THE EFFECTS OF GIBBERELLIC ACID AND A GROWTH RETARDANT ON OVULE FORMATION AND GROWTH OF EXCISED PISTILS OF NIGELLA RANUNCULACEAE

The effects of gibberellic acid (GA₃) and the growth retardant AMO-1618 on ovule formation in excised pistils of Nigella sativa L. were studied by sterile culture techniques. Gibberellic acid promoted pistil growth and inhibited ovule formation. The role of endogenous gibberellins in ovule formation and pistil growth was investigated by adding AMO to the basal medium. Both pistil lengths and ovule formation were reduced significantly with increasing concentrations of AMO. The addition of low concentrations of GA₃ to the medium restored pistil growth but did not reverse the inhibitory effect of AMO on ovule formation. The addition of kinetin or indoleacetic acid (IAA) to the medium containing AMO had no effect on pistil lengths. However, with the addition of 10⁻⁷ m kinetin, the number of ovules in pistils was increased but not to the levels found in pistils grown in the absence of AMO.     

Leaf water and carbohydrate status of VA mycorrhizal rose exposed to drought stress

Summary Shoot water relations and carbohydrate levels were compared for droughted nonmycorrhizal and vesicular-arbuscular (VA) mycorrhizalRosa hybrida L. cv ‘Samantha’ plants grown with high and low phosphorus fertilization. Leaf diffusive conductance (g _i ) of plants colonized byGlomus intraradices Schenk and Smith andGlomus deserticola Trappe, Bloss and Menge were 2 × and 1.5× greater, respectively, than in nonmycorrhizal plants. Regardless of P fertilization, leaf osmotic and bulk water potentials were 0.5 to 1.1 MPa higher in mycorrhizal than in nonmycorrhizal plants. Leaf starch, chlorophyll and water contents while fructose, glucose and total soluble carbohydrates were lower. Level of P fertilization had no effect on water relations or soluble carbohydrate content of nonmycorrhizal roses. The water status of droughted rose was impoved more byG. intraradices than byG. deserticola. Washington State University College of Agriculture and Home Economics Research Center Scientific Paper No. 7375.     

Differential plant growth and osmotic effects of two maize (<Emphasis Type="Italic">Zea mays</Emphasis> L<Emphasis Type="Italic">.</Emphasis>) cultivars to exogenous glycinebetaine application under drought stress

We investigated the effects of exogenous glycinebetaine (GB) and drought stress (DS) on grain yield (GY) and production of dry matter (DM) and osmolytes in two maize (Zea mays L.) cultivars i.e. Shaandan 9 (S₉) and Shaandan 911 (S₉₁₁) during the entire growing period. Drought stress substantially reduced DM and GY but increased free proline, endogenous GB, soluble sugar and K⁺ concentrations in leaves of both cultivars. The DM production, GY, drought index (DI) and concentrations of these osmolytes were greater for S₉ than those for S₉₁₁ under DS. The significant differences in these parameters suggested that S₉ was more drought-tolerant as compared to S₉₁₁. Additionally, foliar application of GB increased the concentrations of all osmolytes measured, DM and GY of both cultivars under DS. These positive responses of exogenous GB spray were more pronounced in S₉₁₁ as compared to those in S₉. Further correlation analysis involving a number of parameters indicated that maize production was tighterly correlated with accumulation of the osmolytes measured during DS rather than well-watered controls. Accordingly, this study demonstrated the notion of an anti-drought role of exogenous GB by osmoregulation under DS, particularly in this drought sensitive cultivar. Thus, exogenous GB application might be firstly used with drought sensitive species/cultivars when exposed to DS.     

