Influence of drought on the concentration and distribution of 2,4-diaminobutyric acid and other free amino acids in tissues of flatpea (Lathyrus sylvestris L.)

2,4-Diaminobutyric acid (A₂bu) may be responsible for the apparent toxicity of flatpea (Lathyrus sylvestris L.) forage to some livestock. To obtain information relative to environmental regulation of A₂bu, 3-month old flatpea plants, cv. “Lathco”, were subjected to water-deficit stress for 1, 2, and 4 weeks. A₂bu, the most abundant free amino acid in roots, stems, and leaves, increased nearly 100% in roots of stressed plants. Increases in the concentrations of asparagine (Asn), proline (Pro), and arginine (Arg) occurred in roots; Asn, Pro, and 4-aminobutyric acid (Abu) in stems; and Pro and homoserine (Hse) in leaves also occurred in response to drought stress. Proline was a minor constituent of the free amino acid pool, even under water-deficit stress. The distribution of A₂bu and Pro in the stressed plants (roots > stems > leaves) was the reverse of that in plants supplied with adequate water (roots < stems < leaves). As concentrations of Asn and Abu decreased from roots to leaves in control tissues, concentrations of Hse and A₂bu increased in roughly the same proportions. This observation suggests that Abu and Asn may be precursors of A₂bu and Hse, respectively. The increase in A₂bu levels in aerial parts of drought-stressed flatpea plants is probably not sufficient to lower the feed value of the forage.     

Biochemical responses in leaves of two apple tree cultivars subjected to progressing drought

The influence of water deprivation on potted apple trees (Malus domestica Borkh.) was investigated. Biochemical responses including ascorbic acid, glutathione (GSH), tocopherols, chlorophylls, carotenoids, free amino acids, and soluble carbohydrates were measured in leaves of diploid ‘Elstar’ and triploid ‘Jonagold Wilmuta’ subjected to progressive drought. Leaf water potential was chosen to be the primary indicator of water stress in tested plants. Time courses of measured biochemical parameters showed that mild drought did not significantly affect the chosen stress indicators. Moderate drought increased the concentrations of ascorbic acid, total GSH, β-carotene, zeaxanthin and -tocopherol, indicating the adaptation to oxidative stress in apple trees. Moderate drought also increased concentration of soluble carbohydrates, mostly due to increased sorbitol concentration. Severe drought negatively affected vitality of apple trees, and caused a decrease in sorbitol concentration. Severe drought also caused decreases in ascorbic acid, total GSH, β-carotene, -tocopherol and chlorophyll concentrations, which, together with the increase in oxidised GSH concentration, indicated severe damage due to oxidative stress. Severe drought increased free amino acid concentration, which was probably the result of increased proteolysis. Zeaxanthin concentration remained high even in leaves of apple trees subjected to severe drought stress. The results were similar for both tested apple cultivars.     

Analysis by high-performance liquid chromatography of free amino acids extracted from needles of drought-stressed and shaded Pinus ponderosa seedlings

Free amino acid concentrations in needles of drought-stressed and shaded seedlings of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) were analyzed to determine the influence of low irradiance on a biochemical response to drought stress. A convenient method was developed for separating and quantitating amino acids in needle extracts using derivatization with dansyl chloride and reversed-phase high-performance liquid chromatography (HPLC). Twenty-one amino acids were separated on an Ultrasphere ODS CIS column and detected with a fluorescence spectrophotometer. As little as 10 pmol of each were detected with good peak separation and reproducible retention times. The results of HPLC analysis showed that drought and shading induced an increase in total amino acid concentrations in needles; shading had the greater effect. Arginine and proline concentrations increased most in needles of drought-stressed seedlings and remained high in unshaded seedlings recovering from drought 48 h after rewatering. Arginine and glutamine increased most in the shaded seedlings, which did not survive severe drought. The large increase in arginine in both drought-stressed and severely shaded seedlings suggests that sequestering and storage of ammonia are important when stress reduces carbon fixation.     

