Chlorophyll a fluorescence, water potential (Ψs), and root system of Juniperus oxycedrus ssp. macrocarpa, Juniperus phoenicea ssp. turbinata, and Pinus pinea were studied in Mediterranean coastal dunes of SW Spain during summer drought and after fall rains in 1999, the driest year in the 90's. A strong and reversible depression in the photochemical efficiency of photosystem 2 of the three species was recorded, which happened concomitantly with the diurnal increase and decrease in radiation. J. phoenicea, with superficial root system, was the most affected species by summer drought. It showed high rates of down-regulation of photosynthesis by photoinhibition and positive correlation between Ψs and Fv/Fp, with Ψs lower than -7 MPa. However, it tolerated this high stress, showing a fast recovery of its physiological state after fall rains. On the other hand, J. oxycedrus and P. pinea, both with deep root systems, kept their Ψs values up to -3 MPa, showing lower stress during summer drought. On the other hand, J. oxycedrus and J. phoenicea were more sensible to changes in edaphic water content than P. pinea. These specific responses to summer drought would be determined by their root distributions and stomatal control of transpiration, conditioning the efficiency in getting and using the available water resources. Ecophysiological responses indicate that these species are well-adapted to long periods of drought in Mediterranean climate areas, developing different strategies: J. phoenicea tolerates high stress with a fast recovery after fall rains, while J. oxycedrus and P. pinea are less affected by summer drought since their deep root systems would allow them to reach deep water resources. and J. M. Castillo ... [et al.].
Seasonal and daily variations in chlorophyll (Chl) fluorescence were studied in two representative species of Mediterranean grasslands, Tuberaria guttata (an annual) and Chamaemelum nobile (a perennial), in order to assess physiological responses to climatically induced stresses during the growing season. The photochemical efficiency of photosystem (PS) 2 in dark-adapted leaves was measured by the Chl fluorescence ratio Fv/Fp. This ratio decreased progressively from December to July, as the effects of increasing solar radiation and summer drought became more severe. The seasonal decline was observed particularly as a depression of morning and midday values, when photoinhibition was more evident. In both species, the extent of this diurnal depression increased with midday irradiance throughout winter and spring. After sunset, there was complete recovery to optimum values. Towards the end of the life cycle, increased irradiance did not affect the midday decline further but Fv/Fp measurements in the morning and evening never regained their optimum values, indicating the accumulation of photodamage in the reaction centres of PS2. The half-rise time of Fp (T1/2), used to estimate the size of the plastoquinone pool, showed little daily variation in C. nobile throughout the most important part of its seasonal cycle. However, towards the end of its life cycle (June and July) T1/2 values ranged from ca. 200 ms before sunrise to near zero at midday on the same day. The annual species, T. guttata, showed similar disregulation in energy transmission rate both at the seedling stage and at end of its life cycle. Thus seedlings and reproductive plants in particular are sensitive to environmental conditions (extremes of temperature and drought) and cannot maintain consistent electron flow throughout the day. and L. Fernández-Baco ... [et al.].
Spartina maritima (Curtis) Fernald, Spartina densiflora Brong, Arthrocnemum perenne (Miller) Moss, and Arthrocnemum fruticosum (L.) Moq are very frequent halophytes on the coasts of SW Europe. The first two are perennial Gramineae with C4 metabolism; the last two are perennial Chenopodiaceae with C3 metabolism. Controlled garden experiments were carried out with the four species to compare their physiological response, i.e., water potential (Ψ), net photosynthetic rate (PN), transpiration rate (E), stomatal conductance (gs), intercellular CO2 concentration (Ci), and chlorophyll fluorescence of photosystem (PS) 2 under saline and non-saline conditions. S. maritima behaves as an osmoconformer species, the other three as osmoregulators. In the four species, PN, E, and gs improved following freshwater irrigation. The variations in PN might be related with biochemical changes (which appear not to affect PS2), but not with significant stomatal fluctuations, which are associated with a lower water use efficiency in the case of Arthrocnemum. The species were segregated into two groups (not depending on their C3 or C4 photosynthetic pathway), in relation with the topographic level of this species in natural conditions: the relative responses of PN in S. maritima and A. perenne were lower than those of S. densiflora and A. fruticosum. The salt-tolerance index supports such segregation. S. densiflora demonstrated the best competitive possibilities against salt-tolerant glycophytes, with its more flexible response in saline or brackish environments, which explains its spreading along the rivers draining into the estuaries of the SW Iberian Peninsula. and F. J. J. Nieva ... [et al.].
Plant invasions may be limited by low radiation levels in ecosystems such as forests. Lantana camara has been classified among the world's 10 worst weeds since it is invading many different habitats all around the planet. Morphological and physiological responses to different light fluxes were analyzed. L. camara was able to acclimate to moderately shaded environments, showing a high phenotypic plasticity. Morphological acclimation to low light fluxes was typified by increasing leaf size, leaf biomass, leaf area index and plant height and by reduced stomatal density and leaf thickness. Plants in full sunlight produced many more inflorescences than in shaded conditions. Physiological acclimation to low radiation levels was shown to be higher stomatal conductance, higher net photosynthetic rates and higher efficiency of photosystem II (PSII). L. camara behaves as a facultative shade-tolerant plant, being able to grow in moderately sheltered environments, however its invasion could be limited in very shady habitats. Control efforts in patchy environments should be mainly directed against individuals in open areas since that is where the production of seeds would be higher and the progress of the invasion would be faster. and J. Carrión-Tacuri ... [et al.].
Chlorophyll fluorescence measurements showed that plasticity to salinity in stems of Salicornia ramosissima is expressed at a modular level, so intraplant variation should be considered in further studies. and S. Redondo-Gómez ... [et al.].
Fluorescence microscopy and physiological examination revealed the presence of an inner cylinder of active photosynthetic cells located below the endodermis-like layer in stems of four of the six taxa of the tribe Salicornieae (Chenopodiaceae). and S. Redondo-Gómez ... [et al.].