Limitations on Leaf Nitrate Reductase Activity during Flowering and Podfill in Soybean

The objective of this study was to identify factors which limit leaf nitrate reductase (NR) activity as decline occurs during flowering and beginning seed development in soybean (Glycine max [L.] Merr. cv Clark). Level of NR enzyme activity, level of reductant, and availability of NO₃⁻ as substrate were evaluated for field-grown soybean from flowering through leaf senescence. Timing of reproductive development was altered within one genotype by (a) exposure of Clark to an artificially short photoperiod to hasten flowering and podfill, and (b) the use of an early flowering isoline. Nitrogen (N) was soil-applied to selected plots at 500 kilograms per hectare as an additional variable. Stem NO₃⁻ concentration and in vivo leaf NR activity were significantly correlated (R² = 0.69 with nitrate in the assay medium and 0.74 without nitrate in the medium at P = 0.001) across six combinations of reproductive and soil N-treatment. The supply of NO₃⁻ from the root to the leaf tissue was the primary limitation to leaf NR activity during flowering and podfill. Levels of NR enzyme and reductant were not limiting to leaf NR activity during this period.     

Soybean mutants lacking constitutive nitrate reductase activity : I. Selection and initial plant characterization

The objectives of this study were to select and initially characterize mutants of soybean (Glycine max L. Merr. cv Williams) with decreased ability to reduce nitrate. Selection involved a chlorate screen of approximately 12,000 seedlings (progeny of mutagenized seed) and subsequent analyses for low nitrate reductase (LNR) activity. Three lines, designated LNR-2, LNR-3, and LNR-4, were selected by this procedure.

In growth chamber studies, the fully expanded first trifoliolate leaf from NO₃⁻-grown LNR-2, LNR-3, and LNR-4 plants had approximately 50% of the wild-type NR activity. Leaves from urea-grown LNR-2, LNR-3, and LNR-4 plants had no NR activity while leaves from comparable wild-type plants had considerable activity; the latter activity does not require the presence of NO₃⁻ in the nutrient solution for induction and on this basis is tentatively considered as a constitutive enzyme. Summation of constitutive (urea-grown wild-type plants) and inducible (NO₃⁻-grown LNR-2, LNR-3, or LNR-4 plants) leaf NR activities approximated activity in leaves of NO₃⁻-grown wild-type plants. Root NR activities were comparable in wild-type and mutant plants grown on NO₃⁻, and roots of both plant types lacked constitutive NR activity when grown on urea. In both growth chamber- and field-grown plants, oxides of nitrogen [NO_(x)] were evolved from young leaves of wild-type plants, but not from leaves of LNR-2 plants, during in vivo NR assays. Analysis of leaves from different canopy locations showed that constitutive NR activity was confined to the youngest three fully expanded leaves of the wild-type plant and, therefore, on a total plant canopy basis, the NR activity of LNR-2 plants was approximately 75% that of wild-type plants. It is concluded that: (a) the NR activity in leaves of NO₃⁻-grown wild-type plants includes both constitutive and inducible activity; (b) the missing NR activity in LNR-2, LNR-3, and LNR-4 leaves is the constitutive component; and (c) the constitutive NR activity is associated with NO_(x) evolution and occurs only in physiologically young leaves.

     

Regulation of Sulfate Assimilation in Plants : XIII. Assimilatory Sulfate Reduction during Ontogenesis of Primary Leaves of Phaseolus vulgaris L

The correlation between the extractable activities of three key enzymes of assimilatory sulfate reduction and the in vivo incorporation of ³⁵SO₄²⁻ into amino acids, proteins, and sulfolipids was investigated from greening to senescence in primary leaves of beans (Phaseolus vulgaris L.). The total extractable activity of ATP sulfurylase (EC 2.7.7.4) and of adenosine 5′-phosphosulfate sulfotransferase reached a maximum in the leaves of approximately 7- and 11-day-old seedlings, respectively. During senescence, there was a decrease in both enzyme activities. After approximately 17 days, no appreciable activities remained. In contrast, total O-acetyl-l-serine sulfhydrylase (EC 4.3.99.8) activity decreased to only approximately 50% of the maximal value during the same period. The in vivo incorporation of ³⁵SO₄²⁻ into amino acid and protein fractions showed a time-course similar to that of the total extractable adenosine 5′-phosphosulfate sulfotransferase activity. Both cysteine and sulfate markedly decreased during senescence. The total extractable activity of ribulosebisphosphate carboxylase (EC 4.1.1.39) was maximal in the primary leaves of 13-day-old seedlings, and approximately 40% of this value was still detectable after 17 days. Taken together with results from the literature, these results show that assimilatory sulfate reduction in primary leaves of P. vulgaris L. stops before CO₂ and nitrate assimilation.     

