intraspecies differences may reflect resource limitation and/or environmental stress factors. Phenotypic plasticity is considered one of the major means by which plants can cope with environmental factor variability. Nevertheless, the extent to which phenotypic plasticity may facilitate survival under environmental condition changes still remains largely unknown because results are sometimes controversial. Thus, it is important to identify plant functional traits in which plasticity may play a determinant role in plant response to global change as well as on the ecological consequences at an ecosystem level for the competition between wild and invasive species, considering that species with a greater adaptive plasticity may be more likely to survive in novel environmental conditions. In the near future, it will be important to increase long-term studies on natural populations in order to understand plant response to environmental factor fluctuations including climate change. There is the necessity to analyze variations at phenotypic and genetic levels for the same species and, in particular, for endemic and rare species because these could have drastic effects at an ecosystem level.

Structural and functional plasticity of Quercus ilex seedlings of different provenances in Italy

Functional and structural leaf traits of Quercus ilex seedlings originated from parent plant acorns from three different localities in Italy were studied. Acorns from three different localities along a gradient from the north to the south of Italy: Nago (site A) in the Garda Lake region at the northernmost limit of holm oak distribution area in Italy, Castelporziano near Rome (site B), at the centre of the distribution area, and Frassanito near Otranto (site C), in a drier area in the south of Italy. Morphological and anatomical leaf traits differed between the provenances with a higher leaf mass area, total leaf thickness and the ratio of palisade to mesophyll thickness in the driest provenance (C seedlings). These traits gave C seedlings a higher water use efficiency, relative water content at predawn and photosynthetic rates than the other provenances in high air temperature conditions. The smaller leaf area of A seedlings seemed to have a higher photosynthetic capacity in low air temperature conditions than B and C seedlings. Growth analysis underlined a higher shoot relative growth rate in B seedlings explaining the highest shoot length and leaf number per shoot. The plasticity index [sensu Valladares et al. (2000) Ecology 81:1925–1936] for physiological traits of the seedlings was higher than morphological and anatomical traits, but the largest differences in plasticity among ecotypes were found for morphological and anatomical traits. The ecotypes of Q. ilex studied here seemed to integrate, at leaf level, functions of growth activity, morphology and physiology related to the climate of the original provenance.

Adaptive photosynthetic strategies of the Mediterranean maquis species according to their origin

In consideration of their origin the adaptive strategies of the evergreen species of the Mediterranean maquis were analysed. Rosmarinus officinalis L., Erica arborea L., and Erica multiflora L. had the lowest net photosynthetic rate (PN) in the favourable period [7.8±0.6 μmol(CO2) m−2s−1, mean value], the highest PN decrease (on an average 86 % of the maximum) but the highest recovery capacity (>70 % of the maximum) at the first rainfall in September. Cistus incanus L. and Arbutus unedo L. had the highest PN during the favourable period [15.5±5.2 μmol(CO2) m−2s−1, mean value], 79 % decrease during drought, and a lower recovery capacity (on an average 54 %). Quercus ilex L., Phillyrea latifolia L., and Pistacia lentiscus L. had an intermediate PN in the favourable period [9.2±1.3 μmol(CO2) m−2s−1, mean value], a lower reduction during drought (on an average 63 %), and a range from 62 % (Q. ilex and P. latifolia) to 39 % (P. lentiscus) of recovery capacity. The Mediterranean species had higher decrease in PN and stomatal conductance during drought and a higher recovery capacity than the pre-Mediterranean species. Among the pre-Mediterranean species, P. latifoliahad the best adaptation to long drought periods also by its higher leaf mass per area (LMA) which lowered leaf temperature thus decreasing transpiration rate during drought. Moreover, its leaf longevity determined a more stable leaf biomass during the year. Among the Mediteranean species, R. officinalis was the best adapted species to short drought periods by its ability to rapidly recover. Nevertheless, R. officinalis had the lowest tolerance to high temperatures by its PN dropping below half its maximum value when leaf temperature was over 33.6°C. R. officinalismay be used as a bioindicator species of global change.

Correlation between leaf age and other leaf traits in three Mediterranean maquis shrub species: Quercus ilex, Phillyrea latifolia and Cistus incanus

