Morphological, physiological, and biochemical changes of herbaceous ornamental plants subjected to drought stress and recovery

Drought episodes, which are very frequent in the Mediterranean region, have negative effects on plant growth. To reduce irrigation, which is no longer sustainable today, it is necessary to identify plants capable of resisting water shortage. The aim of the experiment was to investigate the eco-physiological and biochemical changes in three herbaceous plants, Dimorphotheca hybrida , Pelargonium peltatum , and P. hybridum , which are widely adopted for ornamental purposes in Mediterranean gardening. Their ability and the mechanisms they adopt to recover after water stress, following periodic suspension and restoration of the water supply, were studied. Three water regimes were evaluated: control plants (100% WC) and drought-stressed plants (60% and 40%WC). After one week of submission to drought, stressed plants were re-watered to 100% WC and then recovered for seven days. Subsequently, the plants were subjected to drought exposure for 15 days. The plants, after the second cycle of suspension of water, were kept in the same conditions of control plants for seven days. Plant growth, leaf gas exchanges, chlorophyll a fluorescence, relative water content, chlorophylls and carotenoids, proline, and malondialdehyde content were determined. The total dry biomass was influenced by species and drought treatment, but among the experimental factors there is not interaction. Under severe water stress (40% WC), all species showed significant reductions in total leaf area and leaf number. The greatest decrease in total leaf area was observed in D. hybrida (−41%), followed by P. peltatum (−27%) and P. hybridum (−23%). Leaf number decreased by 11% in D. hybrida , 14% in P. peltatum , and 32% in P. hybridum . The effect of treatments on leaf RWC at different times during the experiment determined values similar to those of the control plants in P. hybridum , and reduced during the second drought event in P. peltatum and D. hybrida . At the end of the trial, net photosynthesis under the most severe stress (40%WC) was significantly reduced in all species by −64% D. hybrida, −76% P. peltatum , −70% P. hybridum . The Fv/Fm ratio in P. peltatum and D. hybrida showed significant differences only during the second drought stress period. By the end of the second re-watering period, the stressed plants exhibited values similar to those of the control plants. Results demonstrated that photosynthetic parameters were the most affected by antioxidants and osmolytes, such as proline. Considering all parameters, the species were ranked according to their drought tolerance as follows: P. hybridum > D. hybrida = P. peltatum , highlighting that each species exhibited distinct adaptations. These findings indicate that the combined evaluation of physiological and biochemical parameters under repeated drought and recovery conditions can enhance the selection of ornamental species better suited to low-water urban landscapes.