Early Drought Research Articles

Response to Early Drought for Traits Related to Nitrogen Fixation and Their Correlation to Yield and Drought Tolerance Traits in Peanut (Arachis hypogaea L.)

This study was aimed to examine the response and contribution of early drought to traits related to N2-fixation and pod yield and their correlation to drought tolerance. The experiment was conducted at the Field Crop Research Station of Khon Kaen University, Khon Kaen Province, Thailand in the dry season of 2007/08. Eleven peanut genotypes (ICGV 98300, ICGV 98303, ICGV 98305, ICGV 98308, ICGV 98324, KK 60-3, Tainan 9, KKU 72-1, KKU 60, KK 4 and KKU 1) and two soil moisture levels [field capacity (FC) and 1/3 available water (1/3 AW)] were laid out in a split-plot design with four replications. Early drought treatment was given by maintaining 1/3 AW from emergence to 40 days after emergence followed by adequate water supply. The data were recorded for nodule dry weight (NDW) and biomass production (BM) as traits related to N2-fixation (TNf) at harvest. In addition to, the data on pod yield, number of pod plant-1, number of seed pod-1 and seed size (SZ) were also collected at harvest. Specific leaf area (SLA) and SPAD chlorophyll meter reading (SCMR) were measured on 20, 40, 50 and 60 days after emergence (DAE) as drought tolerance traits and harvest index (HI) was calculated after harvest. Early drought did not affect NDW and BM. Major variation was found among peanut genotypes and ICGV 98305 showed higher NDW and pod yield under drought condition. Significant and consistent correlation was found between NDW and BM, (r = 0.82*, p < 0.05) and (r = 80*, p < 0.05) under FC and 1/3 AW, respectively. The correlation between TNf and pod yield and yield component parameters varied under the two water regimes. Under 1/3 AW, the only positive correlation observed was between SZ and BM and it might be the only reason for increase in pod yield in some genotypes. SCMR at 60 DAE was strongly related with TNf under both water regimes. There was not any correlation between SLA and HI with NDW and BM. SCMR at 60 DAE is useful to detect chlorophyll density and N2-fixation under both water regimes because of its high and constant correlation with TNf.

Physiological traits associated with drought tolerance in bread wheat (Triticum aestivum L.) under tropical conditions

Although it is generally accepted that drought tolerance is a critical agronomic trait, efficient and predictable improvement in drought tolerance in bread wheat (Triticum aestivum L.), in varying drought stress conditions, has not been fully achieved. This study aimed at assessing the responses of bread wheat to drought, using physiological traits associated with drought tolerance. Two experiments were undertaken in 2001 and 2002. In Experiment I, 17 wheat genotypes were planted in the field in Katumani, Kenya, using a randomised complete block design. Experiment II was conducted under rain shelter (by simulating early season drought at seedling stage) at National Plant Breeding Research Centre, Njoro, Kenya. Treatments were imposed in a split-plot design with water regime (low, medium and high) as main plots and 12 wheat genotypes as sub-plots. Evapo-transpiration (ET) was determined by monitoring soil moisture content at 7 d intervals using a neutron probe, stomatal conductance (g) and instantaneous transpiration (T) rates were measured on the uppermost fully expanded leaves at booting stage using a steady-state porometer, net leaf CO2 exchange rates (CER) was measured on selected leaves using a portable Infrared Gas Analyser, fitted with Parkinson Leaf chamber, and crop biomass, grain yield and harvest index (HI) determined at harvest maturity. Water use efficiency (q), HI, stomatal conductance, and CER were identified as key control points in determining the drought resistance of tolerant genotypes. Therefore it is important to determine the heritability of these traits in order to ascertain their potential usefulness in a wheat breeding program.

