Photosynthetic characteristics of ornamental passion flowers grown under different light intensities

Responses of leaf gas exchange, fluorescence emission, chlorophyll concentration, and morpho-anatomical features to changes in photosynthetic photon flux density (PPFD) were studied in three wild ornamental species of Passiflora L. to select sun and shade species for landscaping projects. Artificial shade was obtained with different shading nylon nets, under field conditions, which allowed the reduction of 25, 50, and 75% of global radiation, along with a control treatment under full sunlight. For Passiflora morifolia the highest mean values of light-saturated net photosynthetic rate (P _Nmax) and light compensation point (LCP) were observed at 50 and 25% shade, respectively, while the highest values of dark respiration rate (R _D) and apparent quantum yield (α) were observed at 75% shade. For Passiflora suberosa litoralis the highest value of P _max was observed at full sunlight. The highest mean values for P _max, R _D, and LCP for Passiflora palmeri var. sublanceolata were obtained at 25% shade. The highest values of net photosynthetic rate (P _N) for P. morifolia, P. palmeri var. sublanceolata, and P. suberosa litoralis were 21.09, 16.15, and 12.36 μmol(CO₂) m⁻² s⁻¹, observed at 50 and 75% shade and full sunlight, respectively. The values of the minimal chlorophyll fluorescence (F₀) were significantly different in P. suberosa litoralis and P. palmeri var. sublanceolata, increasing with the increase of the irradiance. In contrast, the values of maximum photochemical efficiency of PSII (F_v/F_m) were significantly different only in P. suberosa litoralis, being higher at 75%, progressively reducing with the increase of PPFD levels. The total concentration of chlorophyll (Chl) was higher in shaded plants than in the ones cultivated in full sunlight. On the other hand, the values of Chl a/b ratio were reduced in shaded plants. A significant effect of shade levels on leaf area (LA) and specific leaf area (SLA) was found for the three species, whose highest mean values were observed at 75% shade. The thickness of foliar tissues was significantly higher for the three species at full sunlight and 25% shade. These results suggested that P. morifolia and P. palmeri var. sublanceolata appeared to be adapted to moderate shade conditions. P. suberosa litoralis presented higher plasticity to greater variation of the irradiance levels, while the photoinhibition was one of the limiting factors for this species at full sunlight.