Artificial Drought Research Articles

Norway Spruce Seedlings from an Eastern Baltic Provenance Show Tolerance to Simulated Drought

In Northern Europe, an increase in heterogeneity of summer precipitation regime will subject forests to water deficit and drought. This is particularly topical for Norway spruce (Picea abies Karst.), which is a drought sensitive, yet economically important species. Nevertheless, local populations still might be highly plastic and tolerant, supporting their commercial application. Accordingly, the tolerance of Norway spruce seedlings from an Eastern Baltic provenance (western part of Latvia) to artificial drought according to soil type was assessed in a shelter experiment. To simulate drought, seedlings were subjected to reduced amounts (0%, 25%, 50%, 75%, and 100%) of naturally occurring precipitation (irrigation intensity). Three soil types (oligotrophic mineral, mesotrophic mineral, and peat) were tested. Seedling height, chlorophyll a concentration, and fluorescence parameters were measured. Both growth and photochemical reactions were affected by the irrigation intensity, the effect of which experienced an interacted with soil type, implying complex controls of drought sensitivity. Seedlings were more sensitive to irrigation intensity on mesotrophic mineral soil, as suggested by growth and photosynthetic activity. However, the responses were nonlinear, as the highest performance (growth and fluorescence parameters) of seedlings occurred in response to intermediate drought. On peat soil, which had the highest water-bearing capacity, an inverse response to irrigation intensity was observed. In general, fluorescence parameters were more sensitive and showed more immediate reaction to soil water deficit than concentration of chlorophyll on mesotrophic mineral and peat soils, while the latter was a better indicator of seedling performance on oligotrophic soil. This indicated considerable plastic acclimation and hence tolerance of seedlings from the local Norway spruce population to drought, though drought sensitivity is age-dependent.

ASSESS THE DROUGHT TOLERANCE OF BC15 RICE VARIETIES USING INVITRO CULTURE METHOD

BC15 rice is a pure quality rice variety, capable of adapting to large ecological ranges, therefore chosen by farmers to cultivate on large areas. Because global warming and climate change have greatly affected the water source for irrigation, so the rice yield is significantly reduced. This study was conducted to evaluate the drought tolerance of the BC15 variety in vitro culture system to get more information about the characteristics of the BC15 variety when selecting production areas. The object of the study was to determine the most suitable artificial drought conditions for BC15 rice seedlings in vitro. The result shows that in the MS1/2 medium supplemented with PEG with osmotic potential -4 bar; or the MS1/2 supplemented with 7% Saccharose; or the MS1/2 added 3% Mannitol or MS1/2 added 4% Sorbitol, shoots of the rice can be survival. Under these artificial drought conditions caused by Saccharose, Mannitol, Sorbitol, Mannitol expressed the strongest effect on rice sprouts in vitro. Just adding 3% Mannitol to the medium, the root length and shoot height were significantly reduced compared with the control, followed by Sorbitol and finally Saccharose, significantly on the roots and shoots of the seedlings. The results of this study will support necessary information about the suitable environment to evaluate the drought tolerance of rice in vitro and the basic source for in vivo assessment.

Screening cultivars of soybean, contrasting for rapidity maturity, of the degree for heat- and drought resistance, in vivo and in vitro

Aim. Screening for complex heat and drought resistance in varieties of soybeans that differ for maturity in growing experiences and in vitro culture. Methods. To create the conditions of "artificial drought" experimental seedlings and calluses with the addition of 10% mannitol solution were cultured, heat resistance analysis out under the influence of temperature shock (60 ° C) of different duration (10-30 minutes) was carried. Drought and heat resistance by rapid analysis of seed germination and growth inhibition were determined, and in vitro by growth index (RI) and protein coagulation temperature (PCT). Results. During drought and hyperthermia tested varieties with different ripening differ in the degree of resistance to these stressors. Increased drought and heat resistance in conditions, both in vivo and in vitro middle-matured Yatran and middle-early Ustya, lowered – ultra-ripe Anushka were showed. The ripening Hajibey variety the lowest level of complex drought and heat resistance was characterized. Conclusions. It is assumed that the level of drought and heat resistance is determined by the genotypic characteristics of the varieties and may be related to the sign of the duration of the growing season – rapidity of maturity.
 Keywords: Glycine max (L.) Merr., duration of growing season, drought resistance, heat resistance, callus culture.