Adaptation à court et à long terme de l'agriculture au risque de sécheresse : une approche par couplages de modèles biophysiques et économiques

Short and long run adaptation of agriculture, to drought risk : Assessment through coupling of economic and biophysical models. In this article, we analyse the impact of drought risk on agriculture. We use a biophysical crop growth model (STICS) in order to simulate crop yield under various climatic scenarios. A micro-economic model which optimises, under climate uncertainty, land allocation across crops and irrigation is developed to reflect farmer's behaviour. This framework is used to assess the impact of drought risk on a representative French farmer located in the Midi- ? y rénées region (South-West of France). We first show that, on the short run, the private cost of a drought can be high. On the long run, farmers can however significantly reduce this cost by modifying their cropping systems. We finally demonstrate that French farmers may benefit from the implementation of early drought alert mechanisms.

Using AVHRR-based vegetation indices for drought monitoring in the Northwest of Iran

In order to evaluate the capability of NOAA-AVHRR data for drought monitoring in the northwest of Iran having cold semi-arid climate, a study plan was designed involving the production of normalized difference vegetation index (NDVI) and vegetation condition index (VCI) indices and correlating their values to precipitation data. Raw AVHRR images were processed and geometric and radiometric corrections were performed. Seven-day maximum NDVI maps were produced and VCI was calculated using the maximum and minimum NDVI values for the same time period. Precipitation statistics from 19 synoptic meteorological stations were collected. The study covered a five-year time period with three consecutive months in the growing season. Pearson correlation was performed to correlate NDVI and VCI values to precipitation data. Different time lag schemes were tried and the highest correlation coefficients (r values) were obtained while correlating NDVI and VCI to three-month (current plus last two months) precipitation. Better agreement was observed between NDVI and precipitation as compared with that between VCI and precipitation in individual stations. Good correlations were also obtained between average NDVI and VCI of the study area and average three-month precipitation. The results indicated that NOAA-AVHRR derived NDVI well reflects precipitation fluctuations in the study area promising a possibility for early drought awareness necessary for drought risk management.

Stomatal Responses in Rainfed Lowland Rice to Partial Soil Drying ; Evidence for Root Signals

The role of root signals in water deficit responses of rice (Oryza sativa L.) is important in the alternate flooding and drying conditions encountered in the rainfed lowlands, where the abundant roots in shallow soil layers may generate signals when droughted, with consequent reduction in stomatal conductance (gs) and growth, despite the likelihood of additional water in deeper soil layers. This study was conducted to confirm the presence of root signals, explore their nature and plant responses, consider the suitability of the methods, and discuss implications for adaptation under rainfed lowland drought. A split-root technique was used in greenhouse studies, whereby roots were divided into two sections: flooded and droughted. The decrease in gs and transpiration rate (Tr) due to drying of a portion of the roots, and their apparent recovery upon severing of this root portion, were consistent with the role for signals. The field study confirmed the evidence for root signals during progressive soil drying, whereby gs and Tr decreased before leaf water potential (ΨL) started to decline. The increase in leaf ABA concentration under field drought, and its strong association with soil moisture tension and gs, suggested its involvement in mediating stomatal responses during early drought in rice. The recovery in ΨL after severing of droughted roots in the greenhouse could be attributed to increased hydraulic conductance. These responses imply a role for both chemical and hydraulic signals in rice, which have important implications for adaptation and crop performance in contrasting rice ecosystems.

Early drought-induced changes to the needle proteome of Norway spruce

To elucidate early drought responses in needles of Norway spruce (Picea abies (Karst.) L.), we subjected 1-year-old seedlings to gradual desiccation for 6 weeks. Four weeks of drought treatment caused a small but significant decrease in photosystem II quantum yield of light-adapted needles (phi(a)) compared with that of well-watered controls. Six weeks of drought treatment reduced phi(a) and the photosystem II quantum yield of dark-adapted needles (phi) by 50 and 8%, respectively, and reduced shoot water potential by 0.7 MPa, but had no measurable effect on needle relative water content. After two weeks of drought treatment, and before there was a discernible effect of drought on phi or a statistically significant effect on shoot water potential, needles were analyzed for changes in protein composition. Five out of several hundred detected proteins in needles of drought-treated plants showed consistent changes compared with control leaves. The proteins were identified by LC-MS/MS as components of the oxygen-evolving complex (oxygen evolving enhancer protein 2), ribulose-1,5-bisphosphate carboxylase/oxygenase large subunit, and one protein of unknown function, whose mRNA was found in a previous screen of wound- and methyl-jasmonate-induced bark proteins.

