Park S. Nobel

Effect of nurse plants on the microhabitat and growth of cacti

(1) Seedlings of the cactus Carnegiea gigantea at two sites in the Sonoran Desert were found only in sheltered microhabitats, 89% occurring under the canopy of Ambrosia deltoidea and Cercidium microphyllum. In contrast, 29% of the seedlings of Ferocactus acanthodes, which tolerates higher temperatures than C. gigantea, occurred in unsheltered microhabitats, where maximum soil surface temperatures reached 71?C. Most (70%) of the sheltered seedlings of F. acanthodes occurred under the canopy of the perennial bunchgrass Hilaria rigida. (2) Shading by nurse plants reduced the total daily photosynthetically active radiation (PAR) available for their associated seedlings. Near an equinox, a seedling of C. gigantea located at the centre of an A. deltoidea shrub received 77% less total daily PAR, which reduces its predicted net CO2 uptake by 90% compared with an unshaded seedling. Similarly, a seedling of F. acanthodes located at the centre of an H. rigida plant received 64% less total daily PAR and could fix 65% less CO2 than could an unshaded seedling. (3) H. rigida and F. acanthodes have overlapping shallow root systems (mean root depth of 0 08 m for H. rigida and 0 05 m for seedlings of F. acanthodes), which accentuates competition for water. A water uptake model, which closely predicted the soil water potential in the root zone of H. rigida, indicated that a seedling of F. acanthodes located at the centre of the bunchgrass took up 32% less water than did an exposed seedling. (4) Assuming that the effects of temperatures, PAR, and soil water on net CO2 uptake are multiplicative, the predicted net CO2 uptake of a seedling of F. acanthodes under the canopy of H. rigida was only 36% of that of an exposed seedling. However, areole production by seedlings of F. acanthodes located under the nurse plant was 68% of that measured in exposed areas. This discrepancy probably reflects the 60% higher soil nitrogen levels under H. rigida. (5) Therefore, nurse plants facilitate seedling establishment by reducing high temperatures near the soil surface and provide a microhabitat with a higher soil nitrogen level. However, shading and competition for water with the nurse plants markedly reduce seedling growth. The magnitude of the reduction depends on seedling size and location under the nurse plant.

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.

Measurement of leaf relative water content by infrared reflectance

Abstract From basic considerations and Beers law, a leaf water content index incorporating reflectances of wavelengths from 0.76 to 0.90 μm and from 1.55 to 1.75 μm (Landsat Thematic Mapper Bands TM4 and TM5, respectively) was developed that relates leaf reflectance to leaf relative water content. For the leaf succulent, Agave deserti , the leaf water content index was not significantly different from the relative water content for either individual leaves or an entire plant. Also, the relative water contents of intact plants of Encelia farinosa and Hilaria rigida in the field were estimated by the leaf water content index; variations in the proportion of living to dead leaf area could cause large errors in the estimate of relative water content. Thus, the leaf water content index may be able to estimate average relative water content of canopies when TM4 and TM5 are measured at a known relative water content and fraction of dead leaf material.

Water relations and photosynthesis of a barrel cactus ferocactus acanthodes in the colorado desert

SummaryThe structural characteristics, water relations, and photosynthesis of Ferocactus acanthodes (Lemaire) Britton and Rose, a barrel cactus exhibiting Crassulacean acid metabolism (CAM), were examined in its native habitat in the western Colorado desert. Water storage in its succulent stem permitted nighttime stomatal opening ot continue for about 40 days after the soil water potential became less than that of the stem, a period whe the plant would be unable to extract water from the soil. After 7 months of drought and consequent unreplenished water loss from a plant, diurnal stomatal activity was not observed and the stem osmotic pressure was 6.4 bars, more than double the value measured during wet periods with nighttime stomatal opening. F. acanthodes had a shallow root system (mean depth of 8 cm) which responded within 24 h to rainfall.When the nocturnal stem surface temperature was raised from 8.0° C to 35.0° C, the stomatal resistance increased 4-fold, indicating that cool nighttime temperatures are advantageous for gas exchange by F. acanthodes. Moreover, the optimal temperature for CO2 uptake in the dark was only 12.6° C. CO2 uptake at night became maximal for 3.0 mEinsteins cm-2 of photosynthetically active radiation incident during the preceding day, and the minimum number of incident quanta absorbed per CO2 fixed was 68. The transpiration ratio (mass of water transpired/mass of CO2 fixed) had the relatively low value of 70 for an entire year, consistent with values obtained for other CAM plants. The total amount of water annually diverted to the floral structures was about 6% of the stem wet weight. The annual growth increment estimated from the net CO2 assimilation corresponded to about 10% of the stem mass for barrel cacti 34 cm tall, in agreement with measured dimension changes, and indicated that such plants were about 26 years old.