Nitrate Reductase Activity in Shoots and Roots of Maize Seedlings as Affected by the Form of Nitrogen Nutrition and the pH of the Nutrient Solution

The effect of nitrogen form (NH₄-N, NH₄-N + NO₃⁻, NO₃⁻) on nitrate reductase activity in roots and shoots of maize (Zea mays L. cv INRA 508) seedlings was studied. Nitrate reductase activity in leaves was consistent with the well known fact that NO₃⁻ increases, and NH₄⁺ and amide-N decrease, nitrate reductase activity. Nitrate reductase activity in the roots, however, could not be explained by the root content of NO₃⁻, NH₄-N, and amide-N. In roots, nitrate reductase activity in vitro was correlated with the rate of nitrate reduction in vivo. Inasmuch as nitrate reduction results in the production of OH⁻ and stimulates the synthesis of organic anions, it was postulated that nitrate reductase activity of roots is stimulated by the released OH⁻ or by the synthesized organic anions rather than by nitrate itself. Addition of HCO₃⁻ to nutrient solution of maize seedlings resulted in a significant increase of the nitrate reductase activity in the roots. As HCO₃⁻, like OH⁻, increases pH and promotes the synthesis of organic anions, this provides circumstantial evidence that alkaline conditions and/or organic anions have a more direct impact on nitrate reductase activity than do NO₃⁻, NH₄-N, and amide-N.     

The role of nitrate and ammonium ions and light on the induction of nitrate reductase in maize leaves

Corn seedlings (Zea mays cv W64A × W182E) were grown hydroponically, in the presence or absence of NO₃⁻, with or without light and with NH₄Cl as the only N source. In agreement with earlier results nitrate reductase (NR) activity was found only in plants treated with both light and NO₃⁻. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by transfer of the proteins to nitrocellulose paper and reaction with antibodies prepared against a pure NR showed that crude extracts prepared from light-grown plants had a polypeptide of approximately 116 kilodaltons (the subunit size for NR) when NO₃⁻ was present in the growth medium. Crude extracts from plants grown in the dark did not have the 116 kilodalton polypeptide, although smaller polypeptides, which reacted with NR-immunoglobulin G, were sometimes found at the gel front. When seedlings were grown on Kimpack paper or well washed sand, NR activity was again found only when the seedlings were exposed to light and NO₃⁻. Under these conditions, however, a protein of about 116 kilodaltons, which reacted with the NR antibody was present in light-grown plants whether NO₃⁻ was added to the system or not. The NR antibody cross-reacting protein was also seen in hydroponically grown plants when NH₄Cl⁻ was the only added form of nitrogen. These results indicate that the induction of an inactive NR-protein precursor in corn is mediated either by extremely low levels of NO₃⁻ or by some other unidentified factor, and that higher levels of NO₃⁻ are necessary for converting the inactive NR cross-reacting protein to a form of the enzyme capable of reducing NO₃⁻ to NO₂⁻.     

Characterization of Nitrate Reductase from Corn Leaves (Zea mays cv W64A x W182E) : Two Molecular Forms of the Enzyme

The primary leaves from corn seedlings grown for 6 days were harvested, frozen with liquid N₂ and extracted in a Tris buffer (pH 8.5, 250 millimolar) containing 1 millimolar dithiothreitol, 10 millimolar cysteine, 1 millimolar EDTA, 20 micromolar flavin adenine dinucleotide and 10% (v/v) glycerol. Nitrate reductase (NR) in the crude extract was stable for several days at 0°C and for several months at −80°C. The enzyme was purified using (NH₄)₂SO₄ fractionation, brushite-hydroxyl-apatite chromatography and blue-sepharose affinity chromatography. The enzyme was eluted from the blue-sepharose column with a linear gradient of NADH (0-100 micromolar) or with 0.3 molar KNO₃. About 10% of the original activity was recovered with NADH (NADH-NR). It had a specific activity of about 60 to 70 units (micromoles NO₂⁻ per minute per milligram protein). A sequential elution with NADH followed by KNO₃ (0.3 molar) or KCl (0.3 molar) yielded 2 peaks. Rechromatography of each peak gave two peaks again. These results indicate that we are dealing with two forms of the same enzyme rather than two different NR proteins. The two NRs had different molecular weights as judged by chromatography on Toyopearl. The NADH-NR was more sensitive than the NO₃⁻-NR to antibody prepared against barley leaf NR. In Ouchterlony assays a single precipitin line, with completely fused boundaries, was observed.     