Reduced Carbon Availability to Bacteroids and Elevated Ureides in Nodules, But Not in Shoots, Are Involved in the Nitrogen Fixation Response to Early Drought in Soybean

Nitrogen fixation (NF) in soybean (Glycine max L. Merr.) is highly sensitive to soil drying. This sensitivity has been related to an accumulation of nitrogen compounds, either in shoots or in nodules, and a nodular carbon flux shortage under drought. To assess the relative importance of carbon and nitrogen status on NF regulation, the responses to the early stages of drought were monitored with two soybean cultivars with known contrasting tolerance to drought. In the sensitive cultivar ('Biloxi'), NF inhibition occurred earlier and was more dramatic than in the tolerant cultivar ('Jackson'). The carbon flux to bacteroids was also more affected in 'Biloxi' than in 'Jackson', due to an earlier inhibition of sucrose synthase activity and a larger decrease of malate concentration in the former. Drought provoked ureide accumulation in nodules of both cultivars, but this accumulation was higher and occurred earlier in 'Biloxi'. However, at this early stage of drought, there was no accumulation of ureides in the leaves of either cultivar. These results indicate that a combination of both reduced carbon flux and nitrogen accumulation in nodules, but not in shoots, is involved in the inhibition of NF in soybean under early drought.

Agricultural drought assessment at disaggregated level using AWiFS/WiFS data of Indian Remote Sensing satellites

Spatial differences in drought proneness and intensity of drought caused by differences in cropping patterns and crop growing environments within a district indicate the need for agricultural drought assessment at disaggregated level. The objective of this study is to use moderate resolution satellite images for detailed assessment of the agricultural drought situation at different administrative units (blocks) within a district. Monthly time composite NDVI images derived from moderate resolution AWiFS (60 m) and WiFS (180 m) images from Indian Remote Sensing satellites were analysed along with ground data on rainfall and crop sown areas for the kharif seasons (June – November) of 2002 (drought year), 2004 (early season drought) and 2005 (good monsoon year). The impact of the 2002 meteorological drought on crop area in different blocks of the district was assessed. The amplitude of crop condition variability in a severe drought year (2002) and a good year (2005) was used to map the degree of vulnerability of different blocks in the district to agricultural drought. The impact of early season deficit rainfall in 2004 on the agricultural situation and subsequent recovery of the agricultural situation was clearly shown. Agricultural drought assessment at disaggregated level using moderate resolution images is useful for prioritizing the problem areas within a district to undertake, in season drought management plans, such as alternate cropping strategies, as well as for end of the season drought relief management actions. The availability of ground data on rainfall, cropping pattern, crop calendar, irrigation, soil type etc., is very crucial in order to interpret the seasonal NDVI patterns at disaggregated level for drought assessment. The SWIR band of AWiFS sensor is a potential data source for assessing surface drought at the beginning of the season.

Effect of water deficit on respiration of conducting bundles in leaf petioles of sugar beet