Comparative physiological and proteomic analysis of cultivated and wild safflower response to drought stress and re-watering

Drought is one of the major environmental stress that adversely affect the growth and development of oil seed plant, safflower. There is a limited knowledge on proteomic responses to support physiological, biochemical changes in how safflowers can regulate growth and metabolism under drought conditions and followed by re-watering. The changes in morphological, physiological, biochemical and proteomics of safflower genotypes (Carthamus tinctorius L.; Remzibey-05 and Linas, tolerant and sensitive cultivars, respectively, and C. oxyacantha M. Bieb., wild type) after exposure to drought and followed by re-watering have been examined. Drought negatively affected the shoot weight, water content, chlorophyll fluorescence, and biochemical parameters, including photosynthetic pigment, proline, MDA, and H₂O₂ contents and antioxidant enzyme activities in all genotypes, while the re-watering period allowed Remzibey-05 to recover, and it even provided the wild type completely recovered (approximately 100%). A total of 72 protein spots were observed as differently accumulated under treatments. The identified proteins were mainly involved in photosynthesis and carbohydrate, protein, defense, and energy metabolisms. Protein accumulation related to these metabolisms in Remzibey-05 were decreased under drought, while increased following re-watering. However, sensitive cultivar, Linas, could not exhibit an effective performance under drought and recovery when compared with other safflower genotypes. Supplementary InformationThe online version contains supplementary material available at (10.1007/s12298-021-00934-2).     

Recycling of CO2 during induction of CAM by drought in Talinum paniculatum (Portulacaceae)

To investigate the possible induction of Crassulacean acid metabolism (CAM) by drought in Talinum paniculatum ([Jacq.] Gaertn.), a deciduous herb with succulent leaves and lignified stems, nocturnal acid accumulation and CO₂-exchange were studied in watered and droughted greenhouse-grown plants. Watered plants had a typical C3 pattern of CO₂-exchange. When plants were subjected to drought, nocturnal acid accumulation increased significantly from 0.9 to 13.4 μmol H⁺ cm^?2 after 21 days. Water deficit provoked a rapid reduction of daytime CO₂ assimilation of as much as 92% and a slower increase in night-time fixation. A maximum of 24% of the diel carbon gain was contributed by dark fixation in droughted plants. After 34 days of drought, only CO₂ compensation and a small accumulation of acid (idling) was detected during the night. Relative recycling of respiratory CO₂ was approximately 100% for most of the water deficit treatment, the amount of CO₂ recycled showing a high positive correlation with nocturnal acid accumulation. A low rate of nocturnal loss of CO₂ in watered plants did not explain the amount recycled nightly in droughted plants, implying that respiration increased with drought. Leaf lamina area was reduced by 49% during drought due to rolling. Leaf biomass remained unchanged during the water-deficit treatment. Neither apparent quantum yield nor light-saturated photosynthetic rate differed significantly between control and 14-day water-stressed plants rewatered for 20 h. Chlorophyll content did not change with drought. These results confirm that CAM is induced by drought in T. paniculatum; the carbon acquired through this pathway only contributes to maintain, but not to increase, leaf biomass; also, CAM is responsible for a high recycling of respiratory CO₂ during the night. Recycling through CAM, plus the reduction of exposed leaf area during drought, may help explain the maintenance of chlorophyll, quantum yield and saturated photosynthetic rates in water-stressed plants of T. paniculatum.     

Physiological responses of different olive genotypes to drought conditions

Gas exchange rates, chlorophyll fluorescence, pressure–volume relationships, photosynthetic pigments, total soluble sugars, starch, soluble proteins and proline concentrations were investigated in five Olea europaea L. cultivars with different geographical origins (Arbequina, Blanqueta, Cobran?osa, Manzanilla and Negrinha) grown under Mediterranean field conditions. We found considerable genotypic differences among the cultivars. Comparing the diurnal gas exchange rates, we observed that Cobran?osa, Manzanilla and Negrinha had high photosynthetic rate than Arbequina and Blanqueta. The first group reveals to be better acclimated to drought conditions, and appears to employ a prodigal water-use strategy, whereas Blanqueta and Arbequina, with high water-use efficiency, appear to employ a conservative water-use strategy. The degree of midday depression in photosynthesis was genotype dependent, with a maximum in Arbequina and a minimum in Negrinha. The reductions in the photosynthetic rate were dependent from both stomatal and non-stomatal limitations. Elastic adjustment plays an important role as drought tolerance mechanism. The group of cultivars that employ a prodigal water-use strategy revealed high tissue elasticity, whereas Arbequina and Blanqueta revealed high tissue rigidity. We also identified the existence of drought tolerance mechanisms associated with soluble proteins accumulation in the foliage. The high levels of soluble proteins in Arbequina may represent an increased activity of oxidative stress defence enzymes and may also represent a reserve for post stress recovery. In all cultivars, especially in Manzanilla, free proline was accumulated in the foliage. The discussed aspects of drought stress metabolism may have an adaptative meaning, supporting the hypothesis that olive cultivars native to dry regions, such as Cobran?osa, Manzanilla and Negrinha, have more capability to acclimate to drought conditions than cultivars originated in regions with a more temperate climate, like Arbequina and Blanqueta.     