Interactions between seedlings of Agave deserti and the nurse plant Hilaria rigida

Seedlings of the succulent crassulacean acid metabolism (CAM) plant Agave deserti in the northwestern Sonoran Desert were found only in sheltered microhabitats, nearly all occurring under the canopy of a desert bunchgrass, Hilaria rigida. Apparently because soil surface temperatures can reach 71 0C in exposed areas, seedlings were generally located near the center or on the northern side of this nurse plant. Both species have shallow root systems, about half of the roots of H. rigida and all those for seedlings of A. deserti occurring above soil depths of 0.08 m. To examine competition for water between the nurse plant and an associated seedling, a three-dimensional model for root water uptake was developed. The model divided the soil into 17 concentric sheaths each subdivided radially into eight wedges and vertically into nine layers. Predicted pre-dawn soil water potentials at the mean root depth and total shoot transpiration agreed well with field measurements. Simulated annual water uptake by a seedling of A. deserti was reduced 50% when the seedling was moved from an exposed location to the center of the nurse plant. Shading by the nurse plant reduced total daily photosynthetically active radiation (PAR) by up to 74% compared with an exposed seedling. On the other hand, soil nitrogen under the canopy of H. rigida was 60% higher than in exposed locations. Assuming that the effects of nitrogen, temperature, PAR, and soil water on net CO2 uptake are multiplicative, the predicted net CO2 uptake by a seedling of A. deserti under the nurse plant was only -45% of that for an exposed seedling. Thus, although the nurse plant facilitates seedling establishment by reducing maximum soil surface temperatures and provides a microhabitat with higher soil nitrogen levels, its shading and competition for water reduce seedling growth.

Seedling Establishment of Ferocactus Acanthodes in Relation to Drought

The 30 smallest seedlings of Ferocactus acanthodes (Lem.) Britton and Rose (Cactaceae) at a canyon site in the northwestern Sonoran desert averaged 2.5 cm in height. Based on field studies of CO/sub 2/ exchange throughout the year, the estimated time of their germination was late summer 1976. To explan why so little establishment occurred in recent years, the length of lethal drought for seedlings of various ages was calculated from the seedling geometry (volume-to-surface ratio), dehydration tolerance, and transpiration rates. Individual years were identified which had sufficiently long growing seasons relative to the length of the subsequent droughts to allow establishment. Eight of the last 18 y were found to be suitable for establishment of F. acanthodes, the latest year being 1976.

INFLUENCES OF SEASONAL CHANGES IN LEAF MORPHOLOGY ON WATER-USE EFFICIENCY FOR THREE DESERT BROADLEAF SHRUBS'