Decomposition of roots of different diameters in response to different drought periods in a subtropical evergreen broad-leaf forest in Ailao Mountain

Root decomposition plays an essential role in the terrestrial carbon cycle, but the responses of roots of different diameters to varying drought periods are unclear. Therefore, artificial drought experiments were conducted to investigate the effects of different drought periods on the decomposition rates of roots with different diameters. In this study, <2 mm, 2–5 mm and 5–10 mm root decomposition experiments were conducted in control (CK), short-term drought (STD, drought during one rainy season), and long-term drought (LTD, two-year drought) treatments. The drought treatments significantly reduced the soil water content by 52.5% and 83.6%, respectively, but had little effect on the soil temperature. According to the decomposition coefficient (k value), the root decomposition rate decreased significantly with increasing diameter, and STD and LTD significantly reduced root decomposition of different diameters by 30.4–41.7% and 47.8–70.8%, respectively. The analysis of covariance (ANCOVA) revealed that diameter, treatments and their interaction had significant effects on the k values. The linear regression showed that the dependence of the stage root decomposition rate (SRDR) and k value of fine roots on the soil water content were higher than that of coarse roots. Therefore, fine roots were more sensitive to the soil water content and drought, thereby affecting the nutrient cycle and health of forest ecosystems under drought. This study provides new insight into the impact of climate change on root decomposition and the carbon cycle.

Association mapping for yield traits under drought stress in Autumn rice germplasm collection of Assam

Association mapping approach was applied to identify regions linked to SSR markers with yield and related traits in drought stress conditions using 267 autumn rice germplasm collection of Assam. The germplasm exhibited extensive phenotypic and genotypic diversity in both irrigated and artificial drought stress conditions. The drought screening in rainout shelter was successful causing a reduction of mean yield up to 65% in contrast to irrigated conditions. The grain yield showed a positive correlation with traits like productive tiller number, spikelet fertility, and relative leaf water content in drought stress situations suggesting their scope for use as an indirect selection parameter in breeding autumn rice. Many traditional autumn genotypes like ‘Koni ahu’, ‘Garam ahu 2’, ‘Bengonagutiya’, ‘Ahu Joha’ were found to be tolerant in drought stress conditions. The population structure study revealed the presence of structure in the panel dividing 203 accessions into two subgroups and leaving sixty-nine accessions under admixed ancestry. The marker-trait association (MTA) studies using structured mixed linear model (MLM) method unveiled 64 MTA for nine different traits under drought stress conditions. For grain yield, six MTA’s were reported on chromosome 2, 3, 5, 6 and 10 explaining the Phenotypic Variance in the range of 6 to 16%. Seven novel MTA were obtained for their corresponding traits and many of the identified QTL’s were reported previously. Some of the associations were also consistent with General linear model study. Candidate gene analysis of associated marker regions inferred that there are certain stress-responsive genes in its proximity. The tolerant genotypes identified can be employed in different drought breeding programs. The novel associations with major effect offer scope in the mining of new alleles for traits associated with drought tolerance for this class of rice.

Identification of in-vitro PEG Mediated Drought Tolerance Genotype in Rice (Oryza sativa L.)

Drought is a serious bottleneck in the production of rice globally. For this, an experiment was conducted in-vitro on six rice genotypes viz. BRRI Dhan-28, Begunbahar, Burikatari, Pashpai, Dular and Begunbichi to investigate the effect polyethylene glycol (PEG) mediated artificial drought on morpho-physiological parameters such as germination percentage, shoot length, root length, fresh weight, dry weight, turgid weight, relative water content and proline accumulation. Here, different doses of PEG-6000 viz. 0gL-1, 15gL-1, 30gL-1, 45gL-1 and 60gL-1 were used with Murashige and Skoog (MS) medium. The results demonstrated that BRRI Dhan-28, Burikatari and Dular revealed greater performance at control conditions but at the highest degree of water stress conditions only Burikatari showed higher mean value for all parameters studied. Again, Begunbichi followed by BRRI Dhan-28 exhibited the lowest mean value for almost all traits except for proline accumulation. Here, water stress decreased the performance of morpho-physiological characters except proline accumulation in rice. The cluster analysis was performed and distributed into three groups where there was a significant variation among the clusters at different water stress conditions. Here, the genotype Burikatari is more diverse giving maximum Euclidian distances in drought treatments. It could be considered as a parent in the hybridization program against Begunbahar, Dular and Paspai. Therefore, considering the mean performances and cluster analysis, Burikatari exhibited greater performances against the highest degree of drought conditions. This genotype may bear drought-tolerant gene for which could be utilized for further development of drought-tolerant variety and gene transfer.

