Parenchyma-chlorenchyma water movement during drought for the hemiepiphytic cactus Hylocereus undatus.

BACKGROUND AND AIMS: Hylocereus undatus, a hemiepiphytic cactus cultivated in 20 countries for its fruit, has fleshy stems whose water storage is crucial for surviving drought. Inter-tissue water transfer during drought was therefore analysed based on cell volumes and water potential components. METHODS: In addition to determining cell dimensions, osmotic pressures and water potentials, a novel but simple procedure leading to an external water potential of zero was devised by which cells in thin sections were perfused with distilled water. The resulting volume changes indicated that the parenchyma-chlorenchyma water movement was related to more flexible cell walls in the water-storage parenchyma with its lower internal turgor pressure (P) than in the chlorenchyma. KEY RESULTS: Under wet conditions, P was 0.45 MPa in the chlorenchyma but only 0.10 MPa in the water-storage parenchyma. During 6 weeks of drought, the stems lost one-third of their water content, becoming flaccid. About 95 % of the water lost came from cells in the water-storage parenchyma, which decreased by 44 % in length and volume, whereas cells in the adjacent chlorenchyma decreased by only 6 %; the osmotic pressure concomitantly increased by only 10 % in the chlorenchyma but by 75 % in the water-storage parenchyma. CONCLUSIONS: The concentrating effect that occurred as cellular volume decreased indicated no change in cellular solute amounts during 6 weeks of drought. The ability to shift water from the parenchyma to the chlorenchyma allowed the latter tissue to maintain a positive net CO2 uptake rate during such a drought.

Young daughter cladodes affect CO2 uptake by mother cladodes of Opuntia ficus-indica.

BACKGROUND AND AIMS: Drought damages cultivated C3, C4 and CAM plants in the semi-arid lands of central Mexico. Drought damage to Opuntia is common when mother cladodes, planted during the dry spring season, develop young daughter cladodes that behave like C3 plants, with daytime stomatal opening and water loss. In contrast, wild Opuntia are less affected because daughter cladodes do not develop on them under extreme drought conditions. The main objective of this work is to evaluate the effects of the number of daughter cladodes on gas exchange parameters of mother cladodes of Opuntia ficus-indica exposed to varying soil water contents. METHODS: Rates of net CO2 uptake, stomatal conductance, intercellular CO2 concentration, chlorophyll content and relative water content were measured in mature mother cladodes with a variable number of daughter cladodes growing in spring under dry and wet conditions. KEY RESULTS: Daily carbon gain by mother cladodes was reduced as the number of daughter cladodes increased to eight, especially during drought. This was accompanied by decreased mother cladode relative water content, suggesting movement of water from mother to daughter cladodes. CO2 assimilation was most affected in phase IV of CAM (late afternoon net CO2 uptake) by the combined effects of daughter cladodes and drought. Rainfall raised the soil water content, decreasing the effects of daughter cladodes on net CO2 uptake by mother cladodes. CONCLUSIONS: Daughter cladodes significantly hasten the effects of drought on mother cladodes by competition for the water supply and thus decrease daily carbon gain by mother cladodes, mainly by inhibiting phase IV of CAM.

Nectar: properties, floral aspects, and speculations on origin.

Although nectar is crucial for most pollinators, its evolutionary origin has received scant attention. Nectar is derived from the phloem solution. Both have high sugar concentrations (usually 10-30% solutes by fresh mass); the main solute in the phloem is sucrose, whereas nectar can also contain considerable amounts of fructose and glucose. The phloem, not the xylem, is the supplier of water to flowers and certain other organs. Therefore, a 'leaky phloem' hypothesis for the origin of nectar is presented based on the elevated hydrostatic pressure that can occur within the phloem and the structural weakness of developing phloem tissues. A 'sugar excretion' hypothesis is also presented that considers the solute accumulation resulting from the relatively high transpiration rates of flowers.

Carbon and water relations for developing fruits of Opuntia ficus-indica (L.) Miller, including effects of drought and gibberellic acid.