Isolated fibrovascular bundles from source leaf petioles of sugar beet (Beta vulgaris L.) and hog-weed (Heracleum sosnovskyi L.) were used to study the influence of long-term drought on the oxygen uptake rate and activities of mitochondrial oxidases, i.e., cytochrome oxidase and salicylhydroxamic acid-sensitive alternative oxidase (AO). Under normal soil moisture content (70% of full water-retaining capacity, WRC), the oxygen uptake by sugar beet conducting bundles was characterized by a high rate (> 700 μl O2/(g fr wt h)) and by distinct cytochrome oxidase-dependent manner of terminal oxidation (up to 80% inhibition of respiration in the presence of 0.5 mM KCN). After long-term water deficit (40% of WRC), the bundle respiration proceeded at nearly the same rate but featured an elevated resistance to cyanide. At early drought stage (10 days), a decrease in the activity of cytochrome-mediated oxidation pathway was largely counterbalanced by activation of mitochondrial AO, whereas long-term dehydration of plants was accompanied by activation of additional oxidative systems insensitive to both KCN and SHAM. Similar but even more pronounced changes in activities of terminal oxidases were discovered in conducting bundles of wild-grown hogweed plants exposed to long-term natural drought. It is supposed that the suppression of cytochrome-mediated oxidation coupled with ATP synthesis in the cells of sugar beet source leaves impedes the translocation of assimilates and their accumulation in the taproot, which represents an important factor of drastic decrease in the yield of this agricultural crop under conditions of water deficit.

Genotypic response of chickpea (Cicer arietinum L.) cultivars to drought stress implemented at pre- and post-anthesis stages and its relations with nutrient uptake and efficiency

Uptake of mineral nutrients in chickpea cultivars might be an important response in drought tolerance. An experiment under controlled conditions was carried out to study the genotypic response of 11 chickpea (Cicer arietinum&amp;nbsp;L.) cultivars to drought and its relations with N, P, K, Ca, Mg, Fe, Zn, Mn and B uptake and uptake efficiency. Plants were grown either optimal or drought stress implemented at pre- (early drought stress, EDS) and post-anthesis (late drought stress, LDS) stages. Growth reduction of the cultivars as a response to drought significantly differed. The results of the study indicated that EDS had less detrimental effects on growth and nutrient uptake than LDS conditions. In general, drought tolerant chickpea cultivars accumulated more N, P, K, Ca, Zn, Mn and B in both drought stress treatments except for Zn and Mn uptake in LDS treatment. The total nutrient uptake efficiency of the cultivars were also very significantly correlated with the growth reduction ration (GR) both in EDS and LDS treatments giving correlation coefficients (r) of &amp;ndash;7859 and &amp;ndash;0.7678, p &amp;lt; 0.01, respectively.

Growth and Water Use Response of Doubled-Haploid Rice Linesto Drought and Rewatering during the Vegetative Stage

We examined the responses of doubled-haploid lines (DHLs) of rice (Oryza sativa L.) to drought and rewatering in controlled rainfed lowland conditions, to test the the hypothesis that the DHLs would permit trait comparisons with less confounding by unrelated traits than had been reported previously. IR62266 and four DHLs derived from the cross between IR62266 and CT9993 (DHL-32, 51, 54 and 79) were grown in pot experiments in the greenhouse at the IRRI, Los Baños, Philippines. Genotypic variation in leaf and tiller development, transpiration, water use efficiency, osmotic adjustment and leaf water potential was examined in relation to dry matter production. Results revealed that greater seedling vigor through continued leaf expansion in early drought was associated with greater dry matter production after rewatering. A higher water use efficiency was related to a greater increase in dry matter production during drought. Leaf water potential was correlated strongly with dry weight, not only during drought, but especially on rewatering. Therefore, we found that the ability to continue leaf expansion, higher water use efficiency, and a greater osmotic adjustment for maintenance of leaf water potential as drought progressed were desirable traits for improved performance under drought and improved ability to recover on rewatering. These relationships could be analyzed precisely using such genetically-related materials as DHLs, with less confounding effects of plant size and genetic background.