Drought tolerant rice breeding lines developed for rainfed lowland areas

Drought is a major problem in rice production in rainfed areas. Climate change has exacerbated the negative effect of drought on the sustainability of rice production. Development of drought tolerant rice varieties is important strategy to minimize rice yield losses in drought prone areas. This study aimed to determine drought tolerance of twenty-five advanced rice breeding lines under artificial drought environment and to evaluate their agronomic performance in rainfed lowland rice areas. Evaluation of drought tolerance was carried out at seedling stage in greenhouse. Yield trial was conducted in rainfed rice area in Indramayu district, West Java province, during wet season 2017-2018. The trial was designed in randomized complete block design with four replications. Result from this study indicated a variation in drought tolerance of twenty-five breeding lines. Two lines showed highly tolerant response to drought in seedling stage namely B15231-MR-10-1 and B15143C-TGB-12. The grain yield of highly drought tolerant rice line B15231-MR-10-1 (4.81 t ha-1) was comparable to popular rice varieties Situ Bagendit (4.56 t ha-1) and Ciherang (5.09 t ha-1). Drought tolerant rice breeding lines identified from this study have potential to be adopted by farmers in drought prone rainfed areas as an adaptation strategy to the impact of climate change.

Physiological evaluation of drought stress tolerance and recovery in Verbascum sinuatum plants treated with methyl jasmonate, salicylic acid and titanium dioxide nanoparticles

The genus Verbascum L. (Scrophulariaceae) includes medicinal plants, which have several bioactive compounds especially saponins. The possible recovery ability of Verbascum sinuatum from drought stress conditions was assessed by using salicylic acid (SA), methyl jasmonate (MJA) and titanium dioxide nanoparticles (TiO2NPs) as plant growth regulators (PGRs) in liquid culture media. Thirty days-old plants were exposed to different concentrations of polyethylene glycol (PEG-6000) for creating artificial drought conditions (0, −0.3, and −0.6 MPa osmotic potential) and also treated with 200 µM methyl jasmonate (MJA), 100 µM salicylic acid (SA) and 20 ppm TiO2 nanoparticles (TiO2NPs). Results showed that the growth parameters and the content of photosynthetic pigments decreased at higher drought level (−0.6 MPa). However, SA and TiO2NPs alleviated the adverse effects of drought stress by increasing water stress tolerance through promotion of enzymatic and nonenzymatic antioxidant defense systems. MJA negatively affected the growth parameters and increased the content of malondialdehyde (MDA), hydrogen peroxide (H2O2) and total saponin and also the activity of peroxidase (POD) and polyphenol oxidase (PPO). Based on the results obtained from this study, the recovery treatments mainly affected the defense-related metabolism in Verbasum sinuatum plants.

Selection of drought-tolerant rice varieties from the rice variety bank of Can Tho university

Drought has been a big problem and damaged seriously to rice cultivation and production in Vietnam and the Mekong Delta region; evaluating drought tolerance of rice is a major objective for the rice improvement programmes in Can Tho University. Fifty-two collected rice varieties including resistant and susceptible control varieties were screened for water stress under the artificial drought condition. Marker RM223 was used to identify the drought tolerance genotypes for some selected varieties with good and moderate tolerant scores. After 30 days of water stress, the results were 6 varieties of good tolerant, 8 varieties of moderate tolerance, 36 varieties of moderately susceptible and 2 varieties of susceptible to drought. Analyses of PCR showed that 10 varieties expressed the similar bands with the resistant control variety. Four varieties (LH8, MTL812, Lua Canh and VB1) with good tolerant to drought were recommended to use for genetic materials of rice breeding program and applying in alternative wetting and drying irrigation technique for rice cultivation.

Comparative morpho-physiological response of in vitro selected somaclones of wheat (Triticum aestivum L.) and explant donor parent to drought stress