Impact of phosphorus application on drought resistant responses of Eucalyptus grandis seedlings

Eucalyptus grandis is the most widely planted tree species worldwide and can face severe drought during the initial months after planting because the root system is developing. A complete randomized design was used to study the effects of two water regimes (well‐watered and water‐stressed) and phosphorus (P) applications (with and without P) on the morphological and physio‐biochemical responses of E. grandis. Drought had negative effects on the growth and metabolism of E. grandis, as indicated by changes in morphological traits, decreased net photosynthetic rates (P_n), pigment concentrations, leaf relative water contents (LRWCs), nitrogenous compounds, over‐production of reactive oxygen species (ROS) and higher lipid peroxidation. However, E. grandis showed effective drought tolerance strategies, such as reduced leaf area and transpiration rate (E), higher accumulation of soluble sugars and proline and a strong antioxidative enzyme system. P fertilization had positive effects on well‐watered seedlings due to improved growth and photosynthesis, which indicated the high P requirements during the initial E. grandis growth stage. In drought‐stressed seedlings, P application had no effects on the morphological traits, but it significantly improved the LRWC, P_n, quantum efficiency of photosystem II (F_v/F_m), chlorophyll pigments, nitrogenous compounds and reduced lipid peroxidation. P fertilization improved E. grandis seedling growth under well‐watered conditions but also ameliorated some leaf physiological traits under drought conditions. The effects of P fertilization are mainly due to the enhancement of plant N nutrition. Therefore, P can be used as a fertilizer to improve growth and production in the face of future climate change.     

Overaccumulation of glycine betaine enhances tolerance to drought and heat stress in wheat leaves in the protection of photosynthesis

We investigated the different responses of wheat (Triticum aestivum L.) plants to drought- (DS) and heat stress (HS), and analyzed the physiological mechanisms of glycine betaine (GB) involved in the improvement of wheat tolerance to the combination of these stresses. The transgenic wheat T6 line was generated by introducing a gene encoding betaine aldehyde dehydrogenase (BADH) into the wild-type (WT) Shi4185 line. The gene was cloned from the Garden Orache plant (Atriplex hortensis L.). Wheat seedlings were subjected to drought stress (30%, PEG-6000), heat stress (40°C), and their combination. Photosynthetic gas exchange, water status and lipid peroxidation of wheat leaves were examined under different stresses. When subjected to a combination of drought and heat, the inhibition of photosynthesis was significantly increased compared to that under DS or HS alone. The increased inhibition of photosynthesis by the combined stresses was not simply the additive stress effect of separate heat- and drought treatments; different responses in plant physiology to DS and HS were also found. HS decreased the chlorophyll (Chl) content, net photosynthetic rate (P _N), carboxylation efficiency (CE) and apparent quantum yield (AQY) more than DS but DS decreased the transpiration rate (E), stomata conductance (g _s) and intercellular CO₂ concentration (C _i) more than HS. GB over-accumulation led to increased photosynthesis not only under individual DS or HS but also under their combination. The enhancement of antioxidant activity and the improvement of water status may be the mechanisms underlying the improvement of photosynthesis by GB in wheat plants.     

