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Notes: Abstract. The potential influence of tissue tolerances to extreme temperatures on distributional limits was investigated for 15 taxa (14 species) of leaf-succulent agaves from the south-western United States and northern Mexico. As a group, the agaves exhibited a moderate low temperature tolerance of – 11°C (based on a 50% inhibition in the number of mesophyll cells taking up a stain, neutral red). However, nearly all of the species were able to tolerate extremely high tissue temperatures of over 60°C. Nocturnal acid accumulation by these crassulacean acid metabolism plants was about 6°C more sensitive to temperature extremes than was cellular membrane integrity.High and low temperature acclimation in response to changing day/night air temperatures was observed in all 15 taxa, with high temperature acclimation averaging two-fold greater than low temperature acclimation (3.8°C versus 2.0°C per 10°C change in ambient temperature). Species occupying the coldest habitats exhibited the greatest low temperature tolerances and acclimation; several such species, such as Agave utahensis and A. schottii, had small rosette sizes which resulted in higher minimum leaf temperatures. Species from the hottest habitats had among the greatest high temperature tolerances and acclimation; the two species from open desert scrub habitats, A. deserti and A. lecheguilla, had the lowest leaf shortwave absorptances observed, which would result in lower maximum leaf temperatures. Thus morphology and tissue tolerances to stressful temperatures reflect the temperature extremes of a plant's native habitat, although low temperature tolerance appears to limit the distribution of agaves more than high temperature tolerance.

Notes: The influences of various day/night air temperatures on net CO2 uptake and nocturnal acid accumulation were determined for Opuntia ficus-indica, complementing previous studies on the water relations and responses to photosynthetically active radiation (PAR) for this widely cultivated cactus. As for other Crassulacean acid metabolism (CAM) plants, net nocturnal CO2 uptake had a relatively low optimal temperature, ranging from 11°C for plants grown at day/night air temperatures of 10°C/0°C to 23°C at 45°C/35°C. Stomatal opening, which occurred essentially only at night and was measured by changes in water vapor conductance, progressively decreased as the measurement temperature was raised. The CO2 residual conductance, which describes chlorenchyma properties, had a temperature optimum a few degrees higher than the optimum for net CO2 uptake at all growth temperatures. Nocturnal CO2 uptake and acid accumulation summed over the whole night were maximal for growth temperatures near 25°C/15°C, CO2 uptake decreasing more rapidly than acid accumulation as the growth temperature was raised. At day/night air temperatures that led to substantial nocturnal acid accumulation (25°C/15°C.). 90% saturation of acid accumulation required a higher total daily PAR than at non-optimal growth temperatures (10°C/0°C and 35°C/25°C). Also, the optimal temperature of net CO2 uptake shifted downward when the plants were under drought conditions at all three growth temperatures tested, possibly reflecting an increased fractional importance of respiration at the higher temperatures during drought. Thus, water status, ambient PAR, and growth temperatures must all be considered when predicting the temperature response of gas exchange for O. ficus-indica and presumably for other CAM plants.

Notes: The effects of photosynthetically active radiation (PAR), leaf temperature and the leaf-to-air water vapor concentration drop on net CO2 uptake and water vapor conductance were surveyed for 14 species of ferns. Most previous studies indicated that ferns have extremely low maximal rates of net CO2 uptake, below 2 umol m−2 s−1, whereas the average maximal rate observed here at 250 C was 7 umol m−2 s−1. Net CO2 uptake reached 90% of saturation at an average PAR (400 to 700 nm) of only 240 umol m−2 s−1, consistent with the typically shaded habitats of most ferns. Maximal CO2 uptake rates were positively correlated with the PAR for 90% saturation (r2=0.59), the chlorophyII per unit leaf area (r2=0.30), the water vapor conductance (r2=0.65), and the CO2 residual conductance (r2=0.69). A higher water vapor conductance (gwv) was correlated with a greater fractional change in gwv as the leaf-to-air water vapor concentration drop (Δcwv) was raised from 5to20 g m−3 (r2=0.90). Specifically, for species with low gwv of about I mm s−1 the ratio of gwv at Δcwv= 5 g m−3 to that at Δcwv= 20 g m−3 was near 1, but it was near 2 for species with gwv of about 4 mm s−1. Such a relationship, which can prevent excessive transpiration, has apparently not previously been pointed out in surveys of other plant groups.