Early effects of salinity on nitrate assimilation in barley seedlings

The effect of NaCl and Na₂SO₄ salinity on NO₃⁻ assimilation in young barley (Hordeum vulgare L. var Numar) seedlings was studied. The induction of the NO₃⁻ transporter was affected very little; the major effect of the salts was on its activity. Both Cl⁻ and SO₄²⁻ salts severely inhibited uptake of NO₃⁻. When compared on the basis of osmolality of the uptake solutions, Cl⁻ salts were more inhibitory (15-30%) than SO₄²⁻ salts. At equal concentrations, SO₄²⁻ salts inhibited NO₃⁻ uptake 30 to 40% more than did Cl⁻ salts. The absolute concentrations of each ion seemed more important as inhibitors of NO₃⁻ uptake than did the osmolality of the uptake solutions. Both K⁺ and Na⁺ salts inhibited NO₃⁻ uptake similarly; hence, the process seemed more sensitive to anionic salinity than to cationic salinity.

Unlike NO₃⁻ uptake, NO₃⁻ reduction was not affected by salinity in short-term studies (12 hours). The rate of reduction of endogenous NO₃⁻ in leaves of seedlings grown on NaCl for 8 days decreased only 25%. Nitrate reductase activity in the salt-treated leaves also decreased 20% but its activity, determined either in vitro or by the `anaerobic' in vivo assay, was always greater than the actual in situ rate of NO₃⁻ reduction. When salts were added to the assay medium, the in vitro enzymic activity was severely inhibited; whereas the anaerobic in vivo nitrate reductase activity was affected only slightly. These results indicate that in situ nitrate reductase activity is protected from salt injury. The susceptibility to injury of the NO₃⁻ transporter, rather than that of the NO₃⁻ reduction system, may be a critical factor to plant survival during salt stress.

     

Nitrate Reductase Activity in Soybeans (Glycine max [L.] Merr.): II. Energy Limitations

Growth chamber studies with soybeans (Glycine max [L.] Merr.) were designed to determine the relative limitations of NO₃⁻, NADH, and nitrate reductase (NR) per se on nitrate metabolism as affected by light and temperature. Three NR enzyme assays (+NO₃⁻in vivo, −NO₃⁻in vivo, and in vitro) were compared. NR activity decreased with all assays when plants were exposed to dark. Addition of NO₃⁻ to the in vivo NR assay medium increased activity (over that of the −NO₃⁻in vivo assay) at all sampling periods of a normal day-night sequence (14 hr-30 C day; 10 hr-20 C night), indicating that NO₃⁻ was rate-limiting. The stimulation of in vivo NR activity by NO₃⁻ was not seen in plants exposed to extended dark periods at elevated temperatures (16 hr-30 C), indicating that under those conditions, NO₃⁻ was not the limiting factor. Under the latter condition, in vitro NR activity was appreciable (19 μmol NO₂⁻ [g fresh weight, hr]⁻¹) suggesting that enzyme level per se was not the limiting factor and that reductant energy might be limiting.     

Maintenance of low cl concentrations in mesophyll cells of leaf blades of barley seedlings exposed to salt stress

The concentrations of vacuolar Na⁺ and Cl⁻ in the epidermal and mesophyll cells of the leaf blade and sheath of Hordeum vulgare seedlings (cv California Mariout and Clipper) were measured by means of quantitative electron probe x-ray microanalysis. A preferential accumulation of Cl⁻ in vacuoles of epidermal cells in both blade and sheath and a low level in mesophyll cells of the blade were evident in plants grown in full strength Johnson solution. The concentration of Cl⁻ in the mesophyll cells of the blade remained at a low level after exposure to 50 or 100 millimolar NaCl for 1 day or to 50 millimolar for 4 days, while at the same time the concentration of Cl⁻ in the epidermis and mesophyll of the sheath showed a dramatic increase. Clipper generally contained more Cl⁻ in the mesophyll cells of the blade than California Mariout. A greater accumulation of Na⁺ in the mesophyll of the sheath relative to that of the blade was only apparent after treatment with 100 millimolar NaCl for 1 day or 50 millimolar for 4 days. These results confirm the suggestion that sheath tissue is capable of accumulating excess Cl⁻ (and to a lesser extent Na⁺) and suggest that the site of regulation of Cl⁻ concentration in the barley leaf is located in the mesophyll cells of the blade.     