Differential inhibition of photosynthesis during pre-flowering drought stress in Sorghum bicolor genotypes with different senescence traits

Young (16-day-old) Sorghum bicolor plants of a late- and slow-senescing Texas A&M line (B 35) and of an early- and fast-senescing descendant of an Ethiopian landrace (E 36-1) were subjected to drought stress by decreasing the soil water content to 30% field capacity over 6 days. Plant water potentials decreased from − 2 bar (controls) to − 10 to − 18 bar, and this drought stress resulted in: (1) differential phenotypic reactions and (2) differential decreases in photosynthesis rates in the two cultivars. While E 36-1 tended to lose viable leaf area from the leaf tips downwards, B 35 showed a gradual overall drying of the leaf. At the same time, photosynthesis rates decreased from 31.5 ± 1.6 to 12.3 ± 5.0 µmol CO₂ m⁻² s⁻¹ (E 36-1) and from 30.5 ± 1.6 to 3.3 ± 2.6 µmol CO₂ m⁻² s⁻¹ (B 35), respectively. In vitro enzyme activities of phosphoenolpyruvate carboxylase (PEPCase), malate dehydrogenase (MDH) and malic enzyme (ME) on a leaf area basis exceeded the photosynthesis rates. Pyruvate phosphate dikinase (PPDK) activity was close to the photosynthesis rates in control plants and higher than the photosynthesis rates in drought-stressed plants. Thus, none of the enzymes appeared to limit photosynthesis under drought stress, and likely bottleneck enzyme activities of the C3 pathway in the bundle-sheath cells, i.e. ribulose-1,5-bisphosphate carboxylase (RubisCO) and stromal fructose-1,5-bisphosphatase (sFBPase), also showed sufficient activities to sustain higher photosynthesis rates than those observed in the stressed plants. However, under drought stress, total leaf malate concentrations were higher in B 35 (up to 33.1 µmol g⁻¹ FW) than in E 36-1 (up to 22.4 µmol g⁻¹ FW). In particular, at the presumed cytosolic pH of 7.0–7.3, S. bicolor PEPCase was strongly inhibited by malate. In contrast with the in vitro PEPCase enzyme activities, the A/C_i curves suggested a stronger decrease in the in vivo activity of the enzyme in B 35 under drought stress than in E 36-1. It is therefore suggested that photosynthesis under drought stress may be inhibited differentially through feedback malate inhibition of PEPCase in S. bicolor.     

NUTRITIONAL REQUIREMENTS FOR OVULE FORMATION IN EXCISED PISTILS OF NIGELLA

The nutritional requirements for ovule formation in Nigella saliva L. were investigated by growing excised pistils on defined media. Pistils grown on a medium containing the minerals of Murashige and Skoog produced significantly more ovules than on a medium containing the minerals of Bilderback. When the nitrogen, sulfate, and phosphate of the Bilderback medium were adjusted to levels comparable to those of the Murashige and Skoog medium, a similar number of ovules was formed. The effect of different forms and concentrations of nitrogen on ovule formation and pistil growth was investigated. High concentrations of nitrate (40 mil) favored pistil growth and ovule formation, but comparable levels of ammonium were toxic. When ammonium at concentrations above 10 mM was added to nitrate media, ovule formation was inhibited. A medium containing low concentrations of ammonium (10 mM) and nitrate (5 mM) supported more ovule formation and pistil growth in young pistils than a low-nitrate (5 mM) medium without ammonium. However, ovule formation on a medium containing 10 mM ammonium and 5 mM nitrate was significantly less than on a medium containing only 15 mM nitrate. Low concentrations of organic nitrogen in the form of α-alanine (1 mM) and γ-aminobutyric acid (5 mM) supported ovule formation and pistil growth similar to a high nitrate medium.     

Effect of drought on ear and flag leaf photosynthesis of two wheat cultivars differing in drought resistance

We investigated net photosynthetic rate (P_N) of ear and two uppermost (flag and penultimate) leaves of wheat cultivars Hongmangmai (drought resistant) and Haruhikari (drought sensitive) during post-anthesis under irrigated and non-irrigated field conditions. The P_Nof ear and flag leaf were significantly higher and less affected by drought in Hongmangmai than in Haruhikari. The rate of reduction in stomatal conductance (g_s) was similar for the two cultivars, but intercellular CO₂concentration (C_i) in the flag leaf of Hongmangmai was lower than that of Haruhikari in non-irrigated treatment. No differences were observed in leaf water potential (₁) and osmotic adjustment of the flag leaf of the cultivars. These results imply that differences in photosynthetic inhibition on the flag leaf at low leaf ₁between the cultivars were primarily due to non-stomatal effects. Hence the main physiological factor associated with yield stability of Hongmangmai under drought stress may be attributed to the capacity for chloroplast activity in the flag leaf, which apparently allows sustained P_Nof flag leaf during grain filling under drought stress. The higher P_Nof ear in Hongmangmai under drought could also be related to its drought resistance.This revised version was published online in March 2005 with corrections to the page numbers.     

