Responses of photosynthesis and component processes to drought and temperature stress: are Mediterranean trees fit for climate change?

Global warming is raising concerns about the acclimatory capacity of trees and forests, especially in Mediterranean-type ecosystems. The sensitivity of photosynthesis to temperature is a key uncertainty for projecting the magnitude of terrestrial feedbacks on future climate change. While boreal, temperate and tropical species have been comparatively well investigated, our study provides the first comprehensive overview of the seasonal acclimatory responses of photosynthesis and its component processes to temperature in four Mediterranean climax species under natural conditions. We quantified seasonal changes in the responses of net photosynthesis (Anet), stomatal conductance (gs), mesophyllic conductance (gm) and electron-transport rate (Jcf), and investigated their sensitivity to drought and temperature stress in sunlit and shaded leaves of four Mediterranean tree species (Quercus ilex L., Pinus halepensis Mill., Arbutus unedo L. and Quercus pubescens Willd.). Sunlit leaves, but not shaded leaves, showed a pronounced seasonality in the temperature responses of Anet, gs, gm and Jcf. All four species and variables showed a remarkably dynamic and consistent acclimation of the thermal optimum (Topt), reaching peaks in summer ~29–32 °C. Changes in the shape of the response curves were, however, highly species-specific. Under severe drought, Topt of all variables were on average 22–29% lower. This was accompanied by narrower response curves above all in P. halepensis, reducing the optimal range for photosynthesis to the cooler morning or evening periods. Wider temperature-response curves and less strict stomatal control under severe drought were accompanied by wilting and drought-induced leaf shedding in Q. ilex and Q. pubescens and by additional branch dieback in A. unedo. Mild winter conditions led to a high Topt (~19.1–22.2 °C), benefitting the evergreen species, especially P. halepensis. Seasonal acclimation of Anet was explained better by gs and gm being less pronounced in Jcf. Drought was thus a key factor, in addition to growth temperature, to explain seasonal acclimation of photosynthesis. Severe drought periods may exceed more frequently the high acclimatory capacity of Mediterranean trees to high ambient temperatures, which could lead to reduced growth, increased leaf shedding and, for some species such as A. unedo, increased mortality risk.

Does age matter under winter photoinhibitory conditions? A case study in stems and leaves of European mistletoe (Viscum album)

European mistletoe (Viscum album L.) is a hemiparasitic plant with perennial leaves and photosynthetic stems easily discernible according to their age. These properties make V. album the perfect species to (i) compare the mechanisms of seasonal acclimation of photosynthetic stems with those of leaves, and (ii) evaluate the influence of ageing in the efficiency of photosynthetic tissues. To achieve these general objectives, photosynthetic pigments, maximal photochemical efficiency of PSII (Fv/Fm), recovery kinetics and key thylakoidal proteins were analysed during winter and spring in leaves and at different age stems. During winter, some woody species are able to maintain photosynthetic activity, but at lower rates than during spring. In the case of V. album, photosynthetic relevance of green stems appears equal to leaves in terms of total area. Besides, mistletoe stems are able to maintain higher Fv/Fm and lower level of antioxidants than leaves, especially during winter season. The recovery from winter photoinhibition is also faster in stems than in leaves. Thylakoidal protein composition (mainly high levels of D1) also supports the idea of stems as main photosynthetic organs in V. album during winter. Further, in winter, the level of photoinhibition of V. album stems decreased concomitantly with ageing. This work highlights the importance of stem photosynthesis in plant carbon balance and demonstrates that ageing does not necessarily imply a loss of vitality in stems.

Effects of seasonal acclimation on cold tolerance and biochemical status of the carob moth, Ectomyelois ceratoniae Zeller, last instar larvae

The carob moth, Ectomyelois ceratoniae, a pest of Punica granatum, overwinters as a larva. In this study, physiological changes, water content, cold hardiness and supercooling points (SCPs) in relation to ambient temperature in the overwintering period (October to March) and changes of these factors between diapausing (February) and non-diapausing (September) larvae were studied. Pupae that were derived from diapausing larvae (April) and from non-diapausing larvae (August) were also compared. Total body sugar, lipid and protein contents increased with decrease in the temperature and reached the highest levels (12.82, 1.99 and 6.11 mg g−1 body weight, respectively) in February, but glycogen content decreased and reached the lowest level (1.12 mg g−1 body weight) in February. There were significant differences in the levels of these compounds between diapausing and non-diapausing larvae, and pupae that were derived from diapausing and non-diapausing larvae. Trehalose and myo-inositol contents increased during diapause and reached the highest levels (0.50 and 0.07 mg g−1 body weight, respectively) in February. There were significant differences in the levels of these compounds between diapausing and non-diapausing larvae, but the differences between pupae that were derived from diapausing and non-diapausing larvae were not significant. The SCP of diapausing larvae (−17.3 °C) was significantly lower than in the non-diapausing larvae (−12.0 °C). SCP decreased gradually in autumn and reached the lowest level in the middle of winter. Changes of cold hardiness were inversely proportional to SCP changes. The lowest levels of water (65%) and weight (43.13 mg) were recorded in January and March, respectively. Most probably, lipids play a role as energy reserve, and low-molecular weight carbohydrates and polyols provide cryoprotection for overwintering larvae of the carob moth. Since the overwintering larvae die at temperatures above the SCP, the carob moth larvae were found to be a chill-intolerant insect.