Differential effects of soil water deficit on the basic plant functions and their significance to analyse crop responses to water deficit in indeterminate plants

On the basis of a large set of experiments conducted on legumes, cotton, and vineyards, we propose a framework to analyse the functioning of an indeterminate crop in response to soil water deficit. Indicators of basic plant functions (e.g. leaf carbon exchange rate as an indicator of production of assimilates) have been correlated with soil water status in the root zone (quantified with the fraction of transpirable soil water, FTSW), across a range of soil water deficit. Leaf area development was the most sensitive process to soil water deficits, with branching being more sensitive than leaf emission and growth on the main stem. Net carbon exchange rate and the various steps of reproductive development were less sensitive to drought, thereby explaining why grain/fruit yield can be increased by a moderate drought in indeterminate plants. Other traits of plant adaptation to drought at the crop level, such as reduction of flowering duration and increase of harvest index (in case of early drought), can be explained by the observed differential effects of soil dehydration on the various plant functions. These results have been used to develop regulated deficit irrigation strategies using tensiometers and tools for in-field diagnosis, especially for seed production and grape production.

Analysis of early responses to drought associated with field drought adaptation in four Sahelian groundnut ( Arachis hypogaea L.) cultivars

Groundnut ( Arachis hypogaea L.) is the most important oil and cash crop in the sub-Sahelian tropics. Plant adaptation to drought, i.e. cultivars (cvs) that can maintain yield when water is limited, is a complex phenomenon which is not yet fully understood. This study aimed to identify traits expressed at the early stages of the cycle that could reveal cv differences in drought adaptation in the field. The field productivity of four Sahelian groundnut cvs was assessed during three crop seasons in Bambey (Senegal). The same cvs grown in rhizotrons were subjected to early drought stress and to a desiccation test to assess cell membrane tolerance. Between-cv differences were found with respect to pod yield, biomass production, water use efficiency (WUE), stomatal regulation and cell membrane tolerance. Two strategies to cope with water deficit were identified. The first behaviour was characterised by high rapid water loss, late stomatal closure and low cell membrane damage during drought. These traits are all found in the semi-late Virginia cv 57-422 and, into a lesser extent, in the early Spanish cv Fleur 11. For both cvs, biomass production was boosted under favourable conditions in rhizotrons but the semi-late cv had poor pod yield under end-of-season water deficit conditions. The second strategy involved opposite characters, leading to the maintenance of a higher water status, resulting in lower photosynthesis and yield. This characterised the early Spanish cv 73-30, and also, to some extent, the early Spanish cv 55-437. Earliness associated with high WUE, stomatal conductance and cell membrane tolerance, were the main traits of Fleur 11, a cv derived from a Virginia × Spanish cross, which was able to maintain acceptable yield under varying drought patterns in the field. These traits, as they were detectable at an early stage, could therefore be efficiently integrated in groundnut breeding programmes for drought adaptation.

Drought tolerance, traditional crops and biotechnology: breeding towards sustainable development

Germplasm of traditional vegetable crops were screened for drought tolerance. Different physiological, morphological and biochemical traits of drought tolerance were investigated, including enzymes of the antioxidative pathway (SOD, AP and GR), turgor maintenance (LWP, RWC), membrane stability (CMS, TTC), osmoprotection (proline), productivity (photosynthesis), rooting (root architecture), early drought tolerance and leaf area. Different in vitro screening techniques were carried out. Useful traits of drought tolerance were identified in Amaranthus tricolor, A. hybridus, A. hypochondriacus, Vigna unguiculata and V. subterranea, and are currently being applied in a breeding programme in an attempt to develop tolerant genotypes of neglected vegetable and seed crops that might contribute to secure food production in rural areas in Africa.

Small grain screenings in wheat: interactions of cultivars with season, site, and management practices