Effects of drought on non-mycorrhizal and mycorrhizal maize: changes in the pools of non-structural carbohydrates, in the activities of invertase and trehalase, and in the pools of amino acids and imino acids

To study the response of non-mycorrhizal and mycorrhizal maize plants to drought, the changes in the pools of non-structural carbohydrates and amino acids were analysed in leaves and roots of two maize cvs. Plants well colonized by the arbuscular mycorrhizal fungus Glomus mosseae (Nicol. & Gerd.) (60% of root length infected) and comparable non-mycorrhizal plants were subjected to moderate drought stress by reducing the water supply. This stress induced a conspicuous increase in the trehalose pool in the mycorrhizal roots, probably because it was accumulated by the fungal symbiont. Furthermore, glucose and fructose were accumulated in leaves and roots of non-mycorrhizal plants but not in the mycorrhizal ones. Starch disappeared completely from the leaves of both mycorrhizal and non-mycorrhizal plants in response to drought. Activities of soluble acid invertase and trehalase were also measured. Acid invertase activity increased during drought in the leaves of both non-mycorrhizal and mycorrhizal plants whilst in the roots it was unaffected in non-mycorrhizal plants and decreased in the mycorrhizal ones. Without drought stress, trehalase activity was considerably higher in the leaves and roots of mycorrhizal plants than in those of non-mycorrhizal plants. It increased conspicuously during drought, primarily in the leaves of non-mycorrhizal plants. A drought-induced accumulation of amino acids as well as imino acids was found in roots and leaves of both mycorrhizal and non-mycorrhizal plants; leaves of mycorrhizal plants accumulated more imino acids than those of non-mycorrhizal ones. Our results show that drought stress and the presence of a mycorrhizal fungus have a considerable effect on carbon partitioning, imino acid and amino acid accumulation in maize plants.     

Salicylic acid-induced adaptive response to copper stress in sunflower (Helianthus annuus L.)

The ameliorative effect of salicylic acid (SA: 0.5 mM) on sunflower (Helianthus annuus L.) under Cu stress (5 mg l⁻¹) was studied. Excess Cu reduced the fresh and dry weights of different organs (roots, stems and leaves) and photosynthetic pigments (chlorophyll a, b and carotenoids) in four-week-old plants. There was a considerable increase in Chl a/b ratio and lipid peroxidation in both the roots and leaves of plants under excess Cu. Soluble sugars and free amino acids in the roots also decreased under Cu stress. However, soluble sugars in the leaves, free amino acids in the stems and leaves, and proline content in all plant organs increased in response to Cu toxicity. Salicylic acid (SA) significantly reduced the Chl a/b ratio and the level of lipid peroxidation in Cu-stressed plants. Under excess Cu, a higher accumulation of soluble sugars, soluble proteins and free amino acids including proline occurred in plants treated with 0.5 mM SA. Exogenous application of SA appeared to induce an adaptive response to Cu toxicity including the accumulation of organic solutes leading to protective reactions to the photosynthetic pigments and a reduction in membrane damage in sunflower.     

Osmotically Induced Proline Accumulation in <Emphasis Type="Italic">Lotus Corniculatus</Emphasis> Leaves is Affected by Light and Nitrogen Source

Proline accumulation in osmotically stressed leaves of Lotus corniculatus was stimulated by increasing light intensity (photon fluence density, PFD). Treatment with propanil limited proline accumulation in response to light and osmotic stress, indicating a dependence of proline synthesis on photosynthetic NADPH. Drought stress induced proline accumulation in L. corniculatus both in nitrate-fed plant (NFP) and ammonium-fed plants (AFP), although higher proline concentration was observed in AFP than in NFP after 24 h of drought stress. Changes in proline accumulation induced by drought stress in plants grown under different nitrogen regimes could not be explained by changes of either total protein or amino acids, consistent with specifically altered regulation of proline synthesis. Under control conditions, alanine, aspartate and glutamate were the predominant amino acids in NFP; conversely, in AFP, arginine and ornithine were the predominant amino acids. Only the NFP regime showed changes in the concentrations of specific amino acids under drought stress a decrease in alanine, aspartate and glutamate and increased gama-aminobutyric acid. In AFP and especially NFP, proline accumulation under osmotic stress was associated with increased ornithine amino transferase activity. An increase of both activity and protein of ferredoxin-dependent glutamate synthase was observed in osmotic-stressed NFP; inversely both decreased in drought-stressed AFP. PFD and nitrogen source are therefore shown to be regulators of proline accumulation in L. corniculatus osmotically stressed plants.     