Notes: When the day/night air temperatures were raised from 10°C/10°C to 30°C/30°C, the optimal tempearture for nocturnal CO2 uptake by six species of cacti and three species of agave shifted from an average of 12°C to an average of 20°C. The maximum rate of CO2 uptake was higher for Agave americana at the higher ambient temperature, lower for A. deserti, and much lower for A. utahensis, consistent with the relative mean temperatures of their native habitats. For the cactus Coryphantha vivipara, which had the greatest temperature shift observed (13°C), the halftime was 8 days for the upward shift and 4 days for the downward shift. The halftimes for the comparable shifts averaged 1.6 days for three other species of cacti and less than 1 day for two agave species. The shifts in the optimal temperature for nocturnal CO2 uptake were in response to changes in nighttime temperature, at least for C. vivipara, and reflected temperature responses of both the stomates and the chlorenchyma.

Notes: Photosynthetically active radiation (PhAR) is apparently the environmental factor having the greatest influence on leaf thickness for Plectranthus parviflorus Henckel (Labiatae). A four-fold increase in leaf thickness from 280 to 1170 μm occurred as the PhAR was raised from 1.3 to 32.5 mol m−2 day−1. Compared to a constant PhAR of 2.5 mol m−2 day−1, a PhAR of 32.5 mol m−2 day−1 for one week during the first week (with return to 2.5 mol m−2 day−1 during the second and third weeks) led to an increase in final leaf thickness by 323 μm (to 802 μm). When increased PhAR was applied during the second week the increase in final thickness over the control was 217 μm, and when increased PhAR was applied during the third week it was 99 μm. However, leaf thickness was not simply responding to total daily PhAR, since a leaf 450 μm thick could occur at a low instantaneous PhAR for a long daytime (total daily PhAR of 1.5 mol m−2 day−1) and at a high PhAR for a short daytime (4.5 mol m−2 day−1). Total daily CO2 uptake (net photosynthesis) was approximately the same in the two cases, suggesting that this is an important factor underlying the differences in leaf thickness. Leaf thickness is physiologically important, since thicker leaves tend to have greater mesophyll surface area per unit leaf area (Ames/A) and hence higher photosynthetic rates.

Notes: Leaves of twelve C3 species and six C4 species were examined to understand better the relationship between mesophyll cell properties and the generally high photosynthetic rates of these plants. The CO2 diffusion conductance expressed per unit mesophyll cell surface area (gCO2cell) cell was determined using measurements of the net rate of CO2 uptake, water vapor conductance, and the ratio of mesophyll cell surface area to leaf surface area (Ames/A). Ames/A averaged 31 for the C3 species and 16 for the C4 species. For the C3 species gCO2cell ranged from 0.12 to 0.32 mm s-1, and for the C4 species it ranged from 0.55 to 1.5 mm s-1, exceeding a previously predicted maximum of 0.5 mm s-1. Although the C3 species Cammissonia claviformis did not have the highest gCO2cell, the combination of the highest Ames and highest stomatal conductance resulted in this species having the greatest maximum rate of CO2 uptake in low oxygen, 93 μmol m-2 s-1 (147 mg dm-2 h-1). The high gCO2cell of the C4 species Amaranthus retroflexus (1.5 mm s-1) was in part attributable to its thin cell wall (72 nm thick).

Notes: Summary Photosynthetic 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.

Notes: Summary Intraspecific competition in the C4 bunchgrass Hilaria rigida was examined on a Sonoran Desert site in southeastern California. Potential competition within monospecific stands was experimentally altered by removal of the aboveground portions of all plants within a 1.5 m radius of a monitored plant. Compared with unaltered plots, altered plots had less negative soil water potentials during periods of soil drying. Leaf blades on monitored plants of altered plots remained green longer and had greater stomatal conductances than those on monitored plants on unaltered plots. Production of new culms was twice as great on altered plots. Greater root biomass and root length were observed in altered plots, and root extension into soil areas formerly occupied by roots of neighboring plants occurred within one year after treatment. The results indicate that removal of the aboveground biomass of neighboring plants reduces the competition for limited available soil water in this desert environment.

