Progressive Water Stress Research Articles

Respostas funcionais foliares de plantas jovens de <i>Hevea brasiliensis</i> submetidas à deficiência hídrica e à reidratação

A deficiência hídrica é considerada um fator que produz danos significativos durante o período de estabelecimento e de crescimento dos plantios. O objetivo deste estudo foi investigar características morfofisiológicas e bioquímicas de folhas de mudas de seringueira em resposta à deficiência hídrica e, posterior, reidratação. Plantas jovens de Hevea brasiliensis do clone RRIM 600 - Rubber Research Institute of Malaysia foram divididas em dois tratamentos, tratamento 1 = plantas irrigadas (IR) e tratamento 2 = plantas não irrigadas (NI). Nestas plantas, a fotossíntese (A) foi acompanhada durante todo o período experimental (0, 10, 25, 32, 48, 52 e 68 dias). O potencial hídrico foliar (ψw) foi determinado usando-se uma câmara de pressão do tipo Scholander, a fotossíntese (A), condutância estomática (gs) e transpiração (E) foram determinadas usando-se um analisador de gás infravermelho (LI-6400XT, LI-COR, Lincoln, Nebraska, USA) e o conteúdo de carboidratos foliares e de prolina foram determinados por espectrofotometria (Amersham Biosciences Ultrospec 3100 pro). O ψw, A, gs, E e o conteúdo de amido foliar no tratamento NI foram reduzidos durante os primeiros 32 dias, em que A atingiu valores próximos a 0 μmol m-2s-1.Porém, houve aumento da concentração de açúcar solúvel total e de prolina (tratamento NI) neste mesmo período. A recuperação da fotossíntese (NI) ocorreu após 22 dias depois da reidratação, quando comparado ao tratamento (IR). Portanto, pode-se concluir que o clone RRIM 600 possui tolerância ao estresse hídrico em termos de como lidar com o estresse hídrico progressivo, mas a recuperação ao estresse foi relativamente lenta.

Responses of antioxidant enzymes, photosynthetic pigments and carbohydrates in micropropagated Pitcairnia encholirioides L.B. Sm. (Bromeliaceae) under ex vitro water deficit and after rehydration.

In this study, the activities of antioxidant enzymes, photosynthetic pigments, proline and carbohydrate contents in Pitcairnia encholirioides under ex vitro conditions of water deficit were evaluated. Results show that plants under progressive water stress, previously in vitro cultured in media supplemented with 30 g L-1 sucrose and GA3, accumulated more proline and increased peroxidase (POD) activity and the contents of photosynthetic pigments and carbohydrates. For plants previously in vitro cultured with 15 g L-1 sucrose and NAA, no differences were found for proline content and there were reductions in activities of peroxidase (POD), catalase (CAT) and poliphenoloxidase (PPO), and in contents of carbohydrates, with progress of ex vitro water deficit. After rehydration, plants showed physiological recovery, with enzymatic activities and contents of metabolites similar to those found in the controls not submitted to dehydration, regardless of the previous in vitro culture conditions. These results show that micropropagated P. encholirioides has high tolerance to dehydration once in ex vitro conditions, which can ensure the survival of plants from tissue culture when transferred to its natural environment, emphasizing the importance of such biotechnology for the propagation of endangered species.

Relationship between hydraulic and stomatal conductance and its regulation by root and leaf aquaporins under progressive water stress and recovery and exogenous application of ABA in Vitis vinifera L. ‘Syrah’