Drought stress is the most prevalent environmental factor limiting wheat productivity globally and demands integration of new plant improvement approaches. Plant tissue culture, being a mutagenic process induces somaclonal variations, which can be manipulated for improving drought tolerance of wheat. An in vitro selection system was employed to regenerate PEG-6000 tolerant callus lines into R1 somaclones. Comparative morpho-physiologcial responses of R1 somaclones and explant donor parent cultivar to osmotic stress were studied at seedling and booting stages by inducing artificial drought stress along with the control of unstressed. Both R1 somaclones and explant donor parent exhibited differential responses to drought stress at both growth stages and the selected somaclones were less affected by drought stress over their donor parent. At seedling stage, the R1 somaclones exhibited significantly higher root length, shoot length, root to shoot length ratio and relative water contents (RWC) under PEG- 6000 induced osmotic stress. Similarly, at booting stage the R1 somaclones accumulated significantly higher proline, protein, sugar and potassium ion (K+) contents and maintained higher membrane stability index (MSI) than the original cv. GA-2002 under drought stress conditions. The results revealed that the R1 somaclones of wheat regenerated from PEG-6000 tolerant calli have better drought tolerance than the explant donor parent cv. GA-2002, suggesting that in vitro immature embryo culture can be employed to produce drought tolerant plants of wheat.

In Vitro Shoot Regeneration of Rice (&lt;i&gt;Oryza sativa&lt;/i&gt; L.) Varieties under Artificial Drought Stress

The shoot regeneration efficiency of two rice (Oryza sativa L.) varieties (Kasalath and BRRI dhan55) under artificial drought condition created by polyethylene glycol (PEG-6000) was investigated. Calli were induced in Murashige and Skoog (MS) medium supplemented with different concentrations (0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 4.0 mg/L) of 2, 4‐Dichlorophenoxyacetic acid (2, 4‐D). Then, in vitro shoot regeneration under PEG (6000) was conducted on MS medium supplemented with 1.0 mg/L NAA + 2.0 mg/L BAP + 1.0 mg/L kinetin that gave the highest number of shoots. Shoot regeneration in both varieties reduced with increased levels of PEG (0.5, 1.0, 2.0, 3.0 and 4.0%). Regeneration efficiency of BRRI dhan55 in drought stress was higher compared to Kasalath. Thus, in vitro screening of plantlets under PEG can be used to achieve fast and reliable selection for tolerance against drought. The Agriculturists 2018; 16(1) 55-64

THE STUDY RESULTS OF 1000-KERNEL WEIGHT OF WINTER SOFT WHEAT VARIETIES UNDER PROVOKING CONDITIONS “ZASUSHNIK”

In the southern part of theRostovregion high temperature and insufficient humidity in soil are the main unfavorable factors that destroy the conventional physiological and biological processes of winter wheat plants and it results in their productivity decrease. As 1000-kernel weight is one of the principal structural elements that have an effect on productivity, the study of this trait under conditions of artificial drought is of great importance. The article presents the study of 1000-kernel weight in 18 samples of winter soft wheat under provoking conditions (“zasushnik”). As a result we have identified the varieties ‘Asket’, ‘Volny Don’, ‘Volnitsa’ and ‘Don107’which possess high index of 1000-kernel weight in the provoking conditions of severe drought (“zasushnik”) in relation to the optimal conditions of development. These varieties have been suggested to use as the initial material for winter soft wheat breeding on drought tolerance.

Residual Effect of Induced Water Stress and Nitrogen Addition on the Mycobiota in Scots Pine Stands

Mycobiota (fungi, lichens and myxomycetes) was examined in permanent plots following experiments of artificial drought (D) and nitrogen addition (N) and compared with untreated forest (C), in Scots pine stand planted on Arenosols. Species diversity and relationships between fungal community structure and environmental variables (plant species numbers and cover, bryophyte cover, soil and bark pH, tree mortality) were explored. Both D and N treatments lead to decrease of fungal species in general, however, responses of individual trophic and ecological groups varied. The strongest effect of the treatments was observed for soil fungi, especially mycorrhizal: numbers of fruiting species and ectomycorrhizal root tips decreased, and species composition has changed. Saprotrophic fungi reacted by changes in species composition but not in numbers. Of the studied environmental variables, the most significant effect on mycobiota had bryophyte and vascular plant cover as well as vascular plant species numbers.