Growth, gas exchange rates, and carbohydrate content were studied for developing fruits of the cultivated cactus Opuntia ficus-indica (L.) Miller, including effects of drought and exogenous gibberellic acid (GA3). Fruit development required 110 d from the time of bud differentiation to ripening at 80 d after anthesis, when the fruit mass averaged 67 g. Stomatal conductance and net CO2 uptake rates for fruits were higher during the night; they were maximal at 7 d before anthesis and decreased as development progressed. Fruits undergoing drought, imposed by detaching terminal stems bearing fruits, were 50% smaller than the control at 80 d after anthesis and did not ripen. Fruits injected with 2 ml of 500 ppm GA3 were 30% smaller than the control at 80 d after anthesis; they contained a large proportion of aborted seeds that produced a weak sink signal for dry mass accumulation. Gas exchange was higher at 21 d after anthesis for fruits treated with GA3. Total soluble sugars represented 40% of the fruit's dry mass until 45 d after anthesis, when the sugar content rapidly increased, reaching 90% at 73 d after anthesis. Such an increase was not observed for fruits treated with GA3, and the sugar content for fruits undergoing drought remained low throughout development. Starch content increased for developing fruits of O. ficus-indica until 14 d after anthesis and, except for the fruits undergoing drought, decreased thereafter. Fruit development for O. ficus-indica is apparently regulated by water availability as well as hormonal signals originating both within and outside the fruit.

Effects of benomyl and drought on the mycorrhizal development and daily net CO2 uptake of a wild platyopuntia in a rocky semi-arid environment.

The effects of drought and the fungicide benomyl on a wild platyopuntia, Opuntia robusta Wendl., growing in a rocky semi-arid environment were assessed. Cladode phosphorus content, cladode water potential and daily net CO2 uptake were measured monthly in 2000 and 2001 before, during and after the summer rainy period. During 2000, the formation of new roots and new cladodes was severely suppressed in response to a prolonged drought, impairing the development of the symbiotic relationship between the arbuscular mycorrhizal (AM) fungi and the roots. Hence no effect of benomyl application was observed on daily carbon assimilation by this Crassulacean acid metabolism plant. During 2001, drought was interrupted, and new cladodes and roots were formed in response to rainfall. Benomyl was highly effective in suppressing root colonization by AM-fungi; however, daily C assimilation was reduced by benomyl application only in October. Thus, the inhibition of AM-fungal colonization by benomyl did not affect photosynthesis, water uptake and P uptake under prolonged drought.

Tolerances and acclimation to low and high temperatures for cladodes, fruits and roots of a widely cultivated cactus, Opuntia ficus-indica.

• Opuntia ficus-indica , a cactus widely cultivated for fruits and forage/fodder, has shoots composed of flattened stem segments (cladodes) that are relatively sensitive to freezing temperatures below -6°C but extremely tolerant of high temperatures up to 65°C. Based on the uptake of the vital strain neutral red, fruits and roots were damaged by 60 min below -7°C or above 55°C. • Young (6 wk old) and mature (1-yr-old) cladodes had 1.2°C greater low-temperature tolerance at day/night air temperatures of 20 : 10°C compared with 30 : 20°C and 2.8°C lower high-temperature tolerance. Fruits and roots (both 6 wk old) showed no such low-temperature acclimation; roots had high-temperature acclimation similar to that of cladodes, but fruits showed no high-temperature acclimation. • High-temperature tolerance did not change with age for cladodes and fruits up to 10 wk old nor did low-temperature tolerance for cladodes, but fruit low-temperature tolerance decreased by 2.6°C from 4.5 to 10 wk of age, a time when sugars, which can act as cryoprotectants, were increasing. • Cladodes showed 2.0°C greater tolerance of low temperatures with age up to 10 yr and 6.5°C greater high-temperature tolerance, which helps prevent the death of plants, especially during episodic freezing events.

Initial net CO2 uptake responses and root growth for a CAM community placed in a closed environment.