Carbon isotope discrimination in irrigated and droughted potato (Solanum tuberosum L.)

Carbon isotope discrimination (Δ) was measured in irrigated and droughted potato. Under irrigation, Δ in leaflets at given nodes increased (P < 0.001) between 21 and 63 d after emergence (DAE), which was attributed to increasing stomatal conductance (g_s) during leaf expansion. The effect of leaf position on Δ was non-significant in mature leaves. Under drought, Δ decreased (P < 0.001) in successive leaves up the stem, reflecting changes in g_s and water stress. At each node Δ remained constant or decreased, suggesting that effects of water stress were greater than changes with leaf expansion. There were significant differences in Δ between cultivars in both treatments, and in the progressive decrease in Δ up the stem under drought. Differences in Δ between cultivars were consistent with differences in stomatal control of leaf water status following water stress. Values for Δ in tubers were consistently lower than in stem and leaf, and decreased more rapidly. Differences in Δ between cultivars did not reflect dry matter production in either treatment, and differences in water use were non-significant between cultivars under drought. So, plants can achieve similar dry matter production through different growth strategies when irrigated or droughted, and Δ does not provide a simple, indirect method of selecting for dry matter production under water stress.     

Photosynthetic performance and growth traits in Pennisetum centrasiaticum exposed to drought and rewatering under different soil nutrient regimes

Responses of plants exposed to drought and rewatering have been well documented; however, little is known concerning strategies of psammophyte to drought and rewatering under different soil nutrient regimes. For this study, Pennisetum centrasiaticum under two soil nutrient regimes was subjected to progressive drought and subsequent rewatering. Soil water status, gas exchange characteristics, chlorophyll a fluorescence characteristics as well as biomass traits were measured to investigate ecophysiological responses. Net photosynthesis rate (P _n), stomatal conductance (g _s), water use efficiency, maximum quantum efficiency of photosynthesis system II (PSII, F _V/F _M), electron transport flux per cross section (ET₀/CS₀), and performance index on cross section basis (PI_CS) were suppressed during drought periods for both nutrient regimes. Meanwhile, leaf intercellular CO₂ concentration (C _i ), minimal fluorescence intensity (F ₀), and dissipated energy flux per cross section (DI₀/CS₀) increased. Reversible downregulation of PSII photochemistry and enhanced thermal dissipation of excess excitation energy (DI₀/CS₀) contributed to enhanced photo-protection in drought-stressed plants. Thus, the results indicate that P. centrasiaticum is capable of withstanding and surviving extreme drought events, and the recovery pattern of stressed P. centrasiaticum under both nutrient regimes was similar. However, fertilization increased the biomass and the variation in gas exchange and chlorophyll a fluorescence characteristics during drought periods. Additionally, fertilization accelerated the process of drought and aggravated stress under extreme drought events. Thus, the fertilization strategy used in P. centrasiaticum restoration should be carefully selected—fertilization may not always be beneficial.     

Sulfinylated azadecalins act as functional mimics of a pollen germination stimulant in Arabidopsis pistils

