Plant water variables and reproductive traits are influenced by seasonal climatic variables in Prosopis burkartii (Fabaceae) at Northern Chile

- Flower number is negatively correlated with minimum air temperature at the end of winter.
- Midday water potentials during spring are related to flower production.
- Frost and drought periods are critical for maintenance of plant hydraulics and flowering.

Prosopis burkartii is a critically endangered phreatophyte legume endemic to the Atacama Desert of northern Chile where only 50 individuals remain. We studied seasonal water variables and flower parameters in a population of 15 P. burkartii individuals. For each tree, xylem water potential (Ψ) was estimated by measuring two twigs with a Scholander pressure bomb. Each month, we made 3 measurements of xylem Ψ in each tree: pre-dawn (Ψpd), midday (Ψmd), and late-afternoon (Ψla). We also measured pre-dawn hydraulic conductivity (K) and the percent loss of hydraulic conductivity (PLC) on the same trees. Flower parameters were estimated using tagged twigs. The total number of flowers produced by each plant, the number of pods per plants, number of fertile pods and the percentage of abortion of flowers and pods were estimated. Values of Ψ ranged from −2.7 ± 0.1 (July) to −0.8 ± 0.2 MPa (September). On a daily basis, individuals had maximal Ψ at pre-dawn and minimal Ψ at midday, with a recovery of Ψ during the late-afternoon likely due to the low evaporative demand at that time. Furthermore, the native conductance of the stems (Ki) had inverse trends in comparison with PLC data. Number of flowers per plant was strongly correlated with Ψmd estimated during October. During the study period, we observed that Ψmd had higher values during the spring and at the beginning of summer, indicating maintenance of a favorable plant water status during the season of flowering at the time of day when thermal stress is greatest. The differences in PLC suggest that individuals of P. burkartii can maintain functional xylem conduits during summer droughts and extract water from the soil while maintaining high values of midday Ψ, the period of the day with the greatest stress.