Aquaporins (AQPs) are cellular water channel proteins known to play a key role in regulating the hydraulic conductance of roots (Lo) and leaves (Kleaf) in plants. Using potted grapevines (Vitis vinifera L. 'Syrah') on a gravimetric irrigation platform, we examined the effects of progressive water stress and recovery therefrom, and soil application of abscisic acid (ABA) on the relationships between Lo, Kleaf, leaf stomatal conductance (gs) and gene expression of leaf and root AQPs. We test the hypothesis that water stress (potentially resulting in endogenous ABA biosynthesis) and exogenous ABA application increase Lo due to enhanced root AQP activity but decrease g(s) and Kleaf due to stomatal closure and decreased leaf AQP activity. Leaf and stem water potentials, g(s), Lo and Kleaf were measured at different time points along the experiment and leaf and roots samples were taken at the same time to determine AQP expression. Both, water stress and ABA application reduced g(s) to 50 mmol m-2 s-1 three days after the experiment was started. While Kleaf decreased by 75% in the water-stressed plants compared to the well-watered plants, it was unchanged with exogenous ABA. Upon rewatering, the water-stressed plants reached 66% of the initial g(s) and had Kleaf values similar to the well-watered plants. Positive linear relationships were observed between Lo and gs, and between Lo and transpiration that were related to vine water status and ABA application.

Trocas gasosas de plantas de Handroanthus impetiginosus (Mart. ex DC) Mattos submetidas ao déficit hídrico e posterior reidratação

Knowledge of aspects related to the ecophysiology of plant species present in Caatinga is scarce, needing studies that allow the understanding of the adaptations made by plants to be able to survive under conditions of low water availability. Accordingly, this study was developed with the purpose of analyzing the gas exchange of ipe-roxo seedlings subjected to progressive water stress, through the suspension of irrigation, and rehydration. 12 month old plants, growing up in black plastic bags containing 5 kg of substrate, were subjected to two treatments: irrigated and water stress imposed by withholding water. The relative water content, stomatal conductance, transpiration rate, intercellular CO2 concentration, and photosynthetic rates were evaluated daily. After ten days remaining under this stress condition, irrigation was restored, and recovery was evaluated. Drought stress decreased rapidly and also progressively the relative water content, stomatal conductance, transpiration, carboxylation efficiency and photosynthesis of plants. An increase in intercellular CO2 concentration with increase of water stress was observed. After the resumption of water supply, the plants showed recovery of these physiological processes. Water stress did not affect the water efficiency use of plants.

Ancestral QTL Alleles from Wild Emmer Wheat Enhance Root Development under Drought in Modern Wheat.

A near-isogenic line (NIL-7A-B-2), introgressed with a quantitative trait locus (QTL) on chromosome 7AS from wild emmer wheat (Triticum turgidum ssp. dicoccoides) into the background of bread wheat (T. aestivum L.) cv. BarNir, was recently developed and studied in our lab. NIL-7A-B-2 exhibited better productivity and photosynthetic capacity than its recurrent parent across a range of environments. Here we tested the hypothesis that root-system modifications play a major role in NIL-7A-B-2’s agronomical superiority. Root-system architecture (dry matter and projected surface area) and shoot parameters of NIL-7A-B-2 and ‘BarNir’ were evaluated at 40, 62, and 82 days after planting (DAP) in a sand-tube experiment, and root tip number was assessed in a ‘cigar-roll’ seedling experiment, both under well-watered and water-limited (WL) treatments. At 82 DAP, under WL treatment, NIL-7A-B-2 presented greater investment in deep roots (depth 40–100 cm) than ‘BarNir,’ with the most pronounced effect recorded in the 60–80 cm soil depth (60 and 40% increase for root dry matter and surface area, respectively). NIL-7A-B-2 had significantly higher root-tip numbers (∼48%) per plant than ‘BarNir’ under both treatments. These results suggest that the introgression of 7AS QTL from wild emmer wheat induced a deeper root system under progressive water stress, which may enhance abiotic stress resistance and productivity of domesticated wheat.

