Dryland ecosystems impose severe constraints on plant productivity due to intense irradiance, high temperatures, and persistent water scarcity. This study compared seasonal variation in photosynthetic performance, photosystem II (PSII) efficiency, and photoprotective responses between the native thermophilic legume tree Acacia tortilis and the introduced invasive Prosopis juliflora under natural hyper-arid desert conditions. Gas exchange and chlorophyll a fluorescence quenching were assessed alongside leaf pigments, nitrogen content, biomass, and polyphasic OJIP fluorescence transients in winter (February-March) and summer (June-July). A. tortilis consistently exhibited higher net CO2 assimilation rates and maintained water-use efficiency across seasons, whereas P. juliflora showed a pronounced summer decline in both parameters. Chlorophyll fluorescence revealed stable PSII quantum efficiency and photochemical quenching in A. tortilis, with sustained and regulated non-photochemical quenching (NPQ) under summer stress. These traits were consistent with enhanced nitrogen status, pigment contents and leaf mass per area. In contrast, P. juliflora displayed greater photochemical plasticity but suffered structural and functional PSII impairments in summer, linked to reduced NPQ capacity, lower nitrogen, and pigment depletion. OJIP analyses indicated that A. tortilis maintained coordinated intersystem electron transport and efficient overall energy use, resulting in consistently higher energy conservation performance indices (PIABS and PItotal). These findings support the hypothesis that native A. tortilis maintains superior photosynthetic efficiency, pigment stability, and biomass production under extreme summer stress compared with P. juliflora. The results highlight key physiological traits that underpin xerophytic tree resilience and inform strategies for vegetation management and restoration in arid environments.
Ahmad Zia (Sat,) studied this question.