Abstract The anatomical and morphological leaf traits as well as leaf inclination and orientation per different leaf age cohort of Quercus ilex , Phillyrea latifolia and Cistus incanus growing in the Mediterranean maquis along Rome’s coast-line (Italy) were investigated. Specific leaf weight (SLW), total leaf thickness ( L ), leaf density index and leaf inclination ( α ) changed according to leaf age. The maximum values were measured at full leaf expansion, underlining the strong influence of α on the reduction of solar radiation incident on leaf surface and the importance of the received solar radiation by leaf structure during leaf age. C. incanus summer leaves had the lowest surface area, the highest SLW (15±2 mg cm −2 ) and L (244±15 μm) with respect to winter leaves, reducing the evaporative leaf surface during drought. Older leaves of 2–4 years Q. ilex and P. latifolia , shaded by new leaves had lower α than 1 year old leaves. α is a linear function of SLW. By the seasonal leaf dimorphism and the characteristic leaf folding the adjustment of leaf inclination angle from −37° in winter leaves to +44° in summer leaves increased reduction of incident solar radiation during drought. Leaf folding may be related to the less xeromorphic leaf structure of C. incanus . The index of xeromorphism, measured at full leaf expansion and resulting from the surface area of the polygon plotted joining the value of the seven considered xeromorphic leaf traits in the radar graph, is the highest in P. latifolia (0.88), and the lowest in C. incanus (0.44).

Photosynthesis as a temperature indicator in Quercus ilex L.

Net photosynthesis (Pn), stomatal conductance (gs), leaf temperature (LT), transpiration rate (E) and in vivo chlorophyll fluorescence were monitored February 1996–February 1997 in Quercus ilex plants growing in the climax area (Rome) and in the Garda lake region. Photosynthetic activity is an appropriate temperature-dependent functional trait linked to plant metabolism and performance. We employed photosynthesis as a stress temperature indicator. Regression analysis showed that in such regional climatic conditions, net photosynthetic rates were primarily correlated with temperature. The introduction of rainfall in the function did not significantly improve the theoretical prevision, in the range of temperature and rainfall analysed. The favourable leaf temperatures allowing 90–100% of the highest photosynthetic rates were 14–28°C, decreasing over 50% when leaf temperature were respectively below 6°C and over 37°C: the low potential photochemical efficiency of 0.71 PSII (Fv/Fm) at Castelporziano during summer, confirmed the state of stress. Transpiration rates remained high with the increase of leaf temperature in summer, in spite of the 46% decrease of stomatal conductance. Nevertheless, this decrease allowed the maintenance of acceptable Pn rates in stressful conditions. The potential productivity of Q. ilex lied on high peaks of activity during periods of lower evaporative demand and a rapid stomatal response to an increase in air temperature and soil water deficit. Although it is difficult to forecast Q. ilex productivity and plant structure, we may hypothesize its future presence in the Mediterranean Basin by the capability of vegetative activity in a wide range of temperatures, the high stomatal control in stressful conditions, the high plasticity index and water use efficiency (WUE).

Leaf key traits of Erica arborea L., Erica multiflora L. and Rosmarinus officinalis L. co-occurring in the Mediterranean maquis

Summary Leaf morphology and anatomy, water relations, leaf life-span, and gas exchange of Erica arborea L., Erica multiflora L. and Rosmarinus officinalis L. co-occurring in the Mediterranean maquis developing along the coast near Rome were analysed in the period from May 2000 through December 2001. The year 2001 was characterised by a severe drought period from the beginning of June to the middle of September (13.2 mm total rainfall, 32C average maximum air temperature). The highest pre-dawn leaf water potential (ae pd = –1.2 0.2 MPa) and relative water content (RWC = 87 4%) of E. arborea compared to the other two species were indicative of a high leaf water potential recovering capacity, allowing a sufficient photosynthetic activity (Pn = 2.9 0.4 Imol m −2 s −1 ) during the severe drought period. On the contrary, R. officinalis showed 93% Pn rate decrease during drought with respect to the maximum rate. Moreover, the large ae pd variation during the year (from –0.16 to –3.50 MPa) was indicative of a low recovery capacity, which might be related to its narrow root system. From a physiological point of view, E. multiflora had a similar trend to that of R. officinalis with a large ae pd variation during the year and a low ae pd value (–2.50 0.3 MPa) during the severe drought period. The correlation analysis carried out on morphological leaf traits for E. arborea , E. multiflora , R. officinalis and, by previous studies, for the other dominant shrub species of the Mediterranean maquis at the research site of Castelporziano ( Quercus ilex L., Phillyrea latifolia L., Arbutus unedo L., Cistus incanus L.) indicated that variations in leaf mass per unit leaf area (LMA) were associated with variations in leaf tissue density (LTD), leaf life-span (LL), and leaf thickness (L). Discriminant analysis carried out on net photosynthesis (Pn), stomatal conductance (g s ), transpiration rate (E), water relations (ae, RWC), LMA, LL and L showed that g s and LL were the most discriminating leaf traits.

