The Role of Endogenous Abscisic Acid in the Response of Plants to Stress

When a continuous stream of warm air (38°C) was directedon to the leaves of dwarf bean seedlings they wilted and thengradually regained turgor. This process of adaptation was accompaniedby an increasing abscisic acid (ABA) level in the leaves andan increase in leaf resistance (R_L). It is suggested that theleaf-water deficit induced by the warm-air treatment causedthe increase in ABA level and that the latter was responsiblefor stimulating stomatal closure, enabling the plants to regainfull turgor. A similar type of adaptation, brought about byan increased level of ABA in the leaves, is believed to occurin tomato, dwarf bean, and wheat plants when they are flooded.Predictably, in rice, a species adapted to a flooded environment,seedlings showed no increase in ABA level as a result of flooding. It is proposed that adaptation may involve the formation ofan equilibrium between ABA and its conjugate form (i. e. theglucose ester). The ABA-conjugate was observed to disperse slowlyfrom leaves recovering from a water deficit and therefore itmay act as a metabolic ‘back-stop’, enabling the‘free’ ABA level to remain high for a period evenwhen the leaves have regained turgor. Abscisic acid appears to be responsible for alleviating theeffects of water stress in plants, making it possible for plantsto pass through periods of stress with little harm.     

Synthesis and metabolism of abscisic acid in detached leaves of Phaseolus vulgaris L. after loss and recovery of turgor

Metabolism of abscisic acid (ABA) was studied after wilting and upon recovery from water stress in individual, detached leaves of Phaseolus vulgaris L. (red kidney bean). Loss of turgor was correlated with accumulation of ABA and its metabolites, resulting in a 10-fold increase in the level of phaseic acid (PA) and a doubling of the level of conjugated ABA. The level of conjugated ABA in turgid leaves was no higher than that of the free acid. These results indicate that accumulation of ABA in wilted leaves resulted from a stimulation of ABA synthesis, rather than from a release from a conjugated form or from inhibition of the metabolism of ABA. The rate of synthesis of ABA was at its maximum between 2.5 and 5 h after turgor was lost, and slackened there-after. In wilted leaves, the rate of conversion of ABA to PA climbed steadly until it matched the rate of synthesis, after about 7.5 h. Upon rehydration of sections from wilted leaves, the rate of synthesis of ABA dropped close to zero within about 3 h, while the rate of conversion to PA accelerated. Formation of PA was two to four times faster than in sections maintained in the wilted condition; it reached a rate sufficient to convert almost one-half of the ABA present in the tissue to PA within 1 h. In contrast, the alternate route of metabolism of ABA, synthesis of conjugated ABA, was not stimulated by rehydration. The role of turgor in the stimulation of the conversion of ABA to PA was investigated. When leaves that had been wilted for 5 h were rehydrated to different degrees, the amount of ABA which disappeared, or that of PA which accumulated during the next 3 h, did not depend linearly on the water potential of the rehydrated leaf. Rather, re-establishment of the slightest positive turgor was sufficient to result in maximum stimulation of conversion of ABA to PA.Abbreviations ABA abscisic acid - DPA dihydrophaseic acid - PA phaseic acid - _leaf leaf water potential - osmotic pressure     

Relationship between Stress-Induced ABA and Proline Accumulations and ABA-Induced Proline Accumulation in Excised Barley Leaves

When excised second leaves from 2-week-old barley (Hordeum vulgare var Larker) plants were incubated in a wilted condition, abscisic acid (ABA) levels increased to 0.6 nanomole per gram fresh weight at 4 hours then declined to about 0.3 nanomole per gram fresh weight and remained at that level until rehydrated. Proline levels began to increase at about 4 hours and continued to increase as long as the ABA levels were 0.3 nanomole per gram fresh weight or greater. Upon rehydration, proline levels declined when the ABA levels fell below 0.3 nanomole per gram fresh weight.

Proline accumulation was induced in turgid barley leaves by ABA addition. When the amount of ABA added to leaves was varied, it was observed that a level of 0.3 nanomole ABA per gram fresh weight for a period of about 2 hours was required before proline accumulation was induced. However, the rate of proline accumulation was slower in ABA-treated leaves than in wilted leaves at comparable ABA levels. Thus, the threshold level of ABA for proline accumulation appeared to be similar for wilted leaves where ABA increased endogenously and for turgid leaves where ABA was added exogenously. However, the rate of proline accumulation was more dependent on ABA levels in turgid leaves to which ABA was added exogenously than in wilted leaves.

