Terry L. Hartsock

Temperature, water, and PAR influences on predicted and measured productivity of Agave deserti at various elevations

SummaryTo measure productivity of Agave deserti over its elevational range in the northwestern Sonoran Desert, leaf unfolding from its basal rosette was monitored on groups of 10 plants at 13 sites. Based on data from an intermediate elevation (840 m), leaf unfolding proved to be highly correlated (r2=0.88) with an environmental productivity index (EPI) formed as the product of indices for water status, temperature, and photosynthetically active radiation (PAR); each of these latter indices indicated the fraction of maximum net CO2 uptake expected for that parameter based on laboratory measurements of gas exchange and field microclimatic data. At 840 m, the main environmental variable influencing leaf unfolding for A. deserti over a 2-y period was soil water potential. On steep slopes, however, leaf unfolding during the winter ranged from 0.7 leaves per 10 plants for north-facing slopes to 7.3 for south-facing slopes, reflecting the importance of PAR. Summer and winter rainfall increased 3-fold from elevations of 300 m to 1,200 m. Temperatures were more optimal for net CO2 uptake at high elevations in the summer and at low elevations in the winter. Hence EPI increased with elevation in the summer but was maximal at an intermediate elevation in the winter. Moreover, measured leaf unfolding in both the summer and the winter closely followed the changes in EPI with elevation, indicating that productivity could be closely predicted for A. deserti based on physiological CO2 responses and changes in environmental conditions with elevation.

Ecophysiology of Yucca brevifolia, an arborescent monocot of the Mojave Desert

SummaryPhotosynthetic characteristics and transpiration of Yucca brevifolia, an evergreen tree endemic to the Mojave Desert of California and Nevada, were examined in the field and the laboratory. Yucca brevifolia was confirmed to be a C3 plant, with no CAM tendencies observed for its semi-succulent leaves. The species exhibited a maximum net CO2 uptake of 12 μmol m-2 s-1 at 22°C when grown at day/night air temperatures of 31°C/17°C (data expressed on a total area basis for these opaque leaves). The optimum temperature for CO2 uptake shifted 4.5°C per 10°C change in daytime growth temperature, so that observed leaf temperatures in the field were near optimum temperatures throughout the midday period in all but the hottest months of the year. Leaves also acclimated to low and high temperature extremes, tolerances ranging to-11°C and to 59°C, respectively, suggesting that low temperatures limit the distribution of Y. brevifolia but high temperatures do not. Light saturation of photosynthesis occurred at a relatively low PAR of about 500 μmol m-2 s-1, similar to the actual PAR within a rosette. Diurnal patterns of leaf conductance shifted from a broad midday peak in wet seasons to a reduced, narrow, early morning peak in the dry season, indicating effective stomatal control of water use. The approximately 5-month-long winter-spring growth season accounted for 80% of the yearly CO2 uptake, with a predicted annual uptake of about 22 mol m-2 y-1 and a transpiration ratio of 700.

Influence of nitrogen and other nutrients on the growth of Agave deserti

Abstract Although agaves are commercially important worldwide and ecologically interesting, their nutrient responses have not been extensively studied under controlled conditions. Here, nutrient responses of seedlings and adult plants of Agave deserti were examined, with particular emphasis on nitrogen. Growth of seedlings in hydroponics was enhanced by increasing potassium, phosphate, and especially nitrate. Seedling growth in sand culture was also enhanced by adding nitrate, leading to just over 2% N by dry weight in the leaves. Seedlings had optimal growth in soil having about 0.1% N by dry weight and a pH between 6 and 8. In going from irrigation with no added nutrients to full‐strength Hoagland solution for mature plants in soil, leaf unfolding (a non‐destructive measure of productivity) approximately doubled. The rate of leaf unfolding in the field was also doubled by adding 100 kg N hectare‐1, higher levels proving inhibitory.

Light effects on leaf development and photosynthetic capacity of Hydrocotyle bonariensis Lam.

Rates of net CO2 uptake were examined in developing leaves of Hydrocotyle bonariensis. Leaves that developed under high photosynthetically active radiation (48 mol m-2 day-1 PAR) were smaller, thicker, and reached maximum size sooner than did leaves that developed under low PAR (4.8 mol m-2 day-1). Maximum net CO2 uptake rates were reached after 5 to 6 days expansion for both the low and the high PAR leaves. Leaves grown at high PAR had higher maximum photosynthetic rates and a higher PAR required for light saturation but showed a more rapid decline in rate with age than did low PAR leaves. To assess the basis for the difference observed in photosynthetic rates, CO2 diffusion conductances and the mesophyll surface available for CO2 absorption were examined for mature leaves. Stomatal conductance was the largest conductance in all treatments and did not vary appreciably with growth PAR. Mesophyll conductance progressively increased with growth PAR (up to 48 mol m-2 day-1) as did the mesophyll surface area per unit leaf area, but the cellular conductance exhibited most of its increase at low PAR (up to 4.8 mol m-2 day-1).

Zinc, nickel, and cadmium uptake and translocation to seed pods and their effects on gas exchange rates of bush bean plants grown in calcareous soil from the Northern Mojave Desert.

Abstract Bush bean plants (Phaseolus vulgaris L. cv. Improved Tendergreen) were grown in highly calcareous desert soil with high levels of Zn, Ni, and Cd added to them. The purposes were to compare growth and uptake with desert plants grown in the same soil, to assess translocation to fruits, and to measure gas exchange rates. All three trace metals produced toxicity in the highly calcareous soil and the bush beans were less tolerant than previously grown desert plants. The levels used at various points produced 50% or more yield decrease. Zinc and Ni were transported to fruits at concentrations considerably less than in leaves. The transport was greater for Ni than for Zn. Cadmium was not translocated to fruits in detectable quantities. Zinc and nickel supplied in excess restricted Cu movement to fruit. The levels of Zn and Ni had no influence on photosynthesis and transpiration, but the gas exchange rates were decreased by Cd on the dry weight basis.