Nonprotein amino acid composition of flatpea (Lathyrus sylvestris L.) as affected by ethephon seed treatments and seedling fertilization

Use of flatpea (Lathyrus sylvestris L.) as a forage is limited because of nonuniform seed germination and the potentially toxic effects of 2,4-diaminobutyric acid (A₂bu), a nonprotein amino acid found in seeds and vegetative tissues. The effects of ethephon (2-chloroethyl phosphonic acid) on seed germination, amino acid leachates of seeds, and amino acid composition (particularly A₂bu) of seedlings were investigated. Germination of flatpea seeds, imbibed for 16 h in 0, 100, 200, 400, 800, and 1600 mg/L ethephon, did not differ, but amino acid leachates tended to increase up to 200 mg/L ethephon and then decline at higher concentrations. The major amino acid constituents in leachates were A₂bu, 4-aminobutyric acid (Abu), and homoserine (Hse). Dry matter accumulation of seedlings grown from ethephon-treated seeds was reduced for second cuttings grown from ethephon-treated seeds and high nitrogen grown plants. During regrowth, free amino acid accumulation was most pronounced in leaves of plants supplied with high nitrogen. The most abundant free amino acids in flatpea tissues were the same as those in seed leachates, but concentration and relative abundance varied with nitrogen level, plant part, and ethephon treatment. Results suggest that ethephon seed treatments can have persistent effects on the growth and amino acid composition of flatpea seedlings grown under different nitrogen regimes.     

Soil Drench Treatment with ?-Aminobutyric Acid Increases Drought Tolerance of Potato

The non-protein amino acid β-aminobutyric acid (BABA) is known to be a priming agent for a more efficient activation of cellular defence responses and a potent inducer of resistance against biotic and abiotic stresses in plants. Nevertheless, most of the studies on priming have been carried out in Arabidopsis. In potato, the effect of BABA was demonstrated only on biotic stress tolerance. We investigated the effect of BABA on the drought tolerance of potato and found that soil drenched with BABA at a final concentration of 0.3 mM improves the drought tolerance of potato. Water loss from the leaves of the primed plants is attenuated and the yield is increased compared to the unprimed drought-stressed plants. The metabolite composition of the tubers of the BABA-treated plants is less affected by drought than the tuber composition of the non-treated plants. Nitric oxide and ROS (reactive oxygen species) production is increased in the BABA-treated roots but not in the leaves. In the leaves of the BABA-treated plants, the expression of the drought-inducible gene StDS2 is delayed, but the expression of ETR1, encoding an ethylene receptor, is maintained for a longer period under the drought conditions than in the leaves of the non-treated, drought-stressed control plants. This result suggests that the ethylene-inducible gene expression remains suppressed in primed plants leading to a longer leaf life and increased tuber yield compared to the non-treated, drought-stressed plants. The priming effect of BABA in potato, however, is transient and reverts to an unprimed state within a few weeks.     

Nonprotein amino acid composition of flatpea (Lathyrus sylvestris L.) as affected by ethephon seed treatments and seedling fertilization

Use of flatpea (Lathyrus sylvestris L.) as a forage is limited because of nonuniform seed germination and the potentially toxic effects of 2,4-diaminobutyric acid (A₂bu), a nonprotein amino acid found in seeds and vegetative tissues. The effects of ethephon (2-chloroethyl phosphonic acid) on seed germination, amino acid leachates of seeds, and amino acid composition (particularly A₂bu) of seedlings were investigated. Germination of flatpea seeds, imbibed for 16 h in 0, 100, 200, 400, 800, and 1600 mg/L ethephon, did not differ, but amino acid leachates tended to increase up to 200 mg/L ethephon and then decline at higher concentrations. The major amino acid constituents in leachates were A₂bu, 4-aminobutyric acid (Abu), and homoserine (Hse). Dry matter accumulation of seedlings grown from ethephon-treated seeds was reduced for second cuttings grown from ethephon-treated seeds and high nitrogen grown plants. During regrowth, free amino acid accumulation was most pronounced in leaves of plants supplied with high nitrogen. The most abundant free amino acids in flatpea tissues were the same as those in seed leachates, but concentration and relative abundance varied with nitrogen level, plant part, and ethephon treatment. Results suggest that ethephon seed treatments can have persistent effects on the growth and amino acid composition of flatpea seedlings grown under different nitrogen regimes.     

The effects of drought stress on free amino acid accumulation and protein synthesis in Brassica napus

Changes in the concentrations of free amino acids and specific organic acids were analysed during the induction of drought stress in Brassica napus . Most of the amino acids showed a characteristic linear increase with the induction of drought stress in Brassica leaves, increasing an average of 5.9-fold over control levels, followed by a reduction in concentration upon rehydration of the plants. Pyruvate concentrations doubled after 4 days of drought stress whereas 2-oxoglutarate concentrations remained relatively constant. The activities of two of the enzymes involved in amino acid biosynthesis, alanine aminotransferase (EC 2.6.1.2) and aspartate aminotransferase (EC 2.6.1.1), were also measured. Neither enzyme showed any increase in activity, except when the plants were rehydrated. This suggests that the increase in both alanine and aspartate levels results from the increase in their precursors pyruvate and glutamate and may not require increased enzyme activity. The effect of drought stress upon changes in protein synthesis was analysed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. We found that there was an overall decrease in protein synthesis with the induction of drought stress, followed by a resumption of synthesis upon rehydration. In addition, the synthesis of a number of specific polypeptides was found to decrease upon water loss in the leaves.     