Salt-induced proline accumulation was not preceded by increases in ABA levels comparable to those observed in wilted leaves. Levels of less than 0.2 nanomole ABA per gram fresh weight were measured 1 hour after exposure to salt and they declined rapidly to the control level by 3 hours. Proline accumulation commenced at about 9 hours. Thus, ABA accumulation did not appear to be involved in salt-induced proline accumulation.

     

Abscisic Acid accumulation is not required for proline accumulation in wilted leaves

Leaves from dark-grown barley (Hordeum vulgare L. var Larker) seedlings grown in the presence and absence of fluridone were used to determine whether or not abscisic acid (ABA) accumulation was necessary for proline to accumulate in wilted tissue. Wilted tissue (polyethylene glycol-treated) leaves from fluridone-grown seedlings did not accumulate ABA but did accumulate proline at a rate that was not different from the non-fluridone-treated leaves. Thus ABA accumulation is not required for wilting-induced proline accumulation in barley leaves. Proline accumulation in wilted leaves from the wilty tomato (Lycopersicon esculentum) mutant, flacca, was compared to that in the wild type, Rheinlands Ruhm. Proline accumulated in wilted leaves from flacca. The rate of accumulation was faster in flacca compared to the rate in the wild type because the wilty mutant wilted faster. ABA accumulated in wilted leaves from the wild type but not in the wilty mutant. This result is a further confirmation that ABA accumulation is not required for wilting-induced proline accumulation. These results are significant in that proline accumulation in barley leaves can be induced independently by any one of three treatments: wilting, ABA, or salt.     

The effect of wilting on the selection of leaves by the leaf-cutting ant Atta laevigata

Using field assays of leaf preference, we tested the hypothesis that wilting affects the selection of leaves by the leaf-cutting ant Atta laevigata (Fr. Smith). Detached leaves were left to air-dry until noticeably wilted. The area removed by the ants from wilted leaves was significantly greater than the area removed from fresh leaves, this effect being observed in several plant species, in leaves of different age, and in assays with different ant colonies. Leaves collected from water-stressed plants were also preferred to leaves from non-stressed plants. A. laevigata was found to employ a two-stage, size-related, strategy when cutting plants. Larger workers climbed the plant stem and dropped whole leaves to the ground by severing their petioles; smaller workers cut the lamina of the dropped leaves. The ants frequently left dropped leaves on the ground, until the next foraging day or even later, when they were harvested in a wilted condition in preference to newly-dropped leaves.It is possible that during wilting some repellent substances evaporate or become less effective, thus enhancing leaf palatability. Alternatively or in addition, changes in nutrient and water content may have rendered wilted leaves more palatable to leaf-cutting ants.     

Correlation between loss of turgor and accumulation of abscisic acid in detached leaves

Mature leaves of Phaseolus vulgaris L. (red kidney bean), Xanthium strumarium L. (cocklebur), and Gossypium hirsutum L. (cotton) were used to study accumulation of abscisic acid (ABA) during water stress. The water status of individual, detached leaves was monitored while the leaves slowly wilted, and samples were cut from the leaves as they lost water. The leaf sections were incubated at their respecitive water contents to allow ABA to build up or not. At least 8 h were required for a new steady-state level of ABA to be established. The samples from any one leaf covered a range of known water potentials (), osmotic pressures (), and turgor pressures (p). The and p values were calculated from pressure-volume curves, using a pressure bomb to measure the water potentials. Decreasing water potential had little effect on ABA levels in leaves at high turgor. Sensitivity of the production of ABA to changes in progressively increased as turgor approached zero. At p=1 bar, ABA content averaged 4 times the level found in fully turgid samples. Below p=1 bar, ABA content increased sharply to as much as 40 times the level found in unstressed samples. ABA levels rose steeply at different water potentials for different leaves, according to the at which turgor became zero. These differences were caused by the different osmotic pressures of the leaves that were used; must cqual - for turgor to be zero. Leaves vary in , not only among species, but also between plants of one and the same species depending on the growing conditions. A difference of 6 bars (calculated at =0) was found between the osmotic pressures of leaves from two groups of G. hirsutum plants; one group had previously experienced periodic water stress, and the other group had never been stressed. When individual leaves were subsequently wilted, the leaves from stress-conditioned plants required a lower water potential in order to accumulate ABA than did leaves from previously unstressed plants. On the basis of these results we suggest that turgor is the critical parameter of plant water relations which controls ABA production in water-stressed leaves.Abbreviations ABA abscisic acid - me-ABA abscisic-acid methyl ester - leaf water potential - osmotic pressure - p volumeaveraged turgor - volumetric modulus of elasticity     