Adenosine 5'-triphosphate-sulfurylase in corn roots and its partial purification

ATP-sulfurylase (ATP-sulfate adenyltransferase, EC 2.7.7.4) was found in nonparticulate fractions of both roots and leaves of Zea mays L. seedlings using two detection methods. Addition of exogenous pyrophosphatase was essential for maximum rates of conversion of ³⁵SO₄²⁻ to labeled adenosine phosphosulfate in unpurified root extracts, but not in unpurified leaf extracts. In the presence of exogenous pyrophosphatase, the enzyme from roots exhibited specific activities as high as those obtained with the leaf enzyme. The root enzyme was purified 33-fold by centrifugation and column chromatography procedures. Its molecular weight obtained by Sephadex gel filtration was about 42,000. Its Km for pyrophosphate was 7 μm, while for adenosine phosphosulfate, the Km was 1.35 μm. None of the enzyme fractions studied converted adenosine phosphosulfate into detectable amounts of 3′-phosphoadenosine-5′-phosphosulfate. ATP-sulfurylase was also found in roots of corn seedlings grown aseptically. The data suggest that at least the first reaction in sulfate reduction might proceed as effectively in roots as in shoots.     

Comparative Induction of Nitrate Reductase by Nitrate and Nitrite in Barley Leaves

The comparative induction of nitrate reductase (NR) by ambient NO₃⁻ and NO₂⁻ as a function of influx, reduction (as NR was induced) and accumulation in detached leaves of 8-day-old barley (Hordeum valgare L.) seedlings was determined. The dynamic interaction of NO₃⁻ influx, reduction and accumulation on NR induction was shown. The activity of NR, as it was induced, influenced its further induction by affecting the internal concentration of NO₃⁻. As the ambient concentration of NO₃⁻ increased, the relative influences imposed by influx and reduction on NO₃⁻ accumulation changed with influx becoming a more predominant regulant. Significant levels of NO₃⁻ accumulated in NO₂⁻-fed leaves. When the leaves were supplied cycloheximide or tungstate along with NO₂⁻, about 60% more NO₃⁻ accumulated in the leaves than in the absence of the inhibitors. In NO₃⁻-supplied leaves NR induction was observed at an ambient concentration of as low as 0.02 mm. No NR induction occurred in leaves supplied with NO₂⁻ until the ambient NO₂⁻ concentration was 0.5 mm. In fact, NR induction from NO₂⁻ solutions was not seen until NO₃⁻ was detected in the leaves. The amount of NO₃⁻ accumulating in NO₂⁻-fed leaves induced similar levels of NR as did equivalent amounts of NO₃⁻ accumulating from NO₃⁻-fed leaves. In all cases the internal concentration of NO₃⁻, but not NO₂⁻, was highly correlated with the amount of NR induced. The evidence indicated that NO₃⁻ was a more likely inducer of NR than was NO₂⁻.     

Binding of Cysteine Synthase to the STAS Domain of Sulfate Transporter and Its Regulatory Consequences

The sulfate ion (SO₄²⁻) is transported into plant root cells by SO₄²⁻ transporters and then mostly reduced to sulfide (S²⁻). The S²⁻ is then bonded to O-acetylserine through the activity of cysteine synthase (O-acetylserine (thiol)lyase or OASTL) to form cysteine, the first organic molecule of the SO₄²⁻ assimilation pathway. Here, we show that a root plasma membrane SO₄²⁻ transporter of Arabidopsis, SULTR1;2, physically interacts with OASTL. The interaction was initially demonstrated using a yeast two-hybrid system and corroborated by both in vivo and in vitro binding assays. The domain of SULTR1;2 shown to be important for association with OASTL is called the STAS domain. This domain is at the C terminus of the transporter and extends from the plasma membrane into the cytoplasm. The functional relevance of the OASTL-STAS interaction was investigated using yeast mutant cells devoid of endogenous SO₄²⁻ uptake activity but co-expressing SULTR1;2 and OASTL. The analysis of SO₄²⁻ transport in these cells suggests that the binding of OASTL to the STAS domain in this heterologous system negatively impacts transporter activity. In contrast, the activity of purified OASTL measured in vitro was enhanced by co-incubation with the STAS domain of SULTR1;2 but not with the analogous domain of the SO₄²⁻ transporter isoform SULTR1;1, even though the SULTR1;1 STAS peptide also interacts with OASTL based on the yeast two-hybrid system and in vitro binding assays. These observations suggest a regulatory model in which interactions between SULTR1;2 and OASTL coordinate internalization of SO₄²⁻ with the energetic/metabolic state of plant root cells.     

