As established above, ornamental plants used in urban and peri-urban green areas should be adapted to shade and light conditions. Green shaded areas can be generated by buildings or other plants. As a general guide, shade-tolerant plants should be used in shaded areas and sun-tolerant plants in areas of full sunlight exposure. However, shade is not constant throughout the day or during the different seasons. Therefore, it is important to identify the shade intensity of trees and buildings around green areas will be planned. The correct selection of plants in the area should be carried out by matching the light compensation point of each species with the light intensity and photoperiod throughout the daily and seasonal cycles. Figure 1 illustrates the distribution of plants with different light compensation point values.

In summer, more extensive plant canopies and higher temperatures can reduce photosynthetic activity and increase respiration, resulting in a negative net photosynthesis. Under these conditions, plants used more energy than they accumulated over 24 h. Therefore, the selection of plants should account for this, ensuring that the amount of light is sufficient to compensate for the respiration rate within a 24 h interval. The problem is less acute in spring, as plants typically do not show symptoms of shading’ however, with an increase in temperature, plants start to suffer from limited light conditions.

Chlorophyll a fluorescence is a non-destructive measurement that allows the evaluation of light use efficiency, leaf functionality, and the overall leaf health status, under optimal or stress conditions. Chlorophyll a fluorescence is widely used to establish stressful conditions in plants. For example, it has been used to measure the chlorophyll a fluorescence in Ficus elastica Roxb. ex Hornem., Wisteria sinensis (Sims) DC., Hedera helix L., Quercus robur L.

The results showed that F. elastica had the highest induction curve, whereas Q. robur had the lowest. W. sinensis had a similar trend F. elastica and H. helix to Q. robur ( Figure 1 ).

The JIP test allows the calculation of several indices from the data obtained from the data points (Tsimilli-Michael, 2020). These indices can help identify species with the highest variability. The performance index (PI), time to reach the maximal fluorescence (Tfm), and reaction centers per PSII antenna Chl a (RC/ABS) are the parameters that could greatly show the differences in plants under different light conditions ( Figure 2 ). An expanded list of the JIP indices is provided in Supplementary Table 1 .

Plants can adapt to suboptimal conditions. In light compensation points of ornamental plants under low- or high-light conditions are reported in Table 1 . Plants can alter their light compensation points by adapting to various environments. For example, Pseudomonas scutellarioides R. Br. reported an increase in the light compensation points in plants grown under high-pressure sodium lamps (HPS) or light-emitting diodes (LEDs) (Domurath et al., 2012). Lower compensation points are required for plants used in indoor environments, such as Philodendron erubescens K. Koch & Augustin and Dracaena surculosa Lindl. (Tan et al., 2017), whereas an analogous experiment simulating indoor conditions by lowering the light intensity resulted in reduced light compensation points in Leea coccinea Planch and L. rubra Blume ex Spreng. (Sarracino et al., 1992). Different light compensation points can also be observed in the same plant under different light-exposure conditions (Zhao et al., 2012). Leaves of Paeonia lactiflora Pall. exposed to the sun showed double the light compensation point compared with those under shade. This information can be useful when selecting plants for use in shaded locations, as adaptation may be used to lower the light composition of a species at the nursery level by applying different shading nets.

Aeschynanthus longicaulis Wall. ex R.Br., the morphological and photosynthetic responses were measured under high or low irradiance (Li et al., 2014). Under high irradiance, the photosynthetic photon flux density (PPFD) was maintained under 650 µmol·m⁻²·s⁻¹ by internal and external shading of the greenhouse. The light intensity of the low irradiance treatment was maintained under 150 µmol·m⁻²·s⁻¹ by extra shading on the bench (Li et al., 2014). The LCP of Athyrium pachyphlebium C. Chr. at four different shade ranges was 813 μmol m^–2 s^–1 to 886 μmol m^–2 s^–1, 576 μmol m^–2 s^–1 to 633 μmol m^–2 s^–1, 335 μmol m^–2 s^–1 to 402 μmol m^–2 s^–1, 134 μmol m^–2 s^–1 to 175 μmol m^–2 s^–1 and 1,846 μmol m^–2 s^–1 to 1,914 μmol m^–2 s^–1. The LCP under the highest light regime was 33 μmol m^–2 s^–1 to 16 μmol m^–2 s^–1 (Huang et al., 2011). Similar study was performed on Passiflora morifolia Mast., P. suberosa subsp. litoralis (Kunth) Port.-Utl. ex M.A.M.Azevedo, Baumgratz and Gonç.-Estev., and P. palmeri var. sublanceolata Killip (Pires et al., 2011). These ornamental plants showed different LCPs ranging from 36 μmol m^–2 s^–1 to 21 μmol m^–2 s^–1 under reduced light intensities of 25%, 50%, and 75% solar radiation for evaluating the adaptation ability (Pires et al., 2011). Artificial shade was provided using different shading nylon nets, fixed in wooden frames with dimensions of 5 m × 5 m × 2 m, under field conditions, which allowed a 25% reduction (1,000 μmol m^–2 s^–1–1,400 μmol m^–2 s^–1, min–max), 50% reduction (600 μmol m^–2 s^–1 –900 μmol m^–2 s^–1), and 75% reduction (200 μmol m^–2 s^–1 –400 μmol m^–2 s^–1) of solar radiation, along with a control treatment under full sunlight (1,500 μmol m^–2 s^–1–1,900 μmol m^–2 s^–1). The reduced light intensity by 75% allowed the LCP reduction in all three species and the highest plasticity was observed in P. suberosa subsp. litoralis, and P. palmeri var. sublanceolata (Pires et al., 2011). The shading effect of LCP and plant growth were studied in Ficus benjamina L. grown under five shading conditions during two growing periods (Scuderi et al., 2012). The control (0% shading) and 80% shading levels had light intensities ranging from 880 μmol·m⁻²·s⁻¹ to 142 μmol·m⁻²·s⁻¹ in the first growing period and from 1,531 μmol·m⁻²·s⁻¹ to 173 μmol·m⁻²·s⁻¹ in the second growing period. Under these shading regimes, the LCP of F. benjamina was 17 μmol m^–2 s^–1 and 5.9 μmol m^–2 s^–1 (Scuderi et al., 2012).