Changes in abscisic acid and its glucose ester in Phaseolus vulgaris L. during chilling and water stress

Levels of free and conjugated abscisic acid (ABA) were determined in leaves and roots of intact bean (Phaseolus vulgaris L., cv. Mondragone) seedlings under chilling (3C) and drought as well as during recovery from stress. Abscisic acid-glucose ester (ABAGE) was the only conjugate releasing free ABA after alkaline hydrolysis of the crude aqueous extracts. During the first 20–30 h chilled plants rapidly dehydrated and wilted without any change in ABA and ABAGE levels. Subsequently, leaf and root ABA levels increased and plants regained turgor. ABAGE concentration showed a slight increase in leaves but not in roots. Upon recovery from chilling a transient, but significant, rise in leaf ABA content was observed, while no appreciable change in ABAGE was found. Drought triggered ABA accumulation in leaves and roots, while a rise in ABAGE content was detected only in leaf tissues. Recovery from stress caused a drop in ABA levels without a correspondent increase in ABAGE concentration. We conclude that ABAGE is not a source of free ABA during either chilling or water stress and that only a small proportion of the ABA produced under stress is metabolised to ABAGE during recovery.Abbreviations ABA = abscisic acid - ABAGE = abscisic acid-glucose ester - DW = dry weight - FW = fresh weight - RIA = radioimmunoassay - RWC = relative water content - w = water potential - o = osmotic potential - p = turgor potential     

Reduction of turgor induces rapid changes in leaf translatable RNA

The turgor of pea (Pisum sativum) leaves was reduced by exposing excised pea shoots to a stream of 23°C air for 20 min. Poly(A)⁺ RNA was isolated from control and wilted shoots, translated in vitro and radiolabeled translation products separated by electrophoresis on two-dimensional (isoelectric focusing-sodium dodecyl sulfate) polyacrylamide gels. This analysis showed that the levels of several poly(A)⁺ RNAs increased in wilted plants. Most of the poly(A)⁺ RNAs induced in wilted plants did not accumulate in response to heat shock or exogenously applied ABA even though endogenous ABA levels were found to increase in shoots 30 min after wilting and by 4 h had increased 50-fold (1 versus 0.02 microgram per gram fresh weight). A λgt10 cDNA library was constructed using poly(A)⁺ RNA from wilted shoots which had been incubated for 4 hours. Differential screening of the library identified four clones corresponding to poly(A)⁺ RNAs which are induced in wilted shoots.     

Stomatal behaviour and leaf water potential of chilled and water-stressed Solanum melongena, as influenced by growth history

Abstract Leaf diffusion resistance and leaf water potential of intact Solanum melongena plants were measured during a period of chilling at 6 °C. Two pretreatments, consisting of a period of water stress or a foliar spraying of abscisic acid (ABA), were imposed upon the plants prior to chilling. The control plants did not receive a pretreatment. In addition to intact plant studies, stomatal responses to water loss and exogenous abscisic acid were investigated using excised leaves, and the influence of the pretreatment observed. Chilled, control plants wilted slowly and maintained open stomata despite a decline in leaf water potential to –2.2 MPa after 2 d of chilling. In contrast plants that had been water stressed or had been sprayed with abscisic acid, prior to chilling, did not wilt and maintained a higher leaf water potential and a greater leaf diffusion resistance. In plants that had not received a pretreatment, abscisic acid caused stomatal closure at 35 °C, but at 6°C it did not influence stomatal aperture. The two pretreatments greatly increased stomatal sensitivity to both exogenous ABA and water stress, at both temperatures. Stomatal response to water loss from excised leaves was greatly reduced at 6°C. These results are discussed in relation to low temperature effects on stomata and the influence of preconditioning upon plant water relations.     

Abscisic Acid Metabolism in Relation to Water Stress and Leaf Age in Xanthium strumarium

Intact plants of Xanthium strumarium L. were subjected to a water stress-recovery cycle. As the stress took effect, leaf growth ceased and stomatal resistance increased. The mature leaves then wilted, followed by the half expanded ones. Water, solute, and pressure potentials fell steadily in all leaves during the rest of the stress period. After 3 days, the young leaves lost turgor and the plants were rewatered. All the leaves rapidly regained turgor and the younger ones recommenced elongation. Stomatal resistance declined, but several days elapsed before pre-stress values were attained.