Оценка коллекционных образцов ячменя на устойчивость к осмотическому стрессу

The article provides the results of studies on estimation of 50 covered and 37 naked barley collection samples on resistance to osmotic stress. The estimation was done in laboratory conditions. Sucrose with concentration corresponded to pressure of 14 atmospheres was used as osmotic solution. Artificial drought significantly reduced seed germination of most samples. Compared to the control, under osmotic stress only 1.0% of samples had single germinated seeds on the 3-d day of the experiment. Laboratory estimation has shown that differences on germination ability under osmotic stress do not depend on the place of sample origin. For example, among samples from Great Britain there were both forms with resistance higher than average (Canasta) and sensitive to osmotic stress (Crusades). Weak significant correlation was pointed out between drought resistance in laboratory experiment and field productivity (r = 0.28). More close relation was established between resistance to osmotic stress and number of grain in the ear (r = 0.52) as well as ear productivity (r = 0.53). Resistant to osmotic stress samples more than 30% exceeded a standard variety by productivity under drought conditions. For further breeding activity drought resistant samples were selected: of husked barley - Tandem, Chelyabinsky 96, Chelyabinsky 99, Sonet (Russia), Firlbeks Union (Switzerland), Lamba (Denmark), Canasta (Great Britain), Buck (Canada), Bonita (Argentina); of naked barley - Omsky golozerny 1 (Russia), CBSS06YCA 0044S, CBSS06YCA 0019S, CBSS06YCA 0018S (Canada), Azure (USA), k-18079 Local (India).

Effect of moisture stress on growth and yield of cucumber genotypes

The experiment was conducted during spring-summer season with twenty five cucumber genotypessubjected to four levels of drought stress (control, 75%, 50% and 25% of recommended irrigation) in the openfield experiment using completely randomized design with three replications to assess the effects of droughton vegetative growth and fruit yield. Star flow meter instrument used to measure the amount of irrigation wateras calculated to induce an artificial drought stress levels on the plants. ANOVA revealed significant differences amongst genotypes and genotype × drought stress level interaction for all the traits indicating differential response of the genotypes. Decreased amount of water levels resulted in progressive reduction in number ofleaves (19.7, 15.3, 12.4 and 9.0), vine length (55.8, 55.4, 46.7 and 41.3 cm) and fruit yield per vine (0.982, 0.645, 0.546 and 0.487 kg), while increased in affected leaves at 18.8, 58.8, 67.2 and 84.6%, respectively in control(100%), 75%, 50% and 25% of recommended irrigation. Thus, these above phenotypic traits appeared to bepromising as selection criteria for drought tolerance at morphological level. As a result, the genotypes DGC-1and WBC-13 observed drought tolerant, whereas DGC-8 and GS-3 were drought susceptible.

Variability of mesophyll conductance and its relationship with water use efficiency in cotton leaves under drought pretreatment

Drought slows net photosynthetic rate (AN) but increases water use efficiency (WUE). Farmers give an artificial drought pretreatment to some crops in the early growth stage and find that yield increases accompanying with the improvement of WUE. We conducted well-watered, non-drought, mild drought and moderate drought pretreatments of potted cotton cultivars. The aims of the present study were to analyse the importance of mesophyll conductance (gm) as a factor that may simultaneously improve AN and WUE under drought pretreatment conditions, and to analyse the role of anatomical structure and biochemical mechanism in the variability of gm. Our results showed that significant variability of gm estimated by gas exchange and chlorophyll fluorescence was observed between non-drought pretreatment and drought pretreatment associated with change in AN and WUE. There was great difference in anatomical structure and expression of aquaporins (GhAQP1) among all the treatments. In addition, expression of carbonic anhydrase (CA) may not be important in the regulation of gm under drought pretreatment conditions. We concluded that the variability of gm offers a potential target for improving leaf AN and WUE simultaneously by the regulation of anatomical structure and GhAQP1.

Species-specific responses of water use by urban trees to artificial soil drought: Results from a small-scaled study.

Studying tree water relations in response to soil drought improves our understanding of hydrological processes which are central to most ecosystem modelling. Urban greening is an approach to ameliorate rising carbon dioxide emissions whose effects are intensified in cities. Thus, an investigation of water relations with respect to drought of urban trees is of interest for effective water management in urban greening. This study examined responses of sap flux density, water use and stomatal conductance of potted saplings of three common street trees in Bangkok: Pterocarpus indicus ( Pi ), Swietenia macrophylla ( Sm ) and Lagerstroemia speciosa ( Ls ); under one-week non-irrigated treatment. Results showed that Pi and Sm maintained their water use rate while Ls experienced reduction in water use under the soil drying condition. Further analyses on stomatal conductance and leaf area suggested that leaf drop, which may be induced by soil drought, resulted in increases of stomatal conductance. The counteracting effect of increased stomatal conductance and decreased leaf area seems to explain the unaffected water use in Pi and Sm under drought condition. However, less compensatory effect was observed in Ls , resulting in significant reduction of water use in the non-irrigated saplings. Thus, these results indicated that Pi and Sm may be preferred for street tree planting since they would be less affected by drought stress than Ls . Nevertheless, further investigations, such as detailed measurements of tree hydraulic variables and longer experimental duration, are advised to confirm these findings for larger-scaled extrapolation.