Inhibition of Nitrate Transport by Anti-Nitrate Reductase IgG Fragments and the Identification of Plasma Membrane Associated Nitrate Reductase in Roots of Barley Seedlings

Membrane associated nitrate reductase (NR) was detected in plasma membrane (PM) fractions isolated by aqueous two-phase partitioning from barley (Hordeum vulgare L. var CM 72) roots. The PM associated NR was not removed by washing vesicles with 500 millimolar NaCl and 1 millimolar EDTA and represented up to 4% of the total root NR activity. PM associated NR was stimulated up to 20-fold by Triton X-100 whereas soluble NR was only increased 1.7-fold. The latency was a function of the solubilization of NR from the membrane. NR, solubilized from the PM fraction by Triton X-100 was inactivated by antiserum to Chlorella sorokiniana NR. Anti-NR immunoglobulin G fragments purified from the anti-NR serum inhibited NO₃⁻ uptake by more than 90% but had no effect on NO₂⁻ uptake. The inhibitory effect was only partially reversible; uptake recovered to 50% of the control after thorough rinsing of roots. Preimmune serum immunoglobulin G fragments inhibited NO₃⁻ uptake 36% but the effect was completely reversible by rinsing. Intact NR antiserum had no effect on NO₃⁻ uptake. The results present the possibility that NO₃⁻ uptake and NO₃⁻ reduction in the PM of barley roots may be related.     

Regulation of Assimilatory Sulfate Reduction by Herbicide Safeners in Zea mays L

Effects of the herbicide safeners N,N-diallyl-2,2-dichloroacetamide and 4-dichloroacetyl-3,4-dihydro-3-methyl-2H-1,4-benzooxazin (CGA 154281) on the contents in cysteine and glutathione, on the assimilation of ³⁵SO₄²⁻, and on the enzymes of assimilatory sulfate reduction were analyzed in roots and primary leaves of maize (Zea mays) seedlings. Both safeners induced an increase in cysteine and glutathione. In labeling experiments using ³⁵SO₄²⁻, roots of plants cultivated in the presence of safeners contained an increased level of radioactivity in glutathione and cysteine as compared with controls. A significant increase in uptake of sulfate was only detected in the presence of CGA 154281. One millimolar N,N-diallyl-2,2-dichloroacetamide applied to the roots for 6 days increased the activity of adenosine 5′-phosphosulfate sulfotransferase about 20- and threefold in the roots and leaves, respectively, compared with controls. CGA 154281 at 10 micromolar caused a sevenfold increase of this enzyme activity in the roots, but did not affect it significantly in the leaves. A significant increase in ATP-sulfurylase (EC 2.7.7.4) activity was only detected in the roots cultivated in the presence of 10 micromolar CGA 154281. Both safeners had no effect on the activity of sulfite reductase (EC 1.8.7.1) and O-acetyl-l-serine sulfhydrylase (EC 4.2.99.8). The herbicide metolachlor alone or combined with the safeners induced levels of adenosine 5′-phosphosulfate sulfotransferase, which were higher than those of the appropriate controls. Taken together these results show that the herbicide safeners increased both the level of adenosine 5′-phosphosulfate sulfotransferase activity and of the thiols cysteine and glutathione. This indicates that these safeners may be involved in eliminating the previously proposed regulatory mechanism, in which increased concentrations of thiols regulate assimilatory sulfate reduction by decreasing the activities of the enzymes involved.     