Leaf anatomy, inclination, and gas exchange relationships in evergreen sclerophyllous and drought semideciduous shrub species

There are significant differences in leaf life-span among evergreen sclerophyllous species and drought semideciduous species growing in the Mediterranean maquis. Cistus incamus, which has a leaf life-span of four-eight months, was characterised by the highest net photosynthetic rates (PN), while Quercus ilex and Phillyrea latifolia, which maintain their leaves two-three and two-four years, respectively, had a lower PN. The longer leaf life-span of the two evergreen sclerophyllous species may be justified to cover the high production costs of leaf protective structures such as cuticle, hairs, and sclereids: cuticle and hairs screen radiation penetrating into the more sensitive tissues, and sclereids have a light-guiding function. Q. ilex and P. latifolia have the highest leaf mass/area ratio (LMA = 209 g m-2) and a mesophyll leaf density (2065 cells per mm2 of leaf cross section area) about two times higher than C. incanus. In the typical evergreen sclerophyllous species the steepest leaf inclination (α = 56°) reduces 42 % of radiation absorption, resulting in a reduced physiological stress at leaf level, particularly in summer. C. incanus, because of its low leaf life-span, requires a lower leaf investment in leaf protective structures. It exhibits a drastic reduction of winter leaves just before summer drought, replacing them with smaller folded leaves. The lower leaf inclination (α = 44°) and the lower LMA (119 g m-2) of C. incanus complement photosynthetic performance. Water use efficiency (WUE) showed the same trend in Q. ilex, P. latifolia, and C. incanus, decreasing 60 % from spring to summer, due to the combined effects of decreased CO2 uptake and increased transpirational water loss.

Relationship between Photosynthetic Activity and Chlorophyll Content in an Isolated Quercus Ilex L. Tree during the Year

The relationship between chlorophyll (Chl) content and net photosynthetic rate (PN) in an isolated Quercus ilex tree, growing inside Villa Pamphili Park in Rome, was explored. The highest PN was in March, May, and September (10.1 μmol m-2 s-1, maximum rate). PN decreased by 65 % (with respect to the yearly maximum) when leaf temperature reached 34 °C, and by 50 % when leaf temperature was 9 °C. The highest Chl contents were in April, October [1.47 g kg-1 (d.m.), maximum value], and December. The lowest Chl content was found in July (0.78 g kg-1). The decrease of PN in July was in close connection with the decrease of Chl content. On the contrary, the high Chl content during winter did not correspond with PN of this season. Discordances between Chl content and PN over the year influenced the regression analysis, which although positive did not show very high correlation coefficients (r = 0.7). The high Chl (a+b) content during most of the year indicated that the photosynthetic apparatus remained basically intact also during stress periods.

Carbon sequestration by Quercus ilex L. and Quercus pubescens Willd. and their contribution to decreasing air temperature in Rome

Carbon sequestration capability by Quercus ilex L and Quercus pubescens Willd., widely distributed in the city of Rome, and their contribution to decreasing air temperature were investigated. Crown volume is the most significant (p < 0.01) variable explaining variation of air temperature below the tree crown. Q. pubescens gives a higher contribution to decreasing air temperature during the hottest months, due to its inherent larger crown volume than Q. ilex (252 ± 19 and 533 ± 52 m3, respectively for the large size). Moreover, our results show the existence of a strong urban carbon dioxide dome with a peak CO2 concentration (on an average 432 ± 37 ppm) at polluted sites, 16% greater than at control sites. Total carbon sequestration is 84 ± 12 and 111 ± 9 Kg year−1 of CO2 for the small Q. ilex and Q. pubescens tree size, respectively, and 151 ± 10 and 185 ± 7 Kg year−1 of CO2 for the large Q. ilex and Q. pubescen tree size, respectively. Q. pubescens, by its higher total photosynthetic leaf surface area (39% higher than Q. ilex) and its higher mean yearly photosynthetic rates (48% higher than Q. ilex) seems to have a greater role than Q. ilex. However, taking into account the leaf longevity (i.e. 12 ± 3 months for Q. ilex and 4 ± 2 months for Q. pubescens), the evergreen species, by its continuous photosynthetic activity, contributes to reduce CO2 throughout the year, and in particular during the winter months, when traffic volume has a pick, than Q. pubescens.

Responses of Quercus ilex from different provenances to experimentally imposed water stress

Responses of Quercus ilex L. seedlings from three different localities in Italy to experimentally imposed drought stress were analysed. Predawn (Ψpd) and midday (Ψm) leaf water potential of stressed seedlings decreased on an average until −4.0 and −4.2 MPa, respectively, in the severe water stress. At the end of the severe water stress the relative water content (RWC) was 72.5 – 83.6 % and the photosynthetic rates (PN) near zero. The critical threshold value of Ψpd for complete stomatal closure was from −4.0 to −4.5 MPa. The leaf damage after the severe water stress was significantly greater in seedlings originated from the acorns of climax area (45 % total leaf injured area and 40 % fallen leaves) than in the other seedlings (on an average 20.5 % total leaf injured area and 21 % fallen leaves).