Modulation of endogenous levels of some key organic metabolites by exogenous application of glycine betaine in drought stressed plants of sunflower (<Emphasis Type="Italic">Helianthus annuus</Emphasis> L.)

The present study was conducted to examine the changes in some key metabolites in drought-stressed sunflower plants supplied with glycine betaine externally. Imposition of drought stress at the vegetative or reproductive growth stages decreased the plant dry matter production and increased the accumulation of organic solutes (glycine betaine, proline, soluble proteins, free amino acids and soluble sugars) in two sunflower lines, i.e., Glushan-98 and Suncross. In general, decrease in dry matter production and increase in the endogenous levels of organic solutes, were more pronounced when drought stress applied at the vegetative stage than at the reproductive stage. Glycine betaine applied as a pre-sowing seed treatment was not found to be effective in reducing the negative effects of drought stress in sunflower plants. Foliar application of GB further enhanced the leaf endogenous levels of GB, soluble proteins and total soluble sugars in drought stressed plants without exerting any negative effects on other osmotica. However, this GB-induced increase in endogenous levels of organic solutes was found to be not associated with plant dry matter production under stress conditions.     

Growth and 2, 4-Diaminobutyric Acid Composition of Flatpea (Lathyrus sylvestris L.) as Influenced by Mineral- and Symbiotically Fixed-Nitrogen

Flatpea (Lathyrus sylvestris L. cv. ‘Lathco’) plants,grown hydroponically, were inoculated with rhizobia (Rhizobiumleguminosarum strain 92F2), supplied with nitrate nitrogen (2meq 1^–1), or provided with both rhizobia and . Nodulated plants supplied with had the highest biomass and the highest level of free amino acids in leavesand roots. Nitrogen fixation was depressed 80% in inoculatedplants supplemented with . Compared to plants whose nitrogen requirement was satisfied either partially orfully by , plants that were totally dependent upon biological nitrogen fixation had fewer lateral shoots andlower concentrations of nitrogen, soluble protein, and totalfree amino acids in the shoots, especially in the leaves. Thesymbiotic association between plants and rhizobia for nitrogenfixation was not essential to the production and accumulationof 2,4-diaminobutyric acid (A₂bu) and other non-protein aminoacids in flatpea. Levels of A₂bu, and most of the other freeamino acids quantified, were significantly lower in the nodulatedplants grown in the absence of than in the inoculated or noninoculated plants supplied with . Arginine and glutamic acid were exceptions in that levels ofthese two amino acids were higher in roots bearing effectivenodules. Plant responses to the different treatments are attributedto the amount of nitrogen available to the plant, rather thanthe method or form of nitrogen supplied. Key words: 2,4-Diaminobutyric acid, Lathyrus, nitrogen     

Leaf development in Ricinus communis during drought stress: dynamics of growth processes, of cellular structure and of sink-source transition

Dicot leaf growth is characterized by partly transient tip-to-base gradients of growth processes, structure and function. These gradients develop dynamically and interact with dynamically developing stress conditions like drought. In Ricinus communis plants growing under well-watered and drought conditions growth rates peaked during the late night and minimal values occurred in the late afternoon. During this diurnal course the leaf base always showed much higher rates than the leaf tip. The amplitude of this diurnal course decreased when leaves approached maturity and during drought stress without any significant alteration of the diurnal pattern and it increased during the first days after rewatering. Unique relationships between leaf size and cytological structure were observed. This provided the framework for the analysis of changes in assimilation, transpiration and dark respiration, chlorophyll, protein, carbohydrate, and amino acid concentrations, and of activities of sink-source-related enzymes at the leaf tip and base during leaf development in well-watered and drought-stressed plants. Gas exchange was dominated by physiological rather than by anatomical properties (stomatal density). Tip-to-base gradients in carbohydrate concentrations per dry weight and sink-source-related enzymes were absent, whereas significant gradients were found in amino acid concentrations per dry weight. During drought stress, growing leaves developed source function at smaller leaf size, before specific physiological adaptations to drought occurred. The relevance of the developmental status of individual leaves for the drought-stress response and of the structural changes for the biochemical composition changes is discussed.     

