New Strategies to Increase the Abiotic Stress Tolerance in Woody Ornamental Plants in Mediterranean Climate

Decreased Photosynthetic Efficiency in Nicotiana tabacum L. under Transient Heat Stress

Heat stress is an abiotic factor that affects the photosynthetic parameters of plants. In this study, we examined the photosynthetic mechanisms underlying the rapid response of tobacco plants to heat stress in a controlled environment. To evaluate transient heat stress conditions, changes in photochemical, carboxylative, and fluorescence efficiencies were measured using an infrared gas analyser (IRGA Licor 6800) coupled with chlorophyll a fluorescence measurements. Our findings indicated that significant disruptions in the photosynthetic machinery occurred at 45 °C for 6 h following transient heat treatment, as explained by 76.2% in the principal component analysis. The photosynthetic mechanism analysis revealed that the dark respiration rate (Rd and Rd*CO2) increased, indicating a reduced potential for carbon fixation during plant growth and development. When the light compensation point (LCP) increased as the light saturation point (LSP) decreased, this indicated potential damage to the photosystem membrane of the thylakoids. Other photosynthetic parameters, such as AMAX, VCMAX, JMAX, and ΦCO2, also decreased, compromising both photochemical and carboxylative efficiencies in the Calvin-Benson cycle. The energy dissipation mechanism, as indicated by the NPQ, qN, and thermal values, suggested that a photoprotective strategy may have been employed. However, the observed transitory damage was a result of disruption of the electron transport rate (ETR) between the PSII and PSI photosystems, which was initially caused by high temperatures. Our study highlights the impact of rapid temperature changes on plant physiology and the potential acclimatisation mechanisms under rapid heat stress. Future research should focus on exploring the adaptive mechanisms involved in distinguishing mutants to improve crop resilience against environmental stressors.

Does the Use of Perennials in Flower Beds Necessarily Imply Sustainability?

Green spaces are becoming increasingly important for cities due to the growing pressures of urbanization and climate change. Along with trees, shrubs, and lawns, flower beds are an important part of urban green spaces. The majority of flower beds in public spaces consist of annual and biennial flower species. Such seasonal flower beds feature eye-catching colors but require significant effort to maintain and manage. Compared to these conventional flower beds, those with herbaceous perennials are more ecologically effective and less costly to maintain, and therefore more sustainable. The aim of this research was to analyze flower beds with perennials in the public green spaces of the city of Zagreb and to develop a tool based on predefined criteria and indicators to evaluate the sustainability of flower beds. In the context of the research, sustainability meant appropriate selection of flower species based on environmental conditions (temperature, light, precipitation), species diversity, greater ground cover and extensiveness of maintenance. The research results showed that there were 327 flower beds with perennials planted in the ground. The constructed Flower Bed Sustainability Index (FBSI) showed that the majority of these perennial beds (56.3%) had a conventional character, as only 28.1% of the beds had a completely correct species selection. This result indicates that the use of perennials does not necessarily guarantee the sustainability of flower beds, since, as in the case of flower beds with seasonal flowers, it depends, among other things, on the correct selection of species adapted to local environmental conditions. The FBSI is shown to be a suitable tool for assessing the degree of sustainability of a flower bed and could be a useful tool in landscape design and management of such types of green spaces.

Method for selecting ornamental species for different shading intensity in urban green spaces

In urban areas, ornamental plants face different constraints, such as the shading of buildings and trees. Therefore, the selection of suitable species and their integration or combination with pre-existing plants is very important. Trees, shrubs, and herbaceous plant species must be distributed according to plant light requirements and shading intensity. Ornamental plants are classified into two groups based on their light intensity or shade tolerance: sun and shade species. To properly position the plants, especially in the immediate vicinity of buildings, it is necessary to study the projection of shadows during the year and the most critical periods, such as July and August. The position of ornamental species with different shading tolerances can be obtained by characterizing the leaf gas exchange for each species. Among the physiological parameters, the most important is the light compensation point, which is the light intensity corresponding to a net photosynthesis equal to zero. This means that the assimilation of carbon dioxide through photosynthesis is equal to the carbon dioxide emitted by respiration. This steady state represents the most critical condition for plants to endure the summer. The distribution of species inside a green area should be determined by considering the minimum light intensity that allows sufficient photosynthesis to compensate for the respiration rate. In this context, non-destructive leaf gas exchange, chlorophyll a fluorescence, and chlorophyll content can be useful tools for selecting suitable ornamental plants under diverse shading conditions.

Morphological performance and seasonal pattern of water relations and gas exchange in Pistacia lentiscus plants subjected to salinity and water deficit

Soil water deficit and salinity represent a major factor impacting plant survival and agricultural production. The frequency and severity of both abiotic stresses are expected to increase in a context of climate change, especially in arid and semi-arid regions. This work studied the growth pattern, biomass and mineral distribution and the seasonal pattern of water status, photosynthetic rate and stomatal conductance in plant of Pistacia lentiscus grown under different levels of water deficit and salinity. P. lentiscus plants growing under greenhouse conditions were subjected to four irrigation treatments during 11 months: control (C, 1 dS m-1), moderate water deficit (MW, 1dS m-1, 60% of the control), severe water deficit (SW, 1 dS m-1, 40% of the control) and saline (S, 4dS m-1). The results show that Pistacia lentiscus plants were more affected by deficit irrigation than salinity. Deficit irrigation and salinity inhibited plant height, with reductions of 20%, 22% and 35% for S, MW and SW, respectively. Total leaf area was not modified by effect of the treatments, with the result that plant compactness increased in MW. The salt stressed plants only showed lower relative growth rate at the end of the experiment. Plants responded to saline or drought stress by increasing their osmotic adjustment, which was more pronounced under salinity. Saline plants had the highest values in Na+ and Cl- ions and the lowest values for K+/Na+ and Ca2+/Na+ ratios in leaves and stems, which is correlated with a decrease in growth, stomatal conductance, photosynthesis and stem water potential, and can be used as a diagnostic tool to assess plant tolerance to salinity stress. As a measure of plant hydration, relative water content was more sensitive to deficit irrigation than salinity, being a good indicator of water stress. P. lentiscus plants subjected to both deficit irrigation treatments exhibited an increase in their intrinsic water use efficiency, which is an important adaptation for plants growing in environments with water scarcity.