Morphological, anatomical and physiological leaf traits of Q. ilex, P. latifolia, P. lentiscus, and M. communis and their response to Mediterranean climate stress factors

Abstract

Background: Limitations to plant growth imposed by the Mediterranean climate are mainly due to carbon balance in response to stress factors. In particular, water stress associated to high air temperature and irradiance in summer causes a marked decrease in CO₂ assimilation. Air temperature sensitivity of photosynthesis (P_N) differs from that of leaf respiration (R_D). P_N often decreases sharply at temperature above its optimum while R_D increases exponentially over short term rises in temperature. Nevertheless, the impact of water deficit on R_D is still far from clear with reports in literature including decreases, maintenance or increases in its rates. The ratio R_D/P_N can be considered a simple approach to leaf carbon balance because it indicates the percentage of photosynthates that is respired.

Results: The results underline different morphological, anatomical and physiological traits of the evergreen species co-occurring in the Mediterranean maquis which are indicative of their adaptive capability to Mediterranean stress factors. The ratio R_D/P_N varies from 0.15 ± 0.04 in autumn, 0.24 ± 0.05 in spring through 0.29 ± 0.15 in winter to 0.46 ± 0.11 in summer. The lower R_D/P_N in autumn and spring underlines the highest P_N rates during the favorable periods when resources are not limited and leaves take in roughly three to five times more CO₂ than they lose by respiration. On the contrary, the highest R_D/P_N ratio in summer underlines the lowest sensitivity of respiration to drought. Among the considered species, Quercus ilex and Pistacia lentiscus have the largest tolerance to low winter temperatures while Phillyrea latifolia and Myrtus communis to drought, and Phillyrea latifolia the highest recovery capability after the first rainfall following drought.

Conclusions: The Mediterranean evergreen specie shows a different tolerance to Mediterranean climate stress factors. The predicted global warming might differently affect carbon balance of the considered species, with a possible change in Mediterranean shrublands composition in the long-term. Understanding the carbon balance of plants in water limited environments is crucial in order to make informed land management decisions. Moreover, our results underline the importance of including seasonal variations of photosynthesis and respiration in carbon balance models.

Keywords: Air temperature; Drought; LMA; Leaf respiration; Myrtus communis; Phillyrea latifolia; Photosynthesis; Pistacia lentiscus; Quercus ilex.

Figure 1

Trend of a) net photosynthetic rate ( P_N), b) stomatal conductance ( g_s), c) intrinsic water use efficiency (IWUE) of P. lentiscus ( close squares ), P. latifolia ( open circles ), M. communis ( close triangles ), and Q. ilex ( open squares ) during the study period. The mean values for each month (±SE) are shown (n = 40 leaves). Mean values with the same letters are not significantly different (p ≥ 0.05). Lowercase letters indicate the differences among the species for each month, capital letters indicate the intra-specific differences during the study period.

Figure 2

Leaf respiration ( R_D) trend of P. lentiscus ( close squares ), P. latifolia ( open circles ), M. communis ( close triangles ) and Q. ilex ( open squares ) during the study period. The mean values for each month (± SE) are shown (n = 40 leaves). Mean values with the same letters are not significantly different (p ≥ 0.05). Lowercase letters indicate the differences among the species for each month, capital letters indicate the intra-specific differences during the study period.

Figure 3

Leaf to air vapor pressure deficit (VPD_leaf) trend of P. lentiscus ( close squares ), P. latifolia ( open circles ), M. communis ( close triangles ) and Q. ilex ( open squares ) during the study period. The mean values for each month (± SE) are shown (n = 40 leaves). Mean values with the same letters are not significantly different (p ≥ 0.05). Lowercase letters indicate the differences among the species for each month, capital letters indicate the intra-specific differences during the study period.

Figure 4

Regression analysis between leaf respiration ( R_D) and leaf chamber air temperature ( T_ch) for the considered species. Regression equation, determination’s coefficient (R²) and P-level are shown.

Figure 5

Regression analysis between stomatal conductance ( g_s) and leaf to air vapour pressure deficit (VPD_leaf) for the considered species. Regression equation, determination’s coefficient (R²) and P-level are shown.

Figure 6

Principal component analysis (PCA) carried out using physiological traits in winter, spring and summer (photosynthetic rates stomatal conductance transpiration rates intrinsic water use efficiency and leaf respiration) and morphological and anatomical leaf traits (LMA, LTD, leaf thickness, stomatal density, stomatal pore length and stomatal pore area) for the considered species.