XYLEM ANATOMY AND HYDRAULIC CONDUCTANCE OF PSILOTUM NUDUM

Psilotum nudum (L.) Beauv. (Psilotopsida) has a simple, vascularized sporophyte with a dichotomously branching aerial axis. The number and lumen diameters of tracheids in the actinostele decrease in each subsequent branch, leading to an approximate halving of the measured hydraulic conductance (K_h) from segment to segment. To understand how the anatomy of P. nudum affects K_h, a biophysical model based on the Hagen-Poiseuille relation was developed that incorporated lumen diameter, tracheid taper, pit cavities, and pit membranes. Using a technique previously developed for ferns, pit membrane resistance was determined by measuring water flow before and after dissolving the pit membranes with cellulase. Measured K_h was in good agreement with K_h calculated with the model after excluding thick-walled late metaxylem tracheids that dye studies showed were nonconducting. Model simulations showed that the approximately 40% overlap observed for tracheids of P. nudum was in the range leading to greatest conductance and that K_h decreased to half for 20% overlap. The model also showed that the pit membranes account for an increasing percentage of total resistance to water flow as the lumen diameter increases. Thus, the removal of such primary wall material and the evolutionary origin of vessels would have substantially increased K_h.     

Drought-induced shifts in daily CO2 uptake patterns for leafy cacti

For cacti with persistent, relatively large leaves, most shoot CO₂ uptake under well-watered conditions occurs by the leaves using the C₃ pathway. For three species in the primitive subfamily Pereskioideae, droughts of 7 or 14 days decreased leaf daytime net CO₂ uptake by an average of 49 and 88%, respectively; these species always had a net CO₂ release at night by the leaves and both at night and during the day by the stems. For three leafy species in subfamily Opuntioideae, 7 and 14 days of drought reduced leaf daytime net CO₂ uptake by 90 and 100%, respectively. Although drought reduced the total CO₂ uptake over 24 h, the average percentage occurring at night by the leaves of these species increased from 5% under wet conditions to 71% after 7 days of drought to 99% after 14 days of drought. For two of the three species of Opuntioideae, 7 days of drought caused the small net CO₂ uptake by the sterns to shift from the daytime to the nighttime, while for the third species drought caused a reduction of its stem nocturnal net CO₂ uptake. Thus, shifts from predominantly daytime to predominantly nighttime net CO₂ uptake can be induced by drought for the leaves and the stems of leafy cacti in subfamily Opuntioideae, indicating a high degree of biochemical versatility.     

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

Summary 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.     

Nutrient Influences on Leaf Photosynthesis: EFFECTS OF NITROGEN, PHOSPHORUS, AND POTASSIUM FOR GOSSYPIUM HIRSUTUM L

The net rate of CO₂ uptake for leaves of Gossypium hirsutum L. was reduced when the plants were grown at low concentrations of NO₃^-, PO₄^2-, or K⁺. The water vapor conductance was relatively constant for all nutrient levels, indicating little effect on stomatal response. Although leaves under nutrient stress tended to be lower in chlorophyll and thinner, the ratio of mesophyll surface area to leaf area did not change appreciably. Thus, the reduction in CO₂ uptake rate at low nutrient levels was due to a decrease in the CO₂ conductance expressed per unit mesophyll cell wall area (g^cell_CO2). The use of g^cell_CO2 and nutrient levels expressed per unit of mesophyll cell wall provides a new means of assessing nutrient effects on CO₂ uptake of leaves.     

Ozone increases the permeability of isolated pea chloroplasts

The effect of short-term exposure of chloroplasts isolated from the leaves of Pisum sativum to high concentrations of ozone was examined. The inhibitory effect of O₃ on endogenous photophosphorylation was apparently related to an increased permeability of the chloroplast limiting membranes induced by ozone exposure. A 5 min treatment with 50 ppm O₃ reduced the reflection coefficient of meso-erythritol from 0.84 to 0.58 and that of glycerol from 0.26 to 0.03. Such decreases in reflection coefficients indicate that ozone caused a marked increase in the permeability of the limiting membranes of the chloroplasts, which may result from an oxidation of membrane lipids. The decrease in the reflection coefficient of meso-erythritol was proportional both to ozone concentration (up to 30 ppm for 5 min of bubbling) and to time (up to 5 min at 30 ppm). Extrapolating these results to lower concentrations and longer times, ozone injury should be possible for a 2 hr exposure of plants to 0.3 ppm ozone, as is indeed the case.     