Peach yield and fruit quality is maintained under mild deficit irrigation in semi-arid China

We conducted a two-year study of deficit irrigation impact on peach yield and quality in semi-arid northwest China. Over two years, four-year-old peach trees were irrigated at 100, 75, 50 and 25% of peach evapotranspiration (ETc), here, ETc= Coefficient (Kc)×Local reference evapotranspiration (ETo). During the April–July fruit production season we measured root zone soil water depletion, sap flow velocity, net photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs), water use efficiency (WUE=Pn/Tr), fruit quality, and yield under a mobile rain-out shelter. Increased soil water depletion reasonably mirrored decreasing irrigation rates both years, causing progressively greater water stress. Progressive water stress lowered Gs, which in turn translated into lower Tr as measured by sap flow. However, mild deficit irrigation (75% ETc) constricted Tr more than Pn. Pn was not different between 100 and 75% ETc treatments in both years, and it decreased only 5–8% in June with higher temperature than that in May with cooler temperature. Concurrently under 75% ETc treatment, Tr was reduced, and WUE was up to 13% higher than that under 100% ETc treatment. While total fruit yield was not different under the two treatments, because 75% ETc treatment had fewer but larger fruit than 100% ETc trees, suggesting mild water stress thinned fruit load. By contrast, sharply decreased Tr and Pn of the driest treatments (50 and 25% ETc) increased WUE, but less carbon uptake impacted total fruit yield, resulting 13 and 33% lower yield compared to that of 100% ETc treatment. Irrigation rates affected fruit quality, particularly between the 100 and 75% ETc trees. Fewer but larger fruit in the mildly water stressed trees (75% ETc) resulted in more soluble solids and vitamin C, firmer fruit, and improved sugar:acid ratio and fruit color compared to the 100% ETc treatment. Overall, trees deficit irrigated at 75% ETc maintained yield while improving fruit quality and using less water.

Vis/NIR spectroscopy and chemometrics for non-destructive estimation of water and chlorophyll status in sunflower leaves

Vegetation biochemical and biophysical variables are important for many ecological, agronomic, and meteorological applications. Among the main variables, water and chlorophyll are essentials due to directly affect the plant photosynthetic capacity and crop productivity. The objective of this study was develop and validate models capable of estimating water and chlorophyll status in sunflower leaves under progressive water stress, based on the visible/near-infrared region (Vis/NIR) spectral reflectance and chemometric technique. The water and chlorophyll models were adjusted considering the spectral reflectance from the 500–1039 nm wavelengths by using partial least squares regressions (PLSR). In the external validation, high determination coefficient (0.8386 and 0.8097) and low mean bias error (−0.40 dry basis and 0.09 mg g−1) values for water and chlorophyll, respectively, indicating that their predictive capabilities and accuracies of the models were satisfactory. Results showed that spectrometry has potential to be applied as an alternative method in quantifying water and chlorophyll status in sunflower leaves in a non-destructive, quick, and consistent way.

Changes in antioxidant enzyme activities and gene expression in two muskmelon genotypes under progressive water stress

Responses of two muskmelon (Cucumis melo L.) genotypes (drought tolerant SC-15 and drought susceptible EC-564755) were analyzed at 0, 7, 14, and 21 d of progressive water stress. Although water deficit caused a significant decline in relative water content, the magnitude of reduction was lower in SC-15. Electrolyte leakage, hydrogen peroxide, and malonydialdehyde generation were higher in EC-564755, whereas accumulation of proline was higher in SC-15. Higher activities of antioxidant enzymes, such as catalase, superoxide dismutase, ascorbate peroxidase, guaiacol peroxidase, and glutathione reductase, and higher expression of the respective genes were recorded in SC-15 than in EC-564755. Expressions of DREB2C and DREB3 in SC-15 revealed a fluctuating pattern with down-regulation on days 7 and 21 of water stress, whereas up-regulation was observed on day 14. Concurrently, both genes in EC-564755 showed continuous down-regulation on days 7, 14, and 21 of water stress. Expressions of RD22 and dehydrin recorded on days 7, 14, and 21 were lower in SC-15. The cluster analysis showed that, these two genotypes had a clear distinction in physiological and biochemical properties and gene expressions under water stress and the genotype SC-15 had more efficient osmoprotectant mechanism than genotype EC-564755 under water deficit conditions.

Morphological and physiological responses of Heteropogon contortus to drought stress in a dry-hot valley.

