Eficiencia de reabsorción de nitrógeno y fósforo y sus relaciones estequiométricas durante la senescencia en especies del Desierto Sonorense

Antecedentes: Nitrógeno (N) y Fósforo (P) son los nutrientes más limitantes en plantas y su eficiencia de reabsorción sugiere estrategias de conservación. La reabsorción además afecta los cocientes estequiométricos en la hojarasca, modificando las interacciones ecológicas y los ciclos biogeoquímicos en gradientes de aridez. Hipótesis: Existirá mayor eficiencia de reabsorción de nutrientes en especies de sitios con mayor aridez, por lo que esperamos que los cocientes estequiométricos de C:N y C:P sean mayores en hojas senescentes. Especies de estudio: Ipomoea arborescens, Fouquieria macdougalii, Cercidium microphyllum, Encelia farinosa, Mimosa laxiflora, Jatropha cardiophylla, Cenchrus ciliaris, Olneya tesota, Prosopis velutina, Cercidium floridum, Fouquieria splendens, Brongniartia minutifolia, Jatropha cordata, Colubrina viridis, Larrea tridentata y Cercidium praecox. Sitio de estudio y fechas: Tres sitios en un gradiente de aridez del Desierto Sonorense, en la región central de Sonora. Agosto - noviembre 2017 y 2018. Métodos: Se obtuvieron la eficiencia de reabsorción de N y P, y cocientes estequiométricos C:N, C:P y N:P en hojas verdes y senescentes de especies, comparando tipos funcionales y sitios. Resultados: La eficiencia de reabsorción disminuyó con el incremento de aridez así como los cocientes estequiométricos C:N y C:P en hojas senescentes y N:P en leguminosas. Conclusiones: La eficiencia de reabsorción no sugiere una estrategia de conservación de recursos, sin embargo, las diferencias entre tipos funcionales permitieron diferenciar estrategias ecológicas y estequiométricas, en particular las leguminosas, que ayudan a resaltar su influencia en la biogeoquímica de las zonas áridas Sonorenses y posiblemente del país.

Irrigation Regimens Differentially Affect Growth and Water Use Efficiency of Two Southwest Landscape Plants

Growth and water use efficiency (WUE) of two, common Southwest landscape plants, red bird of paradise (Caesalpinia pulcherrima L.) and blue palo verde (Cercidium floridum Benth. Ex A. Gray), were studied in response to three irrigation regimens (frequent, moderate, and infrequent) that mimicked a range of residential landscape watering practices in Phoenix, AZ. During 50 to 58 and 138 to 147 days after the start of irrigation treatments (DAT), mid-day measurements of shoot water potential (Ψ), osmotic potential (Ψ0), and gas exchange were made. Concurrently, diurnal measurements of whole plant transpiration (T) and estimates of dry weight accrual were made to calculate WUE. More frequent irrigations increased shoot length of both species and dry weight of Cercidium. For both species, Ψ and Ψ0 showed patterns of osmotic regulation as the substrate dried between watering events for moderately and infrequently irrigated plants. Infrequently irrigated Caesalpinia and Cercidium had the lowest WUE, except for 138 to 147 DAT during which time infrequently irrigated Cercidium had the highest WUE. Instantaneous transpiration efficiency (ITE) was negatively correlated to the ratio of intracellular to ambient CO2 (Ci/Ca) in all treatments, suggesting that under more frequently irrigated conditions, WUE of Caesalpinia and Cercidium might be reduced by negative feedback effects of high Ci/Ca ratios on stomatal conductance.

432 Irrigation Frequency Affects Growth and Water Use Efficiency of Two Xeriphytic Landscape Plants

Growth and water use efficiency (WUE) of two Southwest landscape plants under various regimes of irrigation frequency was studied in a greenhouse experiment. Red bird of paradise (Caesalpinia pulcherrima L.) and blue palo verde (Cercidium floridum Benth. ex A. Gray) were grown at three levels of irrigation frequency intended to mimic a range of watering practices determined via survey data from the Phoenix, Ariz., metropolitan area. During two irrigation cycles, measurement of mid-day water and osmotic potentials, lysimetric whole-plant transpiration (T), and mid-day shoot gas exchange was made. Irrigation frequency treatments affected Cercidium more than Caesalpinia. Frequent irrigations increased Cercidium shoot length and dry weight. For both species, infrequently irrigated plants showed patterns of osmoregulation in response to drying soil. Transpiration (T) was consistently highest for infrequently irrigated plants. WUE was affected by treatment for Cercidium, but not Caesalpinia. Gas exchange was unrelated to plant growth or T. Instantaneous transpiration efficiency (ITE) was negatively correlated to the ratio of intracellular CO2 to ambient (CICA) in all treatments, suggesting that under well-watered conditions, WUE might be reduced by negative feedback effects of high internal CICA ratios.

Below Ground Pot-in-Pot Effects on Growth of Two Southwest Landscape Trees was Related to Root Membrane Thermostability

Two southwestern desert landscape trees, Acacia smallii L. (sweet acacia) and Cercidium floridum Benth. ex A. Gray (blue palo verde), were grown outdoors in full sun during Summer 1997 in 19-liter (#5) containers placed either pot-in-pot (PIP) below ground or unshielded in above-ground containers (AGC). Soil moisture sensors wired to electronic solenoid valves regulated occurrence of six cyclic micro-irrigation pulses per day (0600, 0900, 1200, 1500, 1800, and 2100 HR) such that container substrate moisture tensions were continuously maintained between −0.005 to −0.01 MPa (90% of water holding capacity) in both PIP and AGC. Mean maximum recorded root-zone temperatures in PIP containers were 19C (34F) lower than for AGC. Micro-irrigation volumes were 40% less for trees grown PIP compared with those in AGC. Growth of sweet acacia was enhanced by PIP placement while in containers and one year after transplanting trees into field plots in 1998. Only caliper growth of blue palo verde was increased by PIP placement while in containers, but had no effect on blue palo verde growth one year after transplanting into field plots. The critical killing temperature (TM) for root tissues of sweet acacia and blue palo verde were 45.3 ± 1.8C (113.5 ± 3.2F) and 49.4 ± 0.8C (120.9 ± 1.4F), respectively, indicating differences in root membrane thermostability. Based on our data, we suggest that sweet acacia trees benefitted from PIP placement more than blue palo verde trees because root-zone temperatures in PIP containers were lower than for AGC in central Arizona, and sweet acacia roots were more susceptible to injury by supraoptimal root zone temperatures.

Use of Crop Water Stress Index to Schedule Irrigation of Freeway Landscape Plants

A study was conducted to determinethe feasibility of using crop water stress index (CWSI) to schedule irrigation of eight species of freeway landscape plants, Acacia redolens B.R. Maslin, Acacia salicina Lindl., Caesalpinia pulcherrima Sw., Cassia nemophila A. Cunn. ex Vogel, Cercidium floridum Benth., Eucalyptus microtheca F.J. Muell., Nerium oleander L., and Prosopis chilensis Mol. Nerium oleander and C. pulcherrima were suited to the use of the CWSI, tolerated repeated exposures to CWSI values of 0.6, and remained aesthetically acceptable. Irrigation of N. oleander via the CWSI resulted in a 19% reduction in water use, compared to the conventional method. CWSI data of other species were too variable, and, thus, irrigation could not be scheduled by CWSI values. Variability was attributed, in part, to lack of a dense canopy, which is necessary to fill the view of the infrared thermometer.