To help understand carbon balance between shoots and developing roots, 41 bare-root crassulacean acid metabolism (CAM) plants native to the Sonoran Desert were studied in a glass-panelled sealable room at day/night air temperatures of 25/15 degrees C. Net CO(2) uptake by the community of Agave schottii, Carnegia gigantea, Cylindropuntia versicolor, Ferocactus wislizenii and Opuntia engelmannii occurred 3 weeks after watering. At 4 weeks, the net CO(2) uptake rate measured for south-east-facing younger parts of the shoots averaged 1.94 micro mol m(-2) s(-1) at night, considerably higher than the community-level nocturnal net CO(2) uptake averaged over the total shoot surface, primarily reflecting the influences of surface orientation on radiation interception (predicted net CO(2) uptake is twice as high for south-east-facing surfaces compared with all compass directions). Estimated growth plus maintenance respiration of the roots averaged 0.10 micro mol m(-2) s(-1) over the 13-week period, when the community had a net carbon gain from the atmosphere of 4 mol C while the structural C incorporated into the roots was 23 mol. Thus, these five CAM species diverted all net C uptake over the 13-week period plus some existing shoot C to newly developing roots. Only after sufficient roots develop to support shoot water and nutrient requirements will the plant community have net above-ground biomass gains.

Plasma membrane aquaporins play a significant role during recovery from water deficit.

The role of plasma membrane aquaporins (PIPs) in water relations of Arabidopsis was studied by examining plants with reduced expression of PIP1 and PIP2 aquaporins, produced by crossing two different antisense lines. Compared with controls, the double antisense (dAS) plants had reduced amounts of PIP1 and PIP2 aquaporins, and the osmotic hydraulic conductivity of isolated root and leaf protoplasts was reduced 5- to 30-fold. The dAS plants had a 3-fold decrease in the root hydraulic conductivity expressed on a root dry mass basis, but a compensating 2.5-fold increase in the root to leaf dry mass ratio. The leaf hydraulic conductance expressed on a leaf area basis was similar for the dAS compared with the control plants. As a result, the hydraulic conductance of the whole plant was unchanged. Under sufficient and under water-deficient conditions, stomatal conductance, transpiration rate, plant hydraulic conductance, leaf water potential, osmotic pressure, and turgor pressure were similar for the dAS compared with the control plants. However, after 4 d of rewatering following 8 d of drying, the control plants recovered their hydraulic conductance and their transpiration rates faster than the dAS plants. Moreover, after rewatering, the leaf water potential was significantly higher for the control than for the dAS plants. From these results, we conclude that the PIPs play an important role in the recovery of Arabidopsis from the water-deficient condition.

Growth and reproductive characteristics of the columnar cactus Stenocereus queretaroensis and their relationships with environmental factors and colonization by arbuscular mycorrhizae.

Three natural populations of pitayo (Stenocereus queretaroensis (Weber) Buxbaum), a columnar arborescent cactus, were studied in their subtropical environments in western Mexico. All of the sites were characterized by shallow, nutrient-poor soils. Percentage of colonization by arbuscular mycorrhizae (AM) fungi, stem growth, fruit mass, and percentage germination were greater in S. queretaroensis at Autlan, Jalisco (AJ) than at Zacoalco de Torres, Jalisco (ZTJ) or Santa Rosa, Zacatecas (SRZ). The onset of root colonization by arbuscular mycorrhizae during the middle of the summer wet period preceded increases in stem extension rate and stem phosphorus concentration. Based on previous studies of effects of environmental factors on photosynthesis, climatic conditions were more favorable for photosynthesis at AJ than at SRZ and ZTJ, as indicated by the amount of summer rainfall, the amount of light, and the moderate air temperatures that prevailed during the fall and winter seasons. There was a significant positive correlation between stem growth and percentage of total root length colonized by arbuscules of AM fungi for S. queretaroensis at SRZ and AJ, but not at ZTJ. A negative significant correlation was observed between stem growth and maximal and minimal air temperatures at the three study sites. Stem growth was positively related to rainfall only at SRZ, and light was statistically related to stem growth only at ZTJ. Among sites, S. queretaroensis at AJ had the highest carbon gain and greatest AM colonization, creating physiological conditions that led to the highest stem growth, fruit mass and percentage of seed germination.