Background Heteropogon contortus is a valuable pasture species that is widely used for vegetation restoration in dry-hot valleys of China. However, to date, its morphological and physiological responses to drought, and the underlying mechanisms are not well understood. This study was aimed to investigate the morphological and physiological changes of H. contortus under drought stress during the dry-hot season. Heteropogon contortus was planted in pots and subjected to four levels of soil water treatments: above 85 % (control), 70–75 % (light stress), 55–60 % (moderate stress) or 35–40 % (severe stress) of field capacity.ResultsWithin the total stress period (0–29 days), H. contortus grew rapidly in the light stress, whereas severe stress had a negative impact on growth. Aboveground biomass decreased together with increasing drought stress, whereas root biomass increased. Consequently, the root/shoot ratio of the severe stress treatment increased by 80 % compared to that of the control treatment. The ratio of bound water/free water (BW/FW) was the most sensitive parameter to drought and showed a value under severe stress that was 152.83 % more than that in the control treatment. Although leaf water potential (LWP) and leaf relative water content (RWC) decreased with progressive water stress, H. contortus managed to maintain a relatively high RWC (nearly 70 %) in the severe stress condition. We also detected a significant reduction (below 0.6) in the ratio of variable fluorescence/maximum fluorescence (Fv/Fm) in the severe stress treatment.ConclusionsOur results show that H. contortus adapts to drought mainly by avoidance mechanisms, and its morphological and physiological characteristics are inhibited under severe stress, but can recover at a certain time after re-watering. These findings might help limited water resources to be fully used for vegetation management in the studied region.

Field phenotyping of water stress at tree scale by UAV-sensed imagery: new insights for thermal acquisition and calibration

Numerous agronomical applications of remote sensing have been proposed in recent years, including water stress assessment at field by thermal imagery. The miniaturization of thermal cameras allows carrying them onboard the unmanned aerial vehicles (UAVs), but these systems have no temperature control and, consequently, drifts during data acquisition have to be carefully corrected. This manuscript presents a comprehensive methodology for radiometric correction of UAV remotely-sensed thermal images to obtain (combined with visible and near-infrared data) multispectral ortho-mosaics, as a previous step for further image-based assessment of tree response to water stress. On summer 2013, UAV flights were performed over an apple tree orchard located in Southern France, and 4 dates and 5 h of the day were tested. The 6400 m2 field plot comprised 520 apple trees, half well-irrigated and half submitted to progressive summer water stress. Temperatures of four different on-ground stable reference targets were continuously measured by thermo-radiometers for radiometric calibration purposes. By using self-developed software, frames were automatically extracted from the thermal video files, and then radiometrically calibrated using the thermal targets data. Once ortho-mosaics were obtained, root mean squared error (RMSE) was calculated. The accuracy obtained allowed multi-temporal mosaic comparison. Results showed a good relationship between calibrated images and on-ground data. Significantly higher canopy temperatures were found in water-stressed trees compared to well-irrigated ones. As high resolution field ortho-mosaics were obtained, comparison between trees opens the possibility of using multispectral data as phenotypic variables for the characterization of individual plant response to drought.

Early Abscisic Acid Accumulation Regulates Ascorbate and Glutathione Metabolism in Soybean Leaves Under Progressive Water Stress

Ascorbate (AsA) and glutathione (GSH) play an important role in improving the tolerance of plants to water stress. The objective of this study was to investigate the effect of early abscisic acid (ABA) accumulation on AsA and GSH metabolism in soybean plants after 24 h of exposure to progressive water stress. The results showed that AsA, total AsA, GSH and total GSH content, and ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), GSH reductase (GR), GSH peroxidase (GPX), l-galactono-1,4-lactone dehydrogenase (GLDH), and γ-glutamylcysteine synthetase (γ-GCS) activities were increased by progressive water stress. The above increases, except for total GSH content and the activities of GLDH and γ-GCS, were blocked by pretreatment with tungstate, an ABA biosynthesis inhibitor, which significantly suppressed the early increase in ABA and reactive oxygen species (ROS) in stressed plants. Application of ABA reversed the effects of tungstate. Pretreatments with several ROS scavengers, such as Tiron and dimethylthiourea (DMTU), and an inhibitor of NADPH oxidase, diphenyleneiodonium (DPI), significantly arrested the early accumulation of ROS but not ABA in stressed plants. Furthermore, the above-mentioned pretreatments remarkably prevented any increase in APX, MDHAR, DHAR, GR, and GPX activities, as well as AsA, total AsA and GSH levels in stressed plants. Our results indicated that early ABA accumulation caused by progressive water stress triggers an early rise in ROS levels, which, in turn, leads to regulation of AsA and GSH metabolism.