Local diversity of heathland Cercozoa explored by in-depth sequencing

Cercozoa are abundant free-living soil protozoa and quantitatively important in soil food webs; yet, targeted high-throughput sequencing (HTS) has not yet been applied to this group. Here we describe the development of a targeted assay to explore Cercozoa using HTS, and we apply this assay to measure Cercozoan community response to drought in a Danish climate manipulation experiment (two sites exposed to artificial drought, two unexposed). Based on a comparison of the hypervariable regions of the 18S ribosomal DNA of 193 named Cercozoa, we concluded that the V4 region is the most suitable for group-specific diversity analysis. We then designed a set of highly specific primers (encompassing ~270 bp) for 454 sequencing. The primers captured all major cercozoan groups; and >95% of the obtained sequences were from Cercozoa. From 443 350 high-quality short reads (>300 bp), we recovered 1585 operational taxonomic units defined by >95% V4 sequence similarity. Taxonomic annotation by phylogeny enabled us to assign >95% of our reads to order level and ~85% to genus level despite the presence of a large, hitherto unknown diversity. Over 40% of the annotated sequences were assigned to Glissomonad genera, whereas the most common individually named genus was the euglyphid Trinema. Cercozoan diversity was largely resilient to drought, although we observed a community composition shift towards fewer testate amoebae.

Proteomics Study of Wheat (Triticum durumL.) Genotypes in Response to Drought Stress

Under present investigation two-dimensional electrophoresis (2-DE) technique was used for finding out the drought responsiveness of differentially expressed protein spots (DEPs) in the three drought susceptible (HI-8498, GW-1139 and MPO-1215) and three drought tolerant wheat genotypes (GW-1, AR-07-30 and AR-07-33). Artificial drought was induced by 20% (−0.52 bar) polyethylene glycol (PEG-6000) treatment. Total 630 protein spots were observed amongst all the genotypes which include 272 droughts responsive protein spots (DRPs), 37 differentially expressed protein spots (DEPs) and 76 cultivar specific protein spots. Susceptible genotypes, namely, HI-8498, GW-1139 and MPO-1215 showed 59.18, 51.37 and 48.71% of DRPs respectively whereas the tolerant genotypes viz. GW-1, AR-07-30 and AR-07-33 expressed 50.89, 25.38 and 40.83% DRPs respectively. Among all the genotypes examined, the susceptible genotypes (HI-8498 and GW-1139) and tolerant genotype (GW-1) showed better performance under the stress conditions. Using this technique the drought responsive proteins were identified in both susceptible and tolerant wheat genotypes which can prove to be an important tool for understanding the biochemical mechanism in the plants under the stress conditions.

Differential controls by climate and physiology over the emission rates of biogenic volatile organic compounds from mature trees in a semi-arid pine forest.

Drought has the potential to influence the emission of biogenic volatile organic compounds (BVOCs) from forests and thus affect the oxidative capacity of the atmosphere. Our understanding of these influences is limited, in part, by a lack of field observations on mature trees and the small number of BVOCs monitored. We studied 50- to 60-year-old Pinus ponderosa trees in a semi-arid forest that experience early summer drought followed by late-summer monsoon rains, and observed emissions for five BVOCs-monoterpenes, methylbutenol, methanol, acetaldehyde and acetone. We also constructed a throughfall-interception experiment to create "wetter" and "drier" plots. Generally, trees in drier plots exhibited reduced sap flow, photosynthesis, and stomatal conductances, while BVOC emission rates were unaffected by the artificial drought treatments. During the natural, early summer drought, a physiological threshold appeared to be crossed when photosynthesis ≅2 μmol m(-2) s(-1) and conductance ≅0.02 mol m(-2) s(-1). Below this threshold, BVOC emissions are correlated with leaf physiology (photosynthesis and conductance) while BVOC emissions are not correlated with other physicochemical factors (e.g., compound volatility and tissue BVOC concentration) that have been shown in past studies to influence emissions. The proportional loss of C to BVOC emission was highest during the drought primarily due to reduced CO2 assimilation. It appears that seasonal drought changes the relations among BVOC emissions, photosynthesis and conductance. When drought is relaxed, BVOC emission rates are explained mostly by seasonal temperature, but when seasonal drought is maximal, photosynthesis and conductance-the physiological processes which best explain BVOC emission rates-decline, possibly indicating a more direct role of physiology in controlling BVOC emission.