RESUMO
The gas exchange and water relations of two Sonoran Desert plants were measured throughout a 12 month period. Seasonal photosynthesis patterns of both species followed the seasonal variation in soil and plant water potential. Acacia greggii, a winter deciduous shrub, appears to be fall active since the day-long mean photosynthesis rate was maximum, i.e., 16 mg CO2 dm-2 h-1, at this time. Cercidium microphyllum, a drought deciduous and chlorophyllous-stemmed tree, also appears to be mainly fall active. For this species the day-long mean photosynthesis rate was not in excess of 14 mg CO2 dm-2 h-1. Both species initiate leaf production in the spring and neither experiences severe plant water stress. Seasonal minima of dawn plant water potential were-44 and-32 bars for the shrub and tree species, respectively. The two species differ slightly in their tolerance of heat and water stress, since foliated plants of Acacia greggii maintain summer gross photosynthesis.All of the aboveground organs for plants of C. microphyllum are capable of exogenous 14CO2 assimilation. This species appears to be unique in the magnitude of the photosynthetic production contributed by stems. Seasonal production by stems, leaves and flowers/fruits averaged 72, 24 and 4% of the total carbon gain per tree, respectively. Aboveground gross primary production was over 4.5-fold greater than aboveground net primary production. This difference between these two production estimates is likely due to the very small foliar biomass maintained throughout the year and the energy expenditure required to maintain the metabolically active cells of the chlorophyllous stems.
RESUMO
The gas exchange and water relations of two Sonoran Desert plants are compared during contrasting periods of water and heat stress. Photosynthesis of Acacia greggii, a winter deciduous shrub, and Cercidium microphyllum, a chlorophyllous stemmed tree, show a moderate correlation with dawn plant water potential. For both species a relationship between stomatal conductance and dawn plant water potential was not apparent, although A. greggii demonstrated a greater overall stomatal conductance. This affected a greater daytime decrease in plant water potential at all levels of water stress and suggests A. greggii is less sensitive to water stress. Our results suggest the lower limit for gross photosynthesis occurs when dawn plant water potentials are less than -44 and -31 bars for the shrub and tree species, respectively. During periods of extreme water and heat stress the photosynthetic capacity of both species is regulated more by mesophyll than stomatal conductance. However, partial stomatal closure causes plant water potential to increase during the day and exceed dawn values. During periods of minimal water and heat stress the daily course of photosynthesis parallels the change in stomatal conductance and irradiance. Maximum gross photosynthesis rates are nearly three-fold higher than the rates observed during periods of stress, with those of A. greggii generally greater than the rates observed in plants of C. microphyllum.
RESUMO
The gas exchange and water relations of two Sonoran Desert plants was measured throughout a 12-month period. Seasonal photosynthesis patterns of both plants followed the seasonal variation in plant water potential. Ambrosia deltoidea, a drought-deciduous shrub, is mainly winter-spring active since maximum photosynthesis rates of 38 mg CO2 dm-2 h-1 were measured at this time. This plant is characterized by marked seasonal variations in plant water potential, and was deciduous for approximately 120 days when plant water potential was below-50 bars. Olneya tesota, a non-riparian microphyllous tree, is evergreen and photosynthetically active throughout the entire year, although demonstrating maximum photosynthesis rates of 12 mg CO2 dm-2 h-1 in spring and summer. The deep-rooted tree species maintains a favorable year-round water balance since minimum plant water potentials were seldom below-33 bars. The two species maintain a relatively high water use efficiency throughout the year, despite the high evaporative gradient characteristic of the Sonoran Desert.The leaves are the major site for carbon assimilation, contributing 87 and 81% of the annual carbon gain for the shrub and tree species, respectively. Above-ground gross primary production throughout the 12-month period was estimated solely from the leaf 14CO2 assimilation studies. This production estimate was compared to above-ground net primary production determined by the harvest method. For both plant species gross production was interpreted to exceed net production by nearly a three-fold difference. On a per plant basis gross production was estimated to be 1.14 and 7.42 kg dry wt plant-1 yr-1 for A. deltoidea and O. tesota. The large difference between net and gross production is probably related to year-round utilization of carbon.
RESUMO
The gas exchange of two Sonoran Desert plants was measured near optimum soil water conditions occurring in the summer and winter. Photosynthesis and stomatal conductance rates of a drought-deciduous shrub, Ambrosia deltoidea, and an evergreen non-riparian tree, Olneya tesota, are mainly affected by plant water potential. During such periods the diurnal gas exchange patterns are characterized by maximum rates of photosynthesis and stomatal conductance occurring early in the day, which decrease progressively thereafter. The effect of plant water potential on gas exchange is both direct and indirect. Decreasing plant water potential indirectly affects 14CO2 assimilation by closure of stomata, and the effect is similar in both species. However, the direct effects of decreasing plant water potential are dissimilar in the two species. Plants of the shrub species have a higher potential maximum photosynthesis, but are more sensitive to plant water stress than are plants of the tree species. Both species respond to favorable growth conditions in the summer and winter, and have the potential for rapid carbon input into the Sonoran Desert ecosystem.