Progressive forest canopy water loss during the 2012–2015 California drought

The 2012-2015 drought has left California with severely reduced snowpack, soil moisture, ground water, and reservoir stocks, but the impact of this estimated millennial-scale event on forest health is unknown. We used airborne laser-guided spectroscopy and satellite-based models to assess losses in canopy water content of California's forests between 2011 and 2015. Approximately 10.6 million ha of forest containing up to 888 million large trees experienced measurable loss in canopy water content during this drought period. Severe canopy water losses of greater than 30% occurred over 1 million ha, affecting up to 58 million large trees. Our measurements exclude forests affected by fire between 2011 and 2015. If drought conditions continue or reoccur, even with temporary reprieves such as El Niño, we predict substantial future forest change.

Physiological responses of selected African sorghum landraces to progressive water stress and re-watering

Sorghum is particularly drought tolerant compared with other cereal crops and is favoured for subsistence farming in water scarce regions of the world. This study was conducted to identify South African sorghum landraces with superior drought tolerance compared with a drought-tolerant breeding line (P898012). Seedlings of 14 South African sorghum landrace accessions were initially screened for drought tolerance by assessing percentage leaf water content (LWC) during progressive water deficit. Four landraces (designated LR5, LR6, LR35, and LR36) recorded higher LWC than P898012. These were subsequently evaluated with P898012 during the reproductive growth stage, for their physiological responses to mild (4days) and severe (6days) water stress treatments and a moderate re-watered treatment on day 7. Plant height, soil moisture, and LWC were measured during harvests. Chlorophyll, carotenoid, and proline contents were quantified. All five genotypes maintained LWC above 80% during mild and severe stress treatments. For LR35 and LR36, LWC were recorded within 8% less in comparison to their well-watered controls following the moderate re-watered treatment. Significantly higher chlorophyll and carotenoid contents were recorded for both LR6 and LR35 in comparison to P898012 during severe stress. When LWC was reduced in LR36 (to 73.68%) and LR35 (to 73.51%), their proline content significantly increased by 14- and 16-fold, respectively. In this study, we have identified four previously uncharacterised sorghum genotypes exhibiting drought tolerance and described their physiological responses during water deficit and moderate re-watering. Aside from their application to breeding, these landraces are valuable resources to elucidate genetic mechanisms that enable drought tolerance in South African sorghum.

Differences among grapevine cultivars in their stomatal behavior and water use efficiency under progressive water stress

Evaluation of different mechanisms adopted by grapevine cultivars to deal with drought is of major importance to carry out more efficient cultivar selection. In this report, 23 cultivars, including local and foreign grape cultivars, were studied under field conditions in order to identify the different behaviors in response to water deficit, and how it can affect water use efficiency (WUE) at leaf level (intrinsic water use efficiency (WUEi) and leaf carbon isotope composition (δ13C)). Ψstem was used to assess plant water status. Under a common environment, a high variability was found in photosynthetic parameters, stomatal response, WUEi and leaf δ13C. This large variability observed represents an opportunity for genotype selection. Using different physiological traits we were able to select suitable cultivars for current and future viticulture. Under non-water stress situation the cultivar Escursac combined high rates of net photosynthesis with low stomatal conductance, resulting in high WUEi. In response to drought, strong, and moderate water saving behavior of local cultivars Manto Negro, Giró Ros, Argamussa and Vinater Negre resulted in higher WUEi. Long-term WUE (leaf δ13C) corroborate these cultivars as high water use efficient.

