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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.Water transport through cellular membranes is facilitated by aquaporins, proteins that form waterselective channels. The presence of aquaporins in a membrane can increase the osmotic hydraulic conductivity of the membrane (L P , meters per second per megapascal) by 10-to 20-fold (Preston et al., 1992). In plants, the physiological importance of aquaporins is currently mainly inferred from their widespread occurrence (Johansson et al., 2000) and the use of HgCl 2 , a nonspecific inhibitor (Tyerman et al., 2002). Aquaporins, which are found in almost all types of tissues (Maurel, 1997), have changed the way we think about plant water relations (Maurel and Chrispeels, 2001).Water movement through a living organ such as a root or a leaf can take an apoplastic route, which has a low resistance to flow, or a transcellular route, which has a higher resistance because water has to move through lipid bilayer membranes . Bulk water flow associated with the transpiration stream is mostly apoplastic, except in the root exo-and endodermis (Zimmermann et al., 2000) and in the leaf bundle sheath (Koroleva et al., 2002), where apoplastic barriers (Casparian band, suberin lamellae, and secondary cell wall thickening) restrict the apoplastic path. Other important processes such as cell enlargement, refilling of embolized vessels, and movement of guard cells and pulvini may require rapid transport of water across membranes. Furthermore, the considerable growth-associated water potential difference (0.1-0.3 MPa) found in most growing organs of herbaceous plants (e.g. Nonami and Boyer, 1...

The influence of illumination level during leaf development on the mesophyll cell surface area per unit leaf area (Ames/A), CO2 resistances, and the photosynthetic rate was determined for leaves of Plectranthus parviflorus Henckel. The relative importance of A"'eS A versus C02 resistances in accounting for observed changes in photosynthesis was quantitatively evaluated using equations based on analogies to electrical circuits.When the illumination during development was raised from 900 to 42,000 lux, the leaves more than tripled in thickness as the mesophyll cells increased in size and frequency, which caused Asses/A to go from 11 to 50. Ignoring respiration and photorespiration, the net rate of photosynthesis per unit leaf area (JC02) is Jco2 = co2/Rco2 = cco2/(1.56 R,, + Rc02) (1) where cCO2 is the CO2 concentration outside the leaf, and RCO2 is a total resistance for CO2 fixation expressed per unit leaf area (3,4,8,13,15,16,20). As equation 1 indicates, RCO2 has a gaseous phase component in common with the water vapor pathway, the factor 1.56 accounting for the ratio of the diffusion coefficient of water vapor to that of CO2 in air at 20 C (16). In the liquid phases, the resistance to CO2 movement (RC2) is composed of contributions from cell walls, plasmalemmas, cytoplasm, chloroplast membranes, and the resistance associated with the carboxylation reaction (3,4,8,14,15,20). This internal resistance, expressed per unit leaf area, is related to the resistance per unit area of mesophyll cells (R"j,) as follows:The internal leaf morphology of many plant species varies from that characteristic of shade leaves at low light levels to that of sun leaves when development occurs at illuminations approaching full sunlight (1,7,(10)(11)(12)(21)(22)(23)(24). Not only do sun leaves tend to have more highly developed palisade and spongy mesophyll regions than shade leaves, but also they have higher photosynthetic rates at light saturation (1, 2, 6, 10). The higher assimilation rates have been suggested to result from changes in activity of enzymes involved in photosynthesis (1,2,9,10,14,18) and to variations in the number of chlorophylls per photosynthetic unit (1, 2, 9, 17). However, the higher rates of photosynthesis could also be a consequence of the changes in internal leaf morphology caused by illumination, a matter which apparently has not been systematically investigated. This MATERIALS AND METHODSCuttings from a single parent plant of Plectranthus parviflorus Henckel, a member of the Labiatae commonly known as "Creeping Charlie," were grown at 20 C and 55%c relative humidity for 5 weeks in sterilized soil. The indicated illumination was provided for 12 hr each day using warm-white fluorescent tubes and neutral density screens (15,000 lux corresponded to 29 neinsteins cm-2 sec-I between 400 and 700 nm). Third node leaves approximately 15 cm2 in area were used for measurements.The Ames A ratio was determined using drawings prepared with the aid of a camera lucida using an over-all magnification of 300 X. S...

summary 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 advantag...