Polarized cell elongation is triggered by small molecule cues during development of diverse organisms. During plant reproduction, pollen interactions with the stigma result in the polar outgrowth of a pollen tube, which delivers sperm cells to the female gametophyte to effect double fertilization. In many plants, pistils stimulate pollen germination. However, in Arabidopsis, the effect of pistils on pollen germination and the pistil factors that stimulate pollen germination remain poorly characterized. Here, we demonstrate that stigma, style, and ovules in Arabidopsis pistils stimulate pollen germination. We isolated an Arabidopsis pistil extract fraction that stimulates Arabidopsis pollen germination, and employed ultra‐high resolution electrospray ionization (ESI), Fourier‐transform ion cyclotron resonance (FT‐ICR) and MS/MS techniques to accurately determine the mass (202.126 Da) of a compound that is specifically present in this pistil extract fraction. Using the molecular formula (C₁₀H₁₉NOS) and tandem mass spectral fragmentation patterns of the m/z (mass to charge ratio) 202.126 ion, we postulated chemical structures, devised protocols, synthesized N‐methanesulfinyl 1‐ and 2‐azadecalins that are close structural mimics of the m/z 202.126 ion, and showed that they are sufficient to stimulate Arabidopsis pollen germination in vitro (30 μm stimulated approximately 50% germination) and elicit accession‐specific response. Although N‐methanesulfinyl 2‐azadecalin stimulated pollen germination in three species of Lineage I of Brassicaceae, it did not induce a germination response in Sisymbrium irio (Lineage II of Brassicaceae) and tobacco, indicating that activity of the compound is not random. Our results show that Arabidopsis pistils promote germination by producing azadecalin‐like molecules to ensure rapid fertilization by the appropriate pollen.     

Pistil induction by hormones from callus of <Emphasis Type="Italic">Oryza sativa</Emphasis> in vitro

This study describes the successful formation of floral organ pistil from the callus of pistil explants of Oryza sativa L. For induction of floral organs, different explants—including young embryo, lemma, palea and pistil—were used for callus induction with different combinations of N⁶-benzyladenine and 2,4-dichlorophenoxyacetic acid (2,4-D). High frequencies of callus formation from pistil and young embryo explants were achieved. Floral organs were induced after calli from pistils were transferred to medium containing both zeatin and 2,4-D. The morphological characteristics of the pistil-like organs are very similar to those formed in planta though with minor differences. Further histological study revealed that the in vitro pistil contains an ovule within its ovary. Furthermore, a pistil-specific gene, OsMADS3 used as a molecular marker for pistil identity, was expressed in the pistil-like organs as it was in pistils in the flower of the plant.     

Seed abortion in the sexual counterpart of <Emphasis Type="Italic">Brachiaria brizantha</Emphasis> apomicts (Poaceae)

Gametophytic organization, fertilization and reproductive success are described for the fertile diploid Brachiaria brizantha accession BRA-002747 which is being raised for use in Brachiaria breeding programs, as well as to understand and control of apomixis in this genus. The current paper reports on reproductive biology and analysis of seed set in field experiments during three consecutive years. Unsuccessful seed production in this plant is believed to correlate with early inbreeding depression, based on the reproductive features analyzed. Caryopsis development was observed using differential interference contrast microscopy with seed set determined by the number of self- and open-pollinated pistils that fully developed into viable seeds. Developing and mature female and male gametophytes were observed in the context of flower phenology, morphology and anthesis patterns. Pollen viability was determined by acetocarmine staining and by observation of germination in vivo, which was also used to observe pollen tube/pistil interaction. Although normal development was observed in floral structures, anthesis and gametophytes, seed set was low, with 2 and 6% in self- and open-pollination, respectively, producing seed. Variations observed in the female organs, such as the presence of a hermaphrodite flower in 50% of the inferior floscules and the presence of multiple embryo sacs of the Polygonum type within the same ovule in 15% of the pistils, are not related to low fertility. The majority of pollen grains are viable, in spite of the reduced number of pollen tubes within the style and ovary carpel, and a developing caryopsis was observed in 70% of self-pollinated pistils, indicating successful double fertilization from 2 days after anthesis (DAA). Nevertheless, abortion gradually increased from 2 until 7 DAA and remains elevated until 12 DAA, when caryopsis maturity is achieved. These data confirm low seed set in this accession and indicate that low fertility is not a consequence of abnormalities, either in the floral or gamete structures, or pollen tube rejection, but most likely a consequence of inbreeding depression.     