Root deployment and shoot growth for two desert species in response to soil rockiness.

Soil texture, as well as the presence of rocks, can determine the water status, growth, and distribution of plants in arid environments. The effects of soil rockiness and soil particle size distribution on shoot and root growth, root system size, rooting depth, and water relations were therefore investigated for the Crassulacean acid metabolism leaf succulent Agave deserti and the C(4) bunchgrass Pleuraphis rigida after precipitation events during the summer and winter/spring rainfall periods in the northwestern Sonoran Desert. The soils at the field site varied from sandy (<3% rocks by volume) to rocky (up to 35% rocks), with greater water availability at higher water potentials for sandy than for rocky soils. Although A. deserti was absent from the sandiest sites, its shoot and root growth during both rainfall periods were greatest in comparatively sandier sites and decreased as the soil rock content increased. Furthermore, A. deserti had twofold greater root surface area, root : leaf area ratio, and mean rooting depth at sandier than at rocky sites. As for A. deserti, shoot growth was greater for P. rigida at the sandier sites than at the rockier sites, even though its root surface area and mean rooting depth did not vary significantly. After early spring rainfall events, the leaf water potential for A. deserti did not differ between rocky and sandy sites, but transpiration rates were almost twofold greater at rocky than at sandy sites. During the same period, P. rigida had lower leaf water potentials and 25% lower transpiration rates at rocky than at sandy sites. The greater variability in the deployment of the root systems of A. deserti in response to soil rockiness may reflect its evergreen habit and slower growth, which allow it to endure periods of lower water availability than does P. rigida, whose leaves die during drought.

Plant frequency, stem and root characteristics, and CO2 uptake for Opuntia acanthocarpa: elevational correlates in the northwestern Sonoran Desert.

A common cylindropuntia in the northwestern Sonoran Desert, Opuntia acanthocarpa, was investigated for the following hypotheses: its lower elevational limit is set by high temperatures, so its seedlings require nurse plants; its upper elevational limit is set by freezing; spine shading is the least at intermediate elevations; and changes in plant size and frequency with elevation reflect net CO2 uptake ability. For four elevations ranging from 230 m to 1,050 m, the mean height of O. acanthocarpa approximately doubled and its frequency increased 14-fold. Nurse plants were associated with only 4% of O. acanthocarpa less than 20 cm tall at the two lower elevations compared with 57% at 1,050 m, where putative freezing damage was especially noticeable, suggesting that nurse plants protect from low temperature damage. Spine shading of the stem doubled from the lowest to the highest elevation. Net CO2 uptake, which followed a Crassulacean acid metabolism pattern, was maximal at day/night air temperatures of 25/15°C and was halved by 4 weeks of drought and by reducing the photosynthetic photon flux from 30 to 12 mol m-2 day-1. The root system of O. acanthocarpa was shallow, with a mean depth of only 9 cm for the largest plants. Root growth was substantial and similar for plants at 25/15°C and 35/25°C, decreasing over 70-fold at 15/5°C and 45/35°C. Based on cellular uptake of the vital stain neutral red, neither roots nor stems tolerated tissue temperatures below -5°C for 1 h while both showed substantial high temperature acclimation, roots tolerating 1 h at 61°C and stems 1 h at 70°C for plants grown at 35/25°C. The increase in height and frequency of O. acanthocarpa with elevation apparently reflected both a greater ability for net CO2 uptake and greater root growth and hence water uptake. This species achieves its greatest ecological success at elevations where it becomes vulnerable to low temperature damage.

Growth and development of the arborescent cactus Stenocereus queretaroensis in a subtropical semiarid environment, including effects of gibberellic acid.