The influences of seasonal variations in leaf length, pubescence, and the ratio of internal to external leaf area (Aᵐ ᵉ ˢ/A) on transpiration and photosynthesis were evaluated for 3 desert broad—leafs (Encelia farinosa [Grey], Hyptis emoryi [Tour.], and Mirabilis tenuiloba [Wats.]). Field data were incorporated into energy balance and gas exchange equations to determine water—use efficiencies (mass CO₂ fixed/mass H₂O transpired) at various times throughout a year. These results were compared with simulated values for leaves without the observed seasonal changes in morphology. For Encelia, leaf lengths were over 3—fold greater during the winter and spring than during the drier summer and fall (4.0 cm vs. 1.2cm). Increases in pubescence during dry periods led to substantial decreases in the leaf absorptance to solar irradiation (from 0.61 to 0.43). Similar alterations occurred for Hyptis, but with less month—to—month variation, while Mirabilis had comparatively small changes in these leaf parameters. For Mirabilis Aᵐ ᵉ ˢ/A was about 75% greater (maximum values of °70 vs. 40 for Encelia and Hyptis) and varied less (9% variation) during the year compared with Encelia and Hyptis (46% and 24% variation, respectively). Minimum leaf resistances to H₂O vapor diffusion (Rw ᵥ) were similar for Encelia and Hyptis throughout most of the year, while Mirabilis had much higher values. The optimal temperature for photosynthesis was several degrees higher for Mirabilis (34°C vs. 30°C for Encelia and Hyptis). The calculated cellular resistance for CO₂ diffusion (Rᶜ ᵉ ˡ ˡCO₂) was lowest for Mirabilis (147 s/cm) and almost identical for Encelia and Hyptis (282 s/cm). Encelia, and to a lesser degree Hyptis, had maximum photosynthetic rates at a lower water—use efficiency when water was available and greatest water—use efficiency at reduced photosynthetic levels during drier periods of the year. During a shorter growth period in the wet portion of the year, Mirabilis had a consistently high water—use efficiency (0.16) and a moderate photosynthetic rate, reflecting its much higher Aᵐ ᵉ ˢ/A and Rw ᵥ combined with a lower Rᶜ ᵉ ˡ ˡCO₂. Simulations for leaves without the observed changes in morphology that occurred during the drier months showed that the mean water—use efficiency for the year would have been °48% less for Encelia, 41% less for Hyptis, and only 3% less for Mirabilis. A decrease in leaf length, a decrease in absorptance, and an increase in Aᵐ ᵉ ˢ/A contributed about equally to the estimated seasonal increases in water—use efficiency for Encelia and Hyptis. Simulated transpiration and photosynthesis for leaves with morphology characteristic of drier periods resulted in water—use efficiencies similar to those for natural conditions, but annual photosynthesis was reduced °24% for Encelia, 17% for Hyptis, and <2% for Mirabilis.

Morphology, nurse plants, and minimum apical temperatures for young Carnegiea gigantea.

The northern limit of Carnegiea gigantea (Engelm) Britton and Rose apparently depends on minimum apical temperatures. Diameters, apical spine coverage, and effects of nurse plants on incoming long-wave (infrared [IR]) radiation, all of which affect apical temperatures, were therefore determined for stems of C. gigantea up to 4 m tall at four sites along a north-south transect in Arizona A simulation model indicated that the increase in diameter accompanying stem growth raised the minimum apical temperature more than 3 C. Thus, plants with the shortest stems would be expected to be the most vulnerable to freezing damage; indeed, freezing damage on stems <0.5 m tall without nurse plants was fairly common at the colder sites Nurse plants obstructed a greater portion of the sky for C. gigantea at the colder sites; e g., the effective environmental temperature for IR radiation at such locations was raised more than 10 C for stems under 1 m tall. If the northern limit of C gigantea reflects wintertime survival of juveniles, nurse plants could extend the range by offering some protection against freezing

Canopy structure and light interception

The rice cultivar I. R.8, so-called ‘miracle rice’, and its derivatives, represent one of the most significant contributions of plant physiology to the improvement of crop yields. Ву comparison to older varieties, the I.R. varieties have higher rates of crop photosynthesis achieved by selection of varieties with a canopy structure allowing more light to reach lower leaves. Indeed, modification of canopy structure can substantially improve crop yield by its influence on light interception by plants [2].

Water Relations of Cacti During Desiccation: Distribution of Water in Tissues

Three species of cacti survived an average stem water loss of 81%. Fractional water loss was greater from water-storage tissue than from the Chaenactis, as documented at the cellular level by determining changes in cell volume and at the tissue level by determining relative water content of chlorenchyma and storage tissues. For Carnegiea gigantea and Ferocactus acanthodes, this differential loss of water resulted from a decrease in the moles of solute per cell for storage tissue; hence, less water was retained at a given osmotic pressure than for the chlorenchyma. Opuntia basilaris lost less water from the chlorenchyma during drought because of a greater initial osmotic pressure in the chlorenchyma than in the storage tissue. Greater retention of water in the chlorenchyma would result in less disruption of photosynthetic activity in these cacti during drought.