Notes: Summary Extreme temperatures near the soil surface, which can reach 70°C at the main study site in the northwestern Sonoran Desert, markedly affect seedling survival. Computer simulations indicated that for the rather spherical barrel cactus Ferocactus acanthodes (Lem.) Britt. & Rose the maximum surface temperature decreased 8°C and the minimum temperature increased 3°C as the seedling height was increased from 1 mm up to 50 mm. Simulated changes in shortwave and longwave irradiation alone showed that shading could decrease the maximum temperature by about 5°C for the common desert agave, Agave deserti Engelm., and raise the minimum 1°C. Actual field measurements on seedlings of both species, where shading would affect local air temperatures and wind speeds in addition to irradiation, indicated that shading decreased the average maximum surface temperature by 11°C in the summer and raised the minimum temperature by 3°C in winter. Seedlings grown at day/iight air temperatures of 30°C/20°C tolerated low temperatures of about -7°C and high temperatures of about 56°C, as measured by the temperature where stain uptake by chlorenchyma cells was reduced 50%. Seedling tolerance to high temperatures increased slightly with age, and F. acanthodes was more tolerant than A. deserti. Even taking the acclimation of high temperature tolerance into account (2.7°C increase per 10°C increase in temperature), seedlings of A. deserti would not be expected to withstand the high temperatures at exposed sites, consistent with previous observations that these seedlings occur only in protected microhabitats. Based primarily on greater high temperature acclimation (4.3°C per 10°C), seedlings of F. acanthodes have a greater high temperature tolerance and can just barely survive in exposed sites. Wide ranges in photoperiod had little effect on the thermal sensitivities of either species. When drought increased the chlorenchyma osmotic pressure from about 0.5 MPa to 1.3 MPa, seedlings of both species became about 2°C less tolerant of high temperatures, which would be nonadaptive in a desert environment, and 2°C more tolerant of low temperatures, which also occurs for other species. In conclusion, seedlings of A. deserti and F. acanthodes could tolerate tissue temperatures over 60°C when acclimated to high temperatures and below -8°C when acclimated to low temperatures. However, the extreme environment adjacent to desert soil requires sheltered microhabitats to protect the plants from high temperature damage and also to protect them from low temperature damage at their upper elevational limits.

Notes: Summary An “environmental productivity index” based on physiological responses to three environmental variables was used to predict the net productivity of a common succulent perennial of the Sonoran Desert, Agave deserti, on a monthly basis. Productivity was also independently measured in the field from dry weight changes. The index was based on soil water availability, day/night air temperatures, and photosynthetically active radiation (PAR), which were individually varied in the laboratory and the effect on net CO2 uptake by the leaves determined. From monthly precipitation, temperature, and PAR at the field site together with the responses measured in the laboratory, an index (maximum value of unity) was assigned to each of these three environmental variables and their product was termed the environmental productivity index. This index indicates the fraction of maximal CO2 uptake expected in the field for each month (well-watered A. deserti assimilated 285 mmol CO2 m-2 leaf area day-1 at PAR saturation and optimal day/night temperatures of 25° C/15° C). The dry weight analysis was based on the monthly unfolding of new leaves from the central spike of the rosette and their seasonal increase in dry weight, which were determined in the field. The production of new leaves was highly correlated with the environmental productivity index (r2=0.93), which in turn was highly correlated with the water status index (r2=0.97). After correction for respiration by folded leaves, stem, and roots, plant productivity predicted by the average environmental productivity index (0.36) over a wet June-to-October period agreed within 4% with the productivity based on the conventional dry weight analysis. The net productivity of A. deserti over this 5-month period was 0.57 kg m-2 ground area (5.7 Mg ha-1), a large value for a desert CAM plant. The environmental productivity index proposed here may provide a reliable means for predicting net productivity on a monthly basis, which may be particularly useful for species in relatively variable environments such as deserts.