Abscisic acid (ABA) and phaseic acid (PA) levels rose in all the leaves after the mature ones wilted. ABA-glucose ester (ABA-GE) levels increased to a lesser extent, and the young leaves contained little of this conjugate. PA leveled off in the older leaves during the last 24 hours of stress, and ABA levels declined slightly. The young leaves accumulated ABA and PA throughout the stress period and during the 14-hour period immediately following rewatering. The ABA and PA contents, expressed per unit dry weight, were highest in the young leaves. Upon rewatering, large quantities of PA appeared in the mature leaves as ABA levels fell to the pre-stress level within 14 hours. In the half expanded and young leaves, it took several days to reach pre-stress ABA values. ABA-GE synthesis ceased in the mature leaves, once the stress was relieved, but continued in the half expanded and young leaves for 2 days.

Mature leaves, when detached and stressed, accumulated an amount of ABA similar to that in leaves on the intact plant. In contrast, detached and stressed young leaves produced little ABA. Detached mature leaves, and to a lesser extent the half expanded ones, rapidly catabolized ABA to PA and ABA-GE, but the young leaves did not. Studies with radioactive (±)-ABA indicated that in young leaves the conversion of ABA to PA took place at a much lower rate than in mature ones. Leaves of all ages rapidly conjugated PA to PA-glucose ester. Furthermore, when half expanded leaves were stressed on the intact plant, their rate of ABA catabolism was enhanced, an effect not observed in the young leaves.

In conclusion, young leaves on intact Xanthium plants produce little stress-induced ABA themselves, but due to import and a low rate of catabolism accumulate more ABA and PA than mature leaves.

     

Acclimation of potato plants to polyethylene glycol-induced water deficit I. Photosynthesis and metabolism

The acclimation of photosynthesis and metabolism in response to water deficit is characterized using hydroponically grown potato plants (Solanum tuberosum cv. Désirée). Plants were subjected to a reduced water potential of the nutrient solution by adding 10% (w/v) PEG 6000. PEG-treated plants were retarded in growth. Leaves which had been fully developed before the PEG treatment and leaves grown during the PEG treatment showed different phenotypes and biochemical and physiological properties. Photosynthesis of all leaves decreased during the whole treatment. However, the decrease of photosynthesis in the two types of leaves had different causes indicated by differences in their metabolism. Leaves which were fully developed at the beginning of the PEG treatment began to wilt starting from the leaf rim. The apoplastic ABA content increased, coinciding with a decreased stomatal conductance. Increased energy charge of the cells indicated impaired chloroplastic metabolism, accompanied by a decrease of amounts of chloroplastic enzymes. The apoplastic and the symplastic ABA content were increased during water deficit and because ABA was concentrated in the cytosolic compartment it is suggested that ABA is involved in decreasing photosynthetic enzyme contents in old leaves. Young leaves, grown after the imposition of water deficit, were smaller than control leaves and had a curly surface. In young leaves apoplastic and cytosolic ABA contents were identical with control values. Carboxylation efficiency of photosynthesis was decreased, but the water use efficiency remained unchanged. Metabolic data of the photosynthetic pathways indicate a down-regulation of chloroplastic metabolism. It is concluded that in young leaves photosynthesis was non-stomatally limited. This limitation was not caused by ABA.     

The effects of benzyladenine, cycloheximide, and cordycepin on wilting-induced abscisic Acid and proline accumulations and abscisic Acid- and salt-induced proline accumulation in barley leaves

Benzyladenine inhibits proline accumulation in wilted, abscisic acid (ABA)-treated, and salt-shocked barley leaves. It does not affect ABA accumulation or disappearance in wilted leaves. Inhibition of proline accumulation in salt-shocked leaves was observed both when benzyladenine was added at the beginning of or after salt treatment. Cycloheximide (CHX) and cordycepin inhibited both ABA and proline accumulations in wilted barley leaves and proline accumulation in ABA-treated leaves. In salt-shocked leaves, cordycepin inhibited proline accumulation when added after salt treatment but before proline began to accumulate but not when added after the onset of proline accumulation. CHX delayed the accumulation of proline in salt-shocked leaves but, after a period of time, proline accumulated in the CHX-treated leaves at rates comparable to the salt-treated control. This delay and subsequent accumulation was observed when CHX was added before, during, and after salt treatment. However, the earlier in the salt treatment period that CHX was given, the longer was the observed delay. These results are interpreted to indicate that gene activation is involved in proline accumulation in response to wilting, to ABA, and to salt in barley leaves. This gene activation is in addition to the gene activation that is required for ABA accumulation in wilted leaves. If ABA accumulation is required for proline accumulation in wilted barley leaves, then two sets of gene activation are involved in wilting-induced proline accumulation. All of our results are consistent with this possibility but do not prove it. The inhibition of proline accumulation by benzyladenine is probably neither due to an effect on gene activation nor to an effect on the ABA level.     