Two species, Physocarpus amurensis (Maxim.) Maxim. and P. opulifolius (L.) Maxim. were exposed to low (100 μmol m^–2 s^–1) or high light intensity (1,000 μmol m^–2 s^–1 –1,500 μmol m^–2 s^–1), and their LCP respected the two conditions. The LCP values were 26 μmol m^–2 s^–1 and 28 μmol m^–2 s^–1 for P. amurensis and P. opulifolius, respectively, under high-irradiance conditions. At low irradiance, both species have an LCP of 20 μmol m^–2 s^–1 (Zhang et al., 2016).

Shade and sun exposure conditions were studied in Calophyllum inophyllum L. (Clusiaceae), Inga spectabilis (Vahl) Willd. (Fabaceae), and Ormosia macrocalyx Ducke (Fabaceae). C. inophyllum and I. spectabilis under shade showed a LCP three times lower than plants exposed to sun. O. macrocalyx only halved the LCP when exposed to shade (Slot et al., 2019).

Several plants have been adapted to indoor conditions to evaluate the ability of some species to remove carbon dioxide from the environment (Treesubsuntorn and Thiravetyan, 2018). The LCP of these species ranges from 10 µmol·m⁻²·s⁻¹ to 15 µmol·m⁻²·s⁻¹ (Torpy et al., 2017).

Based on experimental or literature information (Ignatieva, 2021), plants can be planted in various green areas. If we consider plants with LCP 25 µmol·m⁻²·s⁻¹, 50 µmol·m⁻²·s⁻¹, and 75 µmol·m⁻²·s⁻¹ ( Figure 3A ) the possible planting position/zone in the area mapped for the shade/light intensity are reported in Figure 3B .

Considering the information reported in Table 1 in the shade area with light intensity below 10 µmol·m⁻²·s⁻¹ can be planted the Heuchera americana L. In areas comprised between 10 and 15 can be planted species such as Chlorophytum comosum (Thunb.) Jacques, Epipremnum aureum (Linden ex André) G.S.Bunting, Ficus benjamina L., and Gibasis Raf. spp. In areas with higher light intensities, the number of species that can be used greatly increased ( Table 1 ). Of course, species must be selected based on the temperature of the geographical area in winter and these temperatures must be compatible with species tolerance.

Ornamental plants must be maintained under optimal nutrition and water conditions. The light saturation curve and light compensation point should be measured under specific light intensities that should be set from 1,000 µmol m⁻² s⁻¹ to 1,600 µmol m⁻² s⁻¹ for sun plants, 100 µmol m⁻² s⁻¹–200 µmol m⁻² s⁻¹ for shade plants and of 500 µmol m⁻² s⁻¹–700 µmol m⁻² s⁻¹ for intermediate conditions. The plasticity of plants to adapt to low light regimes must be evaluated by transferring plants under shading and reducing the light intensity using nets with a reduction of 25%, 50%, or 75% solar radiation. Measurements should be carried out after one week of adaptation.

The light saturation curve and light compensation point of a plant can be measured using a leaf gas exchange analyzer. This instrument allows for the identification of the light compensation point and light intensity that allows the achievement of the highest net photosynthesis. The light saturation curve can be determined by measuring the CO₂ assimilation by decreasing the light intensity from 1,600 μmol m⁻² s⁻¹ to 0 μmol m⁻² s⁻¹. The CO₂ concentration should be at ambient level (400 µL L⁻¹). Readings were taken between 9:30 A.M. and 11:30 A.M. from the fully expanded leaves of the three plants.

Chlorophyll a fluorescence can be measured using a portable fluorimeter that allows the determination of plant health status, light-use efficiency, and plant adaptability to different light conditions. The chlorophyll a fluorescence induction curve was measured using a Handy Plant Efficiency Analyzer (PEA, Hansatech, UK). Chlorophyll a fluorescence can be measured in leaves that must be dark-adapted for 30 min by using a leaf clip (4 mm diameter). A rapid pulse of high light intensity of 3,300 μmol m⁻² s⁻¹ (600 W m⁻²) was emitted on the leaf area shaded by the leaf clip ( Figure 4 ). The sensor recorded the leaf fluorescence. Chlorophyll a fluorescence parameters, such as Fo (minimal fluorescence), Fm (maximum fluorescence), Fv (variable fluorescence), and Fv/F_M ratio (maximum quantum efficiency of photosystem II), were automatically calculated. JIP analysis can be performed to determine several indices related to light use efficiency and plant adaptation ( Supplementary Table 1 ).

Based on the data collected regarding the light compensation point, it was possible to distribute the plants in the green areas. Light availability at the planting position must be higher than the light intensity of the light compensation point under the worst conditions, such as in summer with high temperatures.