Photoprotective function of photorespiration in Reaumuria soongorica during different levels of drought stress in natural high irradiance

Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as H₂O₂ content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub. The rate of photorespiration was estimated by combined measurement of gas exchange and Chl fluorescence. The rate of photorespiration increased with the increasing drought stress (DS). The ratio of carboxylation electron flow to oxygenation electron flow (J_c/J_o) and the maximal photochemical efficiency of PS2 (variable to maximum fluorescence ratio, F_v/F_m) decreased with the increasing DS. F_v/F_m in isonicotinic acid hydrazide (INH)-sprayed plants was lower than that in normal plants under moderate DS, but no significant difference was observed under severe DS. H₂O₂ content in INH-sprayed plants was significantly lower than that in normal plants under severe DS. Taken together, photorespiration in R. soongorica consumed excess electrons and protected photosynthetic apparatus under moderate DS, whereas it accelerated H₂O₂ accumulation markedly and induced the leaf abscission under severe DS.     

Starch biosynthesis contributes to the maintenance of photosynthesis and leaf growth under drought stress in maize

To understand the growth response to drought, we performed a proteomics study in the leaf growth zone of maize (Zea mays L.) seedlings and functionally characterized the role of starch biosynthesis in the regulation of growth, photosynthesis and antioxidant capacity, using the shrunken-2 mutant (sh2), defective in ADP-glucose pyrophosphorylase. Drought altered the abundance of 284 proteins overrepresented for photosynthesis, amino acid, sugar and starch metabolism, and redox-regulation. Changes in protein levels correlated with enzyme activities (increased ATP synthase, cysteine synthase, starch synthase, RuBisCo, peroxiredoxin, glutaredoxin, thioredoxin and decreased triosephosphate isomerase, ferredoxin, cellulose synthase activities, respectively) and metabolite concentrations (increased ATP, cysteine, glycine, serine, starch, proline and decreased cellulose levels). The sh2 mutant showed a reduced increase of starch levels under drought conditions, leading to soluble sugar starvation at the end of the night and correlating with an inhibition of leaf growth rates. Increased RuBisCo activity and pigment concentrations observed in WT, in response to drought, were lacking in the mutant, which suffered more oxidative damage and recovered more slowly after re-watering. These results demonstrate that starch biosynthesis contributes to maintaining leaf growth under drought stress and facilitates enhanced carbon acquisition upon recovery.     

Photosynthetic down-regulation in Larrea tridentata exposed to elevated atmospheric CO2: interaction with drought under glasshouse and field (FACE) exposure

The photosynthetic response of Larrea tridentata Cav., an evergreen Mojave Desert shrub, to elevated atmospheric CO₂ and drought was examined to assist in the understanding of how plants from water-limited ecosystems will respond to rising CO₂. We hypothesized that photosynthetic down-regulation would disappear during periods of water limitation, and would, therefore, likely be a seasonally transient event. To test this we measured photosynthetic, water relations and fluorescence responses during periods of increased and decreased water availability in two different treatment implementations: (1) from seedlings exposed to 360, 550, and 700 μmol mol^–1 CO₂ in a glasshouse; and (2) from intact adults exposed to 360 and 550 μmol mol^–1 CO₂ at the Nevada Desert FACE (Free Air CO₂ Enrichment) Facility. FACE and glasshouse well-watered Larrea significantly down-regulated photosynthesis at elevated CO₂, reducing maximum photosynthetic rate (A_max), carboxylation efficiency (CE), and Rubisco catalytic sites, whereas droughted Larrea showed a differing response depending on treatment technique. A_max and CE were lower in droughted Larrea compared with well-watered plants, and CO₂ had no effect on these reduced photosynthetic parameters. However, Rubisco catalytic sites decreased in droughted Larrea at elevated CO₂. Operating C_i increased at elevated CO₂ in droughted plants, resulting in greater photosynthetic rates at elevated CO₂ as compared with ambient CO₂. In well-watered plants, the changes in operating C_i, CE and A_max resulted in similar photosynthetic rates across CO₂ treatments. Our results suggest that drought can diminish photosynthetic down-regulation to elevated CO₂ in Larrea, resulting in seasonally transient patterns of enhanced carbon gain. These results suggest that water status may ultimately control the photosynthetic response of desert systems to rising CO₂.