In Stenocereus queretaroensis (Weber) Buxbaum, an arborescent cactus cultivated in Jalisco, Mexico, for its fruits but studied here in wild populations, stem extension occurred in the autumn at the beginning of the dry season, flowering and fruiting occurred in the spring at the end of the dry season, and new roots grew in the summer during the wet season. The asynchrony of vegetative and reproductive growth reduces competitive sink effects, which may be advantageous for wild populations growing in infertile rocky soils. Seasonal patterns of sugars in the roots and especially the stems of S. queretaroensis were closely related to the main phenological stages, becoming lower in concentration during periods of major stem extension. Cessation of stem extension occurred in 100-year-old plants for which injection of GA(3) reinitiated such growth. Isolated chlorenchyma cylinders had maximum extension in a bathing solution containing 0.1 &mgr;M gibberellic acid.

Root-soil contact for the desert succulent Agave deserti in wet and drying soil.

To investigate the extent and size of root-soil air gaps that develop during soil drying, resin casts of roots of the desert succulent Agave deserti Engelm. were made in situ for container-grown plants and in the field. Plants that were draughted in containers for 7 and 14 d had 24 and 34% root shrinkage, respectively, leading to root-soil air gaps that would reduce the hydraulic conductivity at the root-soil interface by a factor of about 5. When containers were vibrated during drought, root-soil air gaps were greatly diminished, and the predicted conductivity at the interface was similar to that of the control (moist soil). For plants in the field (4 and 6 wk after the last rainfall), root shrinkage was greater than for container-grown plants, but root-soil contact on the root periphery was greater, which led to a higher predicted hydraulic conductivity at the root-soil interface. To test the hypothesis that root-soil air gaps would help to limit water efflux from roots in drying soil, the water potentials of the soil, root, and shoot of plants from vibrated containers (with gaps eliminated or reduced) and non-vibrated containers were compared. The soil water potential was lower for vibrated containers after 14 d of drought, suggesting more rapid depletion of soil water due to better root-soil contact, and the root water potential was lower as well, suggesting greater water loss by roots in the absence of root-soil air gaps. Thus, air gaps could benefit A. deserti by helping to maintain a higher root water potential in the early stages of drought and later by limiting root water loss at the root-soil interface when the water potential exceeds that of the soil.

Light, chlorophyll, carboxylase activity and CO2 fixation at various depths in the chlorenchyma of Opuntia ficus-indica (L.) Miller under current and elevated CO2.

Mature cladodes of Opuntia ficus-indica (L.) Miller have a thick chlorenchyma (about 4 mm) with a relatively high chlorophyll convent (0.65 gm-2 ), suggesting that light may be greatly attenuated and hence CO2 fixation negligible in the inner part of this tissue. Indeed, blue light (400-470 nm) and red light (670-685 nm)were 99% attenuated in the outer 2 mm of the chlorenchyma when the cladodes developed under both current and elevated CO2 Concentrations. Nevertheless, the nocturnal acidity increase and 14 C accumulation following a brief exposure to 14 CO2 at night decreased only 22 to 47% for a layer 2-3 mm deep in the chlorenchyma of this CAM plant. Under a particular growth CO2 , the activities of both ribulose-1,5-bisphosphate carboxylase/oxygenase and phosho-enolyruvate carboxylase were similar for each of the outer three 1-mm-thick layers of the chlorenchyma. Therefore, although the light level and total chlorophyll decreased sharply with depth and the chlorophyll a/b ratio also decreased. Substantial CO2 fixation apparently occurs throughout most of the chlorenchyma. When O. ficus-indica was grown under 720 µmol CO2 mol-1 , the chlorenchyma was 20% thicker but contained 11% less chlorophyll and had a lower absorptance than under the current CO2 concentration (370µmol mol-1 ). Greater nocturnal acidity increases and 14 C accumulation following exposure to 14 CO2 at night occurred at the doubled CO2 concentration despite 29-39% reductions in the activities of the two carboxylating enzymes, the lower absorptance, and a 24% increase in the cladode reflectance from 400-700 nm.