Drought-Stressed Tomato Plants Trigger Bottom–Up Effects on the Invasive Tetranychus evansi

Climate change will bring more drought periods that will have an impact on the irrigation practices of some crops like tomato, from standard water regime to deficit irrigation. This will promote changes in plant metabolism and alter their interactions with biotic stressors. We have tested if mild or moderate drought-stressed tomato plants (simulating deficit irrigation) have an effect on the biological traits of the invasive tomato red spider mite, Tetranychus evansi. Our data reveal that T evansi caused more leaf damage to drought-stressed tomato plants (≥1.5 fold for both drought scenarios). Mite performance was also enhanced, as revealed by significant increases of eggs laid (≥2 fold) at 4 days post infestation (dpi), and of mobile forms (≥2 fold and 1.5 fold for moderate and mild drought, respectively) at 10 dpi. The levels of several essential amino acids (histidine, isoleucine, leucine, tyrosine, valine) and free sugars in tomato leaves were significantly induced by drought in combination with mites. The non-essential amino acid proline was also strongly induced, stimulating mite feeding and egg laying when added to tomato leaf disks at levels equivalent to that estimated on drought-infested tomato plants at 10 dpi. Tomato plant defense proteins were also affected by drought and/or mite infestation, but T. evansi was capable of circumventing their potential adverse effects. Altogether, our data indicate that significant increases of available free sugars and essential amino acids, jointly with their phagostimulant effect, created a favorable environment for a better T. evansi performance on drought-stressed tomato leaves. Thus, drought-stressed tomato plants, even at mild levels, may be more prone to T evansi outbreaks in a climate change scenario, which might negatively affect tomato production on area-wide scales.     

Influence of arbuscular mycorrhiza on organic solutes in maize leaves under salt stress

A pot experiment was conducted to examine the effect of the arbuscular mycorrhizal (AM) fungus, Glomus mosseae, on plant biomass and organic solute accumulation in maize leaves. Maize plants were grown in sand and soil mixture with three NaCl levels (0, 0.5, and 1.0 g kg⁻¹ dry substrate) for 55 days, after 15 days of establishment under non-saline conditions. At all salinity levels, mycorrhizal plants had higher biomass and higher accumulation of organic solutes in leaves, which were dominated by soluble sugars, reducing sugars, soluble protein, and organic acids in both mycorrhizal and non-mycorrhizal plants. The relative abundance of free amino acids and proline in total organic solutes was lower in mycorrhizal than in non-mycorrhizal plants, while that of reducing sugars was higher. In addition, the AM symbiosis raised the concentrations of soluble sugars, reducing sugars, soluble protein, total organic acids, oxalic acid, fumaric acid, acetic acid, malic acid, and citric acid and decreased the concentrations of total free amino acids, proline, formic acid, and succinic acid in maize leaves. In mycorrhizal plants, the dominant organic acid was oxalic acid, while in non-mycorrhizal plants, the dominant organic acid was succinic acid. All the results presented here indicate that the accumulation of organic solutes in leaves is a specific physiological response of maize plants to the AM symbiosis, which could mitigate the negative impact of soil salinity on plant productivity.     

Changes in free amino acids and polyamine levels in Satsuma leaves in response to Asian citrus psyllid infestation and water stress

The effects of biotic and abiotic stresses on changes in amino acids and polyamine levels in Satsuma orange （Citrus unshiu; cultivar Owari） leaves were inves- tigated. Asian citrus psyllids Diaphorina citri （Kuwayama） （ACP） infestation was used to induce biotic stress while a water deficit was imposed to induce abiotic stress. Potted trees were infested by placing 50 psyllids on 3 citrus leaves enclosed in nylon mesh bags for 5 d. A parallel set of plants were kept water stressed by maintaining the soil at 20% water holding capacity for 5 d. Levels of total free amino acids were higher in water stressed and ACP infested leaves. Polyamine putrescine increased in infested leaves but not in water stressed leaves. Proline was the most abundant amino acid and its levels significantly increased by both biotic and abiotic stresses. Proline levels in infested leaves were significantly higher than the water stressed leaves. Histidine, methionine, asparagine, arginine, serine, and leucine levels also increased significantly in infested leaves, but in water stressed leaves only leucine, methionine, and threonine increased. Levels of amino acids, such as tyrosine, isoleucine, phenylalanine, glutamic acid, and alanine, declined in infested leaves. Under water stress asparagine, phenylalanine, serine, and histidine also declined compared to controls. This indicates that while proteolysis occurred under both stresses, metabolic conversion of amino acids was different under the two stresses. In ACP infested leaves some amino acids may be used as feeding material and/or converted into secondary metabolites for defense.     