Phytochrome B increases drought tolerance by enhancing ABA sensitivity in Arabidopsis thaliana

Phytochrome B (phyB) can adjust morphological and physiological responses according to changes in the red : far‐red (R:FR) ratio. phyB‐driven acclimation of plants to open environments (high R:FR ratio) increases carbon gain at the expense of increased water loss. This behaviour alleviates stressful conditions generated by an excess of light, but increases the chances of desiccation. Here we evaluated how phyB modulates this drought‐tolerance response by comparing wild‐type Arabidopsis thaliana adult plants to the null phyB in response to water shortage. phyB wilted before the wild type, and this was due to phyB maintaining open stomata under a reduction in soil water availability. Although phyB presented enhanced ABA levels under well‐watered conditions, this mutant was less sensitive than the wild type in diminishing stomatal conductance in response to exogenous ABA application. Reduced sensitivity to ABA in phyB correlated with a lower expression of ABCG22, which encodes a putative ABA influx transporter, and PYL5, which encodes a soluble ABA receptor. Furthermore, the expression of RAB18 and RD29A, both typical ABA‐induced genes, was lower in phyB than the wild type after ABA treatment. We propose that phyB contributes to the acclimation of plants to open environments by enhancing ABA sensitivity when soil water becomes limiting.     

<Emphasis Type="Italic">SlHDA5</Emphasis>, a Tomato Histone Deacetylase Gene,Is Involved in Responding to Salt,Drought, and ABA

Histone deacetylation catalyzed by histone deacetylases is an important type of histone modification. Histone deacetylases affect various processes of plant development and involve in responding to hormones and biotic and abiotic stresses. Here, we report a tomato PRD3/HDA1 histone deacetylase gene, SlHDA5, which is expressed ubiquitously in different tissues and development stages. Expression profiles in hormone treatments showed that SlHDA5 was induced by abscisic acid (ABA) and methyl jasmonate (MeJA). Seedlings growth of SlHDA5-RNAi lines were more inhibited on the medium containing salt compared with wild type (WT). Under salt stress, chlorophyll in mature leaves degraded earlier in transgenic leaves than that in WT, and transgenic plants displayed wilting earlier and more severe than WT. After drought treatment, transgenic plants wilted and dehydrated earlier than WT, which was confirmed by lower water and chlorophyll content, and higher malondialdehyde (MDA) content in transgenic plants manifesting that the tolerance of transgenic plants to drought receded. Under the treatment of ABA, root length of transgenic seedlings was more strongly repressed by contrast with WT, suggesting repression of SlHDA5 increased seedling sensibility to ABA. Our study indicated that silencing of SlHDA5 resulted in decreasing tolerance to salt, drought, and ABA.     

Abscisic Acid Metabolism in Water-stressed Bean Leaves

Phaseic acid (PA) and dihydrophaseic acid (DPA) are the major metabolites observed when (S)-2-¹⁴C-abscisic acid (ABA) is fed to 14-day excised primary bean leaves (Phaseolus vulgaris L. cv. Red Kidney). The distribution of ¹⁴C in leaves which were wilted after feeding ABA appears to be the same as that observed in unwilted leaves. A reduction in the relative specific radioactivities of the two metabolites after wilting, compared with the specific radioactivities measured in unwilted plants, indicated that these metabolites continue to be formed endogenously after wilting. Estimates of the endogenous ABA levels showed that they rose from 0.04 μg to approximately 0.5 μg/g fresh weight within 4 hours after the beginning of a 10% wilt and remained at that level during a subsequent 20 hours of wilt. In unwilted leaves, the levels of PA and DPA were 5 times and 20 times higher than that of ABA, respectively. Both PA and DPA levels rose throughout the wilt period. PA rose from 0.20 μg to 1.0 μg and DPA from 0.8 μg to over 3 μg/g fresh weight. From these data, we calculated the rate of ABA synthesis to be at least 0.15 μg/hr.g fresh weight during this period. We have interpreted these results to mean that in wilted leaves an elevated level of ABA is maintained because the rate of synthesis and metabolism are both elevated and approximately equal.     