Inhibition of anion transport in corn root protoplasts

The effects of several amino-reactive disulfonic stilbene derivatives and N-(4-azido-2-nitrophenyl)-2-aminoethylsulfonate on Cl⁻, SO₄²⁻, and inorganic phosphate (Pi) uptake in protoplasts isolated from corn root tissue were studied. 4-Acetamido-4′-isothiocyano-2,2′-stilbenedisulfonic acid, 4,4′-diisothiocyano-2,2′-stilbenedisulfonic acid, 4,4′-diamino-2,2′-stilbenedisulfonic acid, and NAP-taurine inhibited Cl⁻ and SO₄²⁻ but not Pi and K⁺ uptake in corn root protoplasts; whereas mersalyl inhibited Pi but not Cl⁻ or SO₄²⁻ uptake. The rate of uptake of all anions decreased with increasing external pH. In addition, these reagents markedly inhibited plasmalemma ATPase activity isolated from corn root tissue. Excised root segments were less sensitive to Cl⁻ and SO₄²⁻ transport inhibitors.     

Effects of the Fungal Endophyte Acremonium coenophialum on Nitrogen Accumulation and Metabolism in Tall Fescue

Infection by the fungal endophyte Acremonium coenophialum affected the accumulation of inorganic and organic N in leaf blades and leaf sheaths of KY 31 tall fescue (Festuca arundinacea Schreb.) grown under greenhouse conditions. Total soluble amino acid concentrations were increased in either the blade or sheath of the leaf from infected plants. A number of amino acids were significantly increased in the sheath, but only asparagine increased in the blade. Infection resulted in higher sheath NH₄⁺ concentrations, whereas NO₃⁻ concentrations decreased in both leaf parts. The effects on amino acid, NO₃⁻, and NH₄⁺ concentrations were dependent upon the level of N fertilization and were usually apparent only at the high rate (10 millimolar) of application. Administration of ¹⁴CO₂ to the leaf blades increased the accumulation of ¹⁴C in their amino acid fraction but not in the sheaths of infected plants. This may indicate that infection increased amino acid synthesis in the blade but that translocation to the sheath, which is the site of fungal colonization, was not affected. Glutamine synthetase activity was greater in leaf blades of infected plants at high and low N rates of fertilization, but nitrate reductase activity was not affected in either part of the leaf. Increased activities of glutamine synthetase together with the other observed changes in N accumulation and metabolism in endophyte-infected tall fescue suggest that NH₄⁺ reassimilation could also be affected in the leaf blade.     

Inorganic Sulfur Oxidation by Aureobasidium pullulans

Aureobasidium pullulans (de Bary) Arnaud isolated from the phylloplane of sycamore exposed to heavy atmospheric pollution oxidized S⁰ to S₂O₃²⁻, S₄O₆²⁻, and SO₄²⁻ in vitro. The intermediates S₂O₃²⁻ and S₄O₆²⁻ were also oxidized to SO₄²⁻. Cell-free extracts of A. pullulans also oxidized reduced forms of S, the oxidation increasing linearly with increasing protein concentration, showing that the process is enzymatic. The possible role of fungi in S oxidation in soils is discussed.     

Soybean Mutants Lacking Constitutive Nitrate Reductase Activity : II. Nitrogen Assimilation, Chlorate Resistance, and Inheritance

Nitrogen assimilation in three nitrate reductase (NR) mutants of soybean (Glycine max L. Merr. cv Williams) was studied in the growth chamber and in the field. These mutants, LNR-2, LNR-3, and LNR-4, lack the non-NO₃⁻-inducible or constitutive fraction of leaf NR activity found in wild-type plants, but this had no effect on the concentration of nitrogen accumulated when grown on NO₃⁻ in the growth chamber. Dry weight accumulation of two of the mutants (LNR-3 and LNR-4) was decreased relative to LNR-2 and wild type. In the field, LNR-2 had dry weights and nitrogen concentrations similar to the wild type at 34 and 61 days after planting, and at maturity. Acetylene reduction activities were also similar at 61 days.     

Leaf water potential, stomatal resistance, and photosynthetic response to water stress in peach seedlings

Individual groups of peach (Prunus persica [L.] Batsch) seedlings stressed to −17, −26 and −36 bars recovered to control levels within 1, 3, and 4 days, respectively. Stomatal resistance was significantly correlated with both leaf water potential and net photosynthesis. In seedlings stressed to −52 bars, leaf water potential and stomatal resistance recovered sooner than net photosynthesis, despite recovery of 0₂ evolution at a rate similar to leaf water potential. Therefore, some nonstomatal factor other than reduction in photochemical activity must be responsible for the lag in recovery of CO₂ assimilation following irrigation.