Inhibition of N2 fixation in soybean is associated with elevated ureides and amino acids

Decreased N2 fixation in soybean (Glycine max) L. Merr. during water deficits has been associated with increases in ureides and free amino acids in plant tissues, indicating a potential feedback inhibition by these compounds in response to drought. We evaluated concentrations of ureides and amino acids in leaf and nodule tissue and the concurrent change in N2 fixation in response to exogenous ureides and soil-water treatments for the cultivars Jackson and KS4895. Exogenous ureides applied to the soil and water-deficit treatments inhibited N2 fixation by 85% to 90%. Mn fertilization increased the apparent catabolism of ureides in leaves and hastened the recovery of N2 fixation following exogenous ureide application for both cultivars. Ureides and total free amino acids in leaves and nodules increased during water deficits and coincided with a decline in N2 fixation for both cultivars. N2 fixation recovered to 74% to 90% of control levels 2 d after rewatering drought-stressed plants, but leaf ureides and total nodule amino acids remained elevated in KS4895. Asparagine accounted for 82% of the increase in nodule amino acids relative to well-watered plants at 2 d after rewatering. These results indicate that leaf ureides and nodule asparagine do not feedback inhibit N2 fixation. Compounds whose increase and decrease in concentration mirrored the decline and recovery of N2 fixation included nodule ureides, nodule aspartate, and several amino acids in leaves, indicating that these are potential candidate molecules for feedback inhibition of N2 fixation.     

Effects of host plant drought stress on the performance of the bird cherry-oat aphid, Rhopalosiphum padi (L.): a mechanistic analysis

Abstract.  1. The growth (increase in height and leaf number) of four grass species was reduced by a −0.5 MPa drought stress, but the performance of an associated herbivore, Rhopalosiphum padi (L.), was not affected consistently. The intrinsic rate of increase of R. padi was reduced by drought stress on three grass species, including Dactylis glomerata (L.), but was unaffected on Arrhenatherum elatius (L.). Therefore, there is no general relationship in the effect of plant drought on an insect herbivore, even among closely related host plant species.
2. Drought stress increased the quality of plant phloem sap, as indicated by increased sieve element osmotic pressure and essential amino acid concentrations. Thus, diet quality could not account for the reduced performance of R. padi under drought stress. The concentration of essential amino acids in the phloem of well-watered A. elatius was, however, lower than that of well-watered D. glomerata , correlating with the decreased performance of aphids on well-watered A. elatius .
3. There were no differences in aphid feeding duration between watering treatments or plant species but sap ingestion rates were reduced significantly under drought stress.
4. Using the measure of dietary amino acid concentrations and the estimate of sap ingestion, the essential amino acid flux through aphids was calculated. Compared with the flux through aphids feeding on well-watered D. glomerata , there was a reduction in aphids feeding on drought-stressed D. glomerata and drought-stressed A. elatius due to lower sap ingestion rates. The flux through aphids on well-watered A. elatius was also reduced due to low phloem essential amino acid concentrations. Thus, the performance of an aphid is correlated with the availability and accessibility of essential amino acids.     

Changes in Mulberry Leaf Metabolism in Response to Water Stress

A series of experiments were conducted to characterize the water stress-induced changes in the activities of RuBP carboxylase (RuBPCO) and sucrose phosphate synthase (SPS), photosystem 2 activity, and contents of chlorophylls, carotenoids, starch, sucrose, amino acids, free proline, proteins and nucleic acids in mulberry (Morus alba L. cv. K-2) leaves. Water stress progressively reduced the activities of RuBPCO and SPS in the leaf extracts, the chlorophyll content, and PS2 activity in isolated chloroplasts. Plants exposed to drought showed lower content of starch and sucrose but higher total sugar content than control plants. While the soluble protein content decreased under water stress, the amino acid content increased. Proline accumulation (2.5-fold) was noticed in stressed leaves. A reduction in the contents of DNA and RNA was observed. Reduced nitrogen content was associated with the reduction in nitrate reductase activity. SDS-PAGE protein profile showed few additional proteins (78 and 92 kDa) in the water stressed plants compared to control plants.