Monitoring the photosystem II behavior of wild and cultivated barley in response to progressive water stress and rehydration using OJIP chlorophyll a fluorescence transient

ABSTRACTTo study the effects of progressive drought stress on photosystem II behavior of wild type (Spantaneum) and cultivated barley (Morocco), different levels of soil water availability, including control, moderate, mild, and severe water stress (70%, 50%, 30%, and 10% water holding capacity of soil, respectively) and rehydration were used. Polyphasic OJIP fluorescence transient of Morocco plants exhibited a considerable increase in fluorescence intensity at O, J, and I steps under mild and severe stress relative to slight increase in wild barley. Values of fluorescence parameters and quantum efficiencies, including minimal fluorescence, relative variable fluorescence at phase J and I, maximal quantum yield of photosystem II (PSII), performance index, electron transport yield, and excitation transfer efficiency were influenced by water stress in both genotypes. These parameters were significantly less affected in wild type barley by progressive drought stress compared to Morocco. After re-watering, both genotypes were able to restore from severe drought in most of the traits. Based on our findings, highly correlated values of relative water content (RWC) and independent JIP-test parameters (P < 0.01) indicate that the chlorophyll a fluorescence induction technique is sensitive to plant water status and performance index represent an accurate and reliable indicator for early stress detection and also explore plant vitality under water stress.

Relationship between drought tolerance with activities of antioxidant enzymes in sugarcane

The objective of this study was to compare capacity of antioxidant enzymes between Khon Kaen 3 (KK3), a drought-tolerant, and Suphanburi 72 (SP72), a drought-sensitive sugarcane variety. Young sugarcane plants were grown under controlled conditions. Water stress was imposed at 4 leaf stage by withholding water supply for 4, 6 and 8 days as compared to control, receiving adequate water supply. First fully expanded leaves were harvested and proteins were extracted for assaying the activities of ascorbate peroxidase (APX), peroxidase (POX) and catalase (CAT). APX was a major antioxidant system in leaves of both the sugarcane varieties as it accounted for 65 % in KK3 and 69 % in SP72 in scavenging H2O2. POX accounted for 27 and 23 % in KK3 and SP72, respectively in the removal of H2O2. Negligible scavenging activities of CAT were observed in both sugarcane varieties. APX activities in KK3 leaves were induced and maintained during progressive water stress and were 15 % higher than that in SP72. POX activities in KK3 were 30 % greater than that in SP72. In conclusion, drought tolerance in KK3 was, at least partially, due to the greater activities of APX and POX under water deficit stress as compared to those of SP72.

Comparison of whole-canopy water use efficiency and vine performance of cv. Sangiovese (Vitis vinifera L.) vines subjected to a post-veraison water deficit

In the present paper a series of WUE expressions, including the physiological and agronomical, were compared in potted cv. Sangiovese (Vitis vinifera L.) grapevines which were either well watered (WW) or subjected to progressive post-veraison water stress (WS) by supplying decreasing fractions (i.e. 70%, 50% and 30%) of daily canopy transpiration (Tc) concurrently recorded via a plant-enclosure gas exchange system. While single-leaf intrinsic water-use efficiency (WUEi) increased with water stress severity, seasonal whole-canopy WUE were similar at pre-stress, 70% Tc, and upon rewatering but dropped during severe water stress. WUE calculated as mass of dry weight stored in annual biomass (leaves, canes and clusters) per liter of water used, did not differ on a seasonal basis, whereas WW and WS showed similar grape composition despite the latter suffered a 25% yield reduction. Results confirm that whole-canopy WUE is a much better index than single-leaf based WUE parameter for extrapolation to agronomic WUE and actual grape composition. Under our specific case study, due to high reactivity of Sangiovese to early yellowing and shedding of basal leaves, it can be recommended that post-veraison water supply to Sangiovese vines should not be lower than 70% of daily vine water use.