Light-Induced Chloroplast Shrinkage in vivo Detectable After Rapid Isolation of Chloroplasts From Pisum sativum

A light-induced shrinkage of chloroplasts in vivo could be detected with chloroplasts isolated within 2 minutes of harvesting pea plants. As determined both by packed volume and Coulter counter, the mean volume of chloroplasts from plants in the dark was 39 μ³, whereas it was 31 μ³ for chloroplasts from plants in the light. Upon illumination of the plants, the half-time for the chloroplast shrinkage in vivo was about 3 minutes, and the half-time for the reversal in the dark was about 5 minutes. A plant growth temperature of 20° was optimal for the volume change. The chloroplast shrinkage was half-maximal for a light intensity of 400 lux incident on the plants and was light-saturated near 2000 lux. The light-absorbing pigment responsible for the volume change was chlorophyll. This light-induced shrinkage resulted in a flattening and slight indenting of the chloroplasts. This chloroplast flattening upon illumination of the plants may accompany an increase in the photosynthetic efficiency of chloroplasts.     

A rapid technique for isolating chloroplasts with high rates of endogenous photophosphorylation

The main features of the procedure developed for rapid chloroplast isolation are: 1) gentle grinding of the plant material in a special nylon bag which retains nearly all whole cells and large debris, 2) osmoticum concentration chosen on the basis of the measured endogenous photophosphorylation, 3) a single, brief, low-speed centrifugation, 4) pellet resuspension by means of a vortex mixer, and 5) a total elapsed time from harvesting the plants to the obtaining of a resuspended chloroplast pellet of only 2 minutes. The usual isolation medium consists of an osmoticum (0.2 m sucrose) and a buffer (0.02 m N-tris-(hydroxymethyl) methyl-2-aminoethanesulfonate-NaOH, pH 7.9). In addition to these, the incubation medium contains only 200 μm ADP and 200 μm phosphate. Photophosphorylation rates of 24 μmoles ATP formed per mg chlorophyll per hour are consistently obtained using chloroplasts isolated from peas (Pisum sativum var. Laxton's Superb). The rate of endogenous photophosphorylation is maximal when the isolation and incubation media have an osmolarity of about 0.19 made up either with sucrose or with NaCl. The high rates and ease of measurement of endogenous photophosphorylation may facilitate the study of certain soluble components of chloroplasts as well as the general state of the photosynthetic ability of the plant.     

Physiological responses of Opuntia ficus-indica to growth temperature

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.     

Monoclonal antibodies directed against the sexual binding site of Chlamydomonas eugametos gametes

Monoclonal antibodies were raised against the mt- sexual agglutinin of Chlamydomonas eugametos gametes. Those that blocked the agglutination site were selected. They were divided into two classes dependent upon whether they gave a weak (class A) or clear positive (class B) reaction with mt- flagellar membranes in an ELISA and an indirect immunofluorescence test using glutaraldehyde-fixed mt- gametes. Class A antibodies were shown to be specific for the agglutinin in an extract of mt- gametes, based on results from immunoblotting, immunoprecipitation, affinity chromatography, and the absence of a reaction with nonagglutinable cells. Surprisingly, class A mAbs also recognized two mt+ glycoproteins, one of which is the mt+ agglutinin. Class B antibodies were shown to bind to several glycoproteins in both mt- and mt+ gametes, including the mt- agglutinin. Fab fragments from class A mAbs blocked the sexual agglutination process, but those from class B did not, even though the parent antibody did. We conclude that the class A epitope lies in or close to the agglutination site of the mt- agglutinin, whereas the class B epitope lies elsewhere on the molecule. We also conclude that the mt- agglutinin is the only component on the mt- flagellar surface directly involved in agglutination. Class A mAbs were found to elicit several reactions displayed by the mt+ agglutinin. They bound to the mt- agglutinin on gamete flagella and induced most of the reactions typical of sexual agglutination, with the exception of flagellar tip activation. None of these reactions was induced by Fab fragments. High concentrations of class A mAbs completely repressed the sexual competence of live mt- gametes, but low concentrations stimulated cell fusion.