Irrigation and Phosphorus-driven adaptations in Rosemary and Thyme: a comparative morphophysiological and biochemical assessment

This study compares the adaptive response of rosemary (Rosmarinus officinalis) and thyme (Thymus vulgaris) to the combined effects of irrigation regimes and phosphorus (P) levels on morphological, physiological, and biochemical stress indicators, nutrient uptake, and essential oil yield. Both species were subjected to well-watered regime (80% field capacity, I1), moderate drought (50% FC, I2), and severe drought (30% FC, I3). Three P levels were applied for rosemary (10, 25, and 50 kg/ha as P1, P2, and P3) and thyme (10, 20, and 40 kg/ha as P1, P2, and P3), under greenhouse conditions. Rosemary employed a flexible growth strategy. It reallocated resources between shoots and roots depending on the levels of irrigation and P. Under I3, shoot biomass declined by 41%, but root elongation increased, especially in I3P3. Phosphorus improved leaf area index (LAI) and chlorophyll pigments, but its effect was constrained under I3. Chlorophyll a dropped to 0.5-1.0 mg g⁻¹ FW under I3, accompanied by a 150% rise in hydrogen peroxide (H₂O₂) and a 36% increase in glycine betaine in I3P3. Thyme displayed a more conservative strategy. Under I3, shoot biomass decreased by 28%, but LAI and plant height stayed stable. Pigments were better preserved under I3P3 compared to rosemary. However, thyme recorded higher lipid peroxidation, with malondialdehyde (MDA) increasing by 28% compared to I2. Nutrient uptake patterns differed, rosemary absorbed significantly more nitrogen, phosphorus, and potassium (up to 316 mg N, 22 mg P, and 341 mg K per plant under I1P1) and phosphorus use efficiency (PUE), peaking at 9 g biomass mg⁻¹ P applied. In rosemary, essential oil yield increased by 37% under I2P2, while thyme peaked under I3P2, with a 29% increase. These findings illustrate that R. officinalis and T. vulgaris used different adaptive responses to irrigation and phosphorus. Rosemary exhibits enhanced morphological plasticity and nutrient use efficiency, facilitating increased biomass and essential oil yield under moderate stress, whereas thyme sustains physiological stability through conservative resource utilization. This comparative study emphasizes the importance of species-specific irrigation and phosphorus control in increasing the resilience and productivity of aromatic crops.