Foliar sprays with ABA promote growth of Ilex paraguariensis by alleviating diurnal water stress

Stomatal closure, relative water content (RWC) and vegetative growth were monitored in Ilex paraguariensis plants grown under well-watered conditions with a photosynthetic photon flux density (PPFD) varying from 100% to 1.5%, and sprayed weekly with either distilled water (control) or 1.89 mM abscisic acid (ABA). ABA treatments caused stomatal closure, ranging from 62% to 73%. These treatments also increased RWC in the early evening from 82% to 92% and 88% to 94% in mature and immature leaves, respectively. Such alleviation of the water stress was correlated with increases in leaf area, leaf dry weight (DW), shoot length and shoot DW. On day 35 from the beginning of the experiment, the increases in DW of both leaves and shoots were 1.5-fold at the 1.5% PPFD and 3-fold (for leaves) and 4.5-fold (for shoots) under 100% PPFD. In water-sprayed control plants grown under 1.5% PPFD shoot length also increased significantly, although these shoots contained more ABA (assessed by capillary gas chromatography–mass spectrometry) than those of plants grown under 100% PPFD. These results show that ABA sprayed on to leaves promotes growth in I. paraguariensis plants by alleviating diurnal water stress.     

Overexpression of a 9-cis-epoxycarotenoid dioxygenase gene in Nicotiana plumbaginifolia increases abscisic acid and phaseic acid levels and enhances drought tolerance

The plant hormone abscisic acid (ABA) plays important roles in seed maturation and dormancy and in adaptation to a variety of environmental stresses. An effort to engineer plants with elevated ABA levels and subsequent stress tolerance is focused on the genetic manipulation of the cleavage reaction. It has been shown in bean (Phaseolus vulgaris) that the gene encoding the cleavage enzyme (PvNCED1) is up-regulated by water stress, preceding accumulation of ABA. Transgenic wild tobacco (Nicotiana plumbaginifolia Viv.) plants were produced that overexpress the PvNCED1 gene either constitutively or in an inducible manner. The constitutive expression of PvNCED1 resulted in an increase in ABA and its catabolite, phaseic acid (PA). When the PvNCED1 gene was driven by the dexamethasone (DEX)-inducible promoter, a transient induction of PvNCED1 message and accumulation of ABA and PA were observed in different lines after application of DEX. Accumulation of ABA started to level off after 6 h, whereas the PA level continued to increase. In the presence of DEX, seeds from homozygous transgenic line TN1 showed a 4-d delay in germination. After spraying with DEX, the detached leaves from line TN1 had a drastic decrease in their water loss relative to control leaves. These plants also showed a marked increase in their tolerance to drought stress. These results indicate that it is possible to manipulate ABA levels in plants by overexpressing the key regulatory gene in ABA biosynthesis and that stress tolerance can be improved by increasing ABA levels.     

Solute accumulation and decreased photosynthesis in leaves of potato plants expressing yeast-derived invertase either in the apoplast, vacuole or cytosol

Potato (Solanum tuberosum cv. Désirée) plants expressing yeast invertase directed either to the apoplast, vacuole or cytosol were biochemically and physiologically characterised. All lines of transgenic plants showed similarities to plants growing under water stress. Transformants were retarded in growth, and accumulated hexoses and amino acids, especially proline, to levels up to 40-fold higher than those of the wild types. In all transformants rates of CO₂ assimilation and leaf conductance were reduced. From the unchanged intercellular partial pressure of CO₂ and apoplastic cis-abscisic acid (ABA) content of transformed leaves it was concluded that the reduced rate of CO₂ assimilation was not caused by a limitation in the availability of CO₂ for␣the ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco). In the transformants the amount of Rubisco protein was not reduced, but both activation state and carboxylation efficiency of photosynthesis were lowered. In vacuolar and cytosolic transformants this inhibition of Rubisco might be caused by a changed ratio of organic bound and inorganic phosphate, as indicated by a doubling of phosphorylated intermediates. But in apoplastic transformants the pattern of phosphorylated intermediates resembled that of leaves of water-stressed potato plants, although the cause of inhibition of photosynthesis was not identical. Whereas in water-stressed plants increased contents of the phytohormone ABA are supposed to mediate the adaptation to water stress, no contribution of ABA to reduction of photosynthesis could be detected in invertase transformants. Received: 29 May 1996 / Accepted: 30 December 1996