Hydraulic conductivity and anatomy along lateral roots of cacti: changes with soil water status.

To help understand root function for desert plants at different levels of water availability, cellular and water-conducting properties were examined for young lateral roots of Ferocactus acanthodes (Lem.) Britton & Rose and Opuntia ficus-indica (L.) Miller. Hydraulic conductivity (Lp ), radial conductivity (LR ), and axial conductance (Kh ) were determined for root segments in wet soil, during drought, and after rewetting. All three parameters increased with distance from the root tip under wet conditions and during drought for both species, with larger increases occurring under wet conditions. Lp decreased with drought duration up to 35 d for both species, due to dehydration of cortical cells, suberization of periderm, and embolism of xylem vessels; Lp and LB decreased more for O. ficus-indica, and Kh decreased more for F. acanthodes. The increase in Kh with distance from the root tip and the decrease with duration of drought were associated with the changes in the number of conducting xylem vessels. After 7 d of rewetting, new secondary lateral roots developed, enhancing water uptake, especially for O. ficus-indica. Rewetting also caused water uptake for primary lateral roots to return to approximately the original value under wet conditions, mainly because decreases in emboli led to recovery of axial flow.

Influence of rocks on soil temperature, soil water potential, and rooting patterns for desert succulents.

At a site in the Sonoran Desert, subterranean rocks and exposed boulders affected soil water potential as well as root morphology and distribution. For Agave deserti, the number of lateral roots per unit length of main root was 11 times higher under rocks and six times higher alongside rocks than in rock-free regions. Total root length per unit soil volume for Echinocereus engelmannii averaged 3-fold higher within 1 cm of boulders than 5 cm away, where the soil was drier. The total length of lateral roots per unit length of main root for Ferocactus acanthodes was 4.2 m m-1 under rocks but only 0.8 m m-1 in rock-free regions. The number of lateral roots per unit length of main root for Opuntia acanthocarpa was 7-fold higher alongside rocks than in rock-free regions and even higher under rocks. For transplanted and watered A. deserti, the number of new main roots produced per 1-2 month interval averaged 13 for five plants on the north side of boulders, 8 on the south side, 11 for five plants with half of their roots under rocks, 2 for those with half of their roots over rocks, and 3 for the control plants without rocks. Laboratory experiments showed that the soil water potential under rocks for 10 and 30 mm waterings stayed above -0.5 MPa for 13 and 19 d longer, respectively, than for regions away from rocks. The shortwave absorptance of granitic rocks from the field site was 0.82, the thermal conductivity coefficient was 1.50 W m-1 °C-1, and the volumetric heat capacity was 1.75 MJ m-3 °C-1. Field measurements indicated that 5-cm-thick buried rocks decreased the diel variation in soil temperatures on their undersurface by only 0.4° C compared with soil. Thus, the primary influence of rocks at the field site on root proliferation and branching for the four species was apparently caused by influences on soil water content.

Water relations and mucopolysaccharide increases for a winter hardy cactus during acclimation to subzero temperatures.

Thickness, relative water content (RWC), osmotic pressure, water potential isotherms, and mucopolysaccharide content were measured for the photosynthetic chlorenchyma and the water-storage parenchyma of the winter hardy cactus, Opuntia humifusa, after shifting from day/night air temperatures of 25° C/15° C to 5° C/-5° C. After 14 d at 5° C/-5° C, the average fraction of water contained in the symplast decreased from 0.92 to 0.78, the water potential of saturated (fully hydrated) tissue was essentially unchanged, but the osmotic pressure of saturated tissue decreased (by 0.15 MPa for the chlorenchyma and 0.12 MPa for the water-storage parenchyma). After 7 weeks at 5° C/-5° C, tissue thickness was reduced by 61% for the chlorenchyma and 65% for the water-storage parenchyma, and the RWC decreased by 42% and 68%, respectively; these changes contributed to an osmotic pressure increase of 0.55 MPa for the chlorenchyma and 0.34 MPa for the water-storage parenchyma. During the 7 week acclimation to low temperature, mucopolysaccharide increased by 114% for the chlorenchyma and by 89% for the water-storage parenchyma. The water potential of the extracted mucopolysaccharide was relatively constant for an RWC between 1.00 and 0.30, decreasing abruptly below 0.30. Changes in water relations parameters and in mucopolysaccharide content during low-temperature acclimation may reduce water efflux from the cells, and thus reduce damage due to rapid dehydration during extracellular freezing.