A Device Enabling Fully Automated Water-Deficit Experiments with Potted Grapevines

Here, we describe a novel device for programming and replenishing water transpired by potted plants. To test the robustness of the system, vines were subjected to progressive water stress (WS), the severity of which was maintained in relation to transpiration (Tc) of well-watered (WW) plants. Throughout the 40-day experiment, water supply in the WS treatment was progressively lowered to 70, 50, and 30% of WW Tc prior to rewatering. During the same stages, mean Tc of WS plants was 74, 48, 28, and 93% that of WW plants. Linear relationships between vine transpiration and water supply during the 40-day experiment (R2 = 0.95 for WW and 0.94 for WS) confirmed the reliability of the system in providing a water supply that closely tracked measured transpiration. The emptying volume of the cylinder tank was set at 265 mL and proved to be adequate for daily water losses, which ranged from ~300 to 2300 mL. In addition to relieving operators of laborious and time-consuming manual irrigation, the system provides the ability to adjust water supply to actual water use as measured concurrently in a grapevine-enclosure system and enables customization of the water supply according to the size and transpiration potential of each vine.

Photosynthetic acclimation of sweet sorghum under progressive water stress

This study addressed photosynthetic acclimation of sweet sorghum under progressive soil drying at different growth stages: (i) three fully expanded leaves (D1); (ii) growing point differentiation (D2); (iii) booting (D3); (iv) half-blooming (D4). Soil water potential (SWP), photosynthetic traits and soluble sugars accumulation were determined. Under progressive drought stress, D1 and D2 photosynthesized until 20 days after stress imposition (SWP≈−3.5MPa), while, D3 and D4 were phothosynthetically inactive 12 days after irrigation suspension (SWP≈−1.5MPa). Photoprotection mechanisms also changed with plant age: in mature plants, electron transport rate per cross section (ETo/CSm), and silent PSII reaction centers (RCs) drastically declined at SWP of −1.0MPa, while remained unchanged in young plants until −1.5MPa. As water stress increased (SWP<−1.8MPa), the light energy absorption (ABS/CSm), the trap energy flux (TRo/CSm) and ETo/CSm dramatically decreased in mature plants (−70% compared to control). Soluble sugars significantly correlated with thermal energy dissipation (DIo/CSm) and density of active reaction centers (RCo/CSm) at young stages. The acceptor side of Photosystem I (PSI) was affected by plant age resulting in a lower efficiency than donor side of PSII.

The ABA-Deficiency Suppressor Locus HAS2 Encodes the PPR Protein LOI1/MEF11 Involved in Mitochondrial RNA Editing

The hot ABA-deficiency suppressor2 (has2) mutation increases drought tolerance and the ABA sensitivity of stomata closure and seed germination. Here we report that the HAS2 locus encodes the MITOCHONDRIAL EDITING FACTOR11 (MEF11), also known as LOVASTATIN INSENSITIVE1. has2/mef11 mutants exhibited phenotypes very similar to the ABA-hypersensitive mutant, hai1-1 pp2ca-1 hab1-1 abi1-2, which is impaired in four genes encoding type 2C protein phosphatases (PP2C) that act as upstream negative regulators of the ABA signaling cascade. Like pp2c, mef11 plants were more resistant to progressive water stress and seed germination was more sensitive to paclobutrazol (a gibberellin biosynthesis inhibitor) as well as mannitol and NaCl, compared with the wild-type plants. Phenotypic alterations in mef11 were associated with the lack of editing of transcripts for the mitochondrial cytochrome c maturation FN2 (ccmFN2) gene, which encodes a cytochrome c-heme lyase subunit involved in cytochrome c biogenesis. Although the abundance of electron transfer chain complexes was not affected, their dysfunction could be deduced from increased respiration and altered production of hydrogen peroxide and nitric oxide in mef11 seeds. As minor defects in mitochondrial respiration affect ABA signaling, this suggests an essential role for ABA in mitochondrial retrograde regulation.