Achievable productivities of certain CAM plants: basis for high values compared with C3 and C4 plants.

CAM species, which taxonomically are at least five times more numerous than C4 species, often grow-slowly, as is the case for various short-statured cacti and many epiphytes in several families, However, slow growth is not a necessary corollary of the CAM photosynthetic pathway, as can be appreciated by considering the energetics of CO2 fixation. For every CO2 fixed photosynthetically, C3 plants require 3 ATP and 2 NADPH, whereas the extra enzymatic reactions and compartmentation complexity for C4 plants require 4 or 5 ATP and 2 NADPH, and CAM plants require 5.5-6.5 ATP and 2 NADPH. Photorespiration in C8 plants can release some of the CO2 , fixed and also has an energetic-cost, whereas photorespiration is much less in C4 and CAM plants. Therefore, CAM plants can perform net CO2 fixation 15% more efficiently than C3 , plants, although 10% less efficiently than C4 plants. Using a simple model that assumes 8 photons per CO2 fixed and a processing time per excitation of 5 ms, a maximum instantaneous rate for net CO2 , uptake of 55 µmol m-2 s-1 is predicted. Measured maximal rates average 48µmol m-2 s-1 for leaves of six C3 species with the highest rates and 64 µmol m-2 s-1 for six such C4 species; CAM plants take up CO2 mainly at night, which is not directly related to the instantaneous rate of photon absorption. Net CO2 uptake integrated over 24 h, which is more pertinent to productivity than are instantaneous CO2 uptake rates, is similar for the three pathways, although the higher water-use efficiency of CAM plants can be an advantage during drought. Canopy architecture is crucial for the distribution of the photosynthetic photon flux density (PPFD) over the shoot, which determines net CO2 uptake per unit ground area and hence determines productivity. Maximal productivity for idealized canopies under optimal conditions is predicted to be about 100 Mg d. wt ha-1 yr-1 (1 Mg = 1 tonne), whereas actual values of environmental factors in the field approximately halve this prediction. The influence of environmental factors on net CO2 uptake can be quantified using an environmental productivity index (EPI), which predicts the fractional limitation on net CO2 uptake and is the product of a water index, a temperature index, and a PPFD index (nutrient effects can also be included). Using EPI with a ray-tracing technique to determine the PPFD index and taking into account respiration and carbon incorporated structurally, maximal productivity of CAM plants is predicted to occur at leaf or stem area indices of 4-5. In experiments designed using such shoot area indices, annual above-ground dry-weight productivities averaging 43 Mg ha-1 yr-1 have recently been observed for certain agaves and plutyopuntias. In comparison, the measured average annual productivity of the most productive plants is 49 Mg ha-1 yr-1 for six agronomic C4 species, 35 Mg ha-1 yr-1 for sis agronomic C3 species, and 39 Mg ha-1 yr-1 for six C3 tree species. Thus, CAM plants are capable of similar high productivities, which can become especially advantageous in regions of substantial water stress. Recognition of the high potential productivity of certain CAM species under optimal environmental conditions, exceeding that of most C3 species, may increase the cultivation of such CAM plants in various areas in the future. CONTENTS Summary 183 I. Introduction 184 II. Biochemistry of C3 , C4 , and CAM plants 185 III. CO2 uptake rates 188 IV. Canopy architecture and light absorption 193 V. Measured biomass productivity 198 VI. Conclusions 200 Acknowledgement 202 References 202.

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

To 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 (r 2=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.

Extreme temperatures and thermal tolerances for seedlings of desert succulents.

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.