Small grains that pass through a 2-mm slotted screen (sievings or screenings) are one of the most important causes of price dockages of wheat in Australia because grain size variation greatly affects flour yield and commercial value. The aims of this study were to examine the effects of season, time of sowing, plant population, and applied nitrogen, and their interactions with cultivars, on small grain screenings. Twenty-one field experiments involving 16 new cultivars and elite crossbreds, and various management variables, were conducted in the medium (annual rainfall 325–450 mm) and low (annual rainfall &lt;325 mm) rainfall zones of the Northern Agricultural Region of Western Australia over 3 diverse cropping seasons (1999–2001). Rainfall events towards the end of the season were critical to the level of screenings. Screenings were higher in season 2000 with terminal drought stress, but were low in 2001 despite severe drought stress during early growth. Delayed seeding caused higher screenings in 1999 (average rainfall with even distribution) and in 2000 (terminal drought) but not consistently in 2001 when early drought stress restricted tillering and spike size thereby constraining the yield level. Strong varietal and time of sowing interactions were evident but the relationship between maturity group and the level of screenings was not consistent. Rather, the ability of cultivars to adjust yield components was more important; 82% of the total variance in small grain screenings was accounted for by a regression model based on variety-specific kernel weight, post-heading rainfall (from about 2 weeks before anthesis), and location factors. The effect of increasing plant population on screenings was mostly negative, with some minor exceptions for a few cultivars in the low-rainfall zone. As applied nitrogen was increased, screenings generally increased and cultivar influenced this trend more than rainfall zone. It is postulated that for a cultivar to be unaffected by applied nitrogen, it should have inherently higher grain weight as well as high stability of grain weight across nitrogen levels. Applied nitrogen had a significant effect on screenings only at higher plant populations. In experiments where the level of screenings exceeded 5%, the yield components that were significantly associated with screenings, in order of relative importance, were grain weight &gt; grain number/area &gt; grain number/head &gt; grain yield. Cultivars differed in production of screenings in response to plant population, nitrogen fertiliser and sowing time. Harrismith was the most sensitive cultivar and Wyalkatchem was overall the most tolerant cultivar. Delayed seeding had the least effect on the screenings of cultivars Westonia, Carnamah, and Wyalkatchem. Carnamah was the most stable cultivar against higher levels of applied nitrogen, whereas Westonia required high plant numbers to contain screenings. It is concluded that cultivars can be classified according to specific sensitivities, and appropriate management practices may be suggested to growers.

Responses of ABA and CTK to soil drought in leafless and leafy apple tree.

The authors tested the contents of ABA (abscisic acid), ZR (zeatin riboside), DHZR (dihydrozeatin riboside) and iPA (isopentenyl adenosine) in leafless and leafy apple trees (Red Fuji/Malus micromalus Makino) during soil drought stress. ABA concentration in drought stressed leafless trees increased significantly compared to the controls. ABA both in roots and xylem rose steadily in the earlier drought stage, reaching a maximum of 1.46 +/- 0.35 nmol g(-1) FW and 117 nmol l(-1) after the 8th day. Similar change patterns of ABA concentration was observed in the leafy trees during soil drought stress; ABA concentrations in roots and xylem sap increased and reached the maximum in the first three days; after 8th day, it decreased slightly, whereas leaf ABA concentration increased steadily in drought stressed plants throughout the duration of the experiment. Between drought stressed and control trees, no significant differences were observed in concentration of ZR and DHZR in both leafless and leafy trees; whereas iPA concentration of the drought stressed leafless and leafy plants decreased markedly in the later stage of drought. These results showed that endogenous ABA originated mainly from the roots in the earlier drought stage, and mainly from the leaves in the later drought stage. Total CTK showed no reduction in the earlier drought stage and decreased in the later drought stage.

Climatic control of stand thinning in unmanaged spruce forests of the southern taiga in European Russia

The demography of Picea abies trees was studied over a period of about 30 yr on permanent plots in six forest types of an unmanaged forest located in a forest reserve of the Southern Taiga, NW of Moscow. This study encompassed a broad range of conditions that are typical for old growth spruce forests in the boreal region, including sites with a high water table and well drained sites, podzolic soils, acidic soils and organic soils. At all sites stand density, tree height, breast height diameter and age has been periodically recorded since 1968. Tree density ranged between 178 and 1035 trees ha−1 for spruce and between 232 and 1168 trees ha−1 for the whole stand, including mainly Betula and Populus. Biomass ranged between 5.4 and 170 tdw ha−1 for spruce and between 33 to 198 tdw ha−1 for the whole stand. Averaged over a long period of time, biomass did not change with stand density according to the self-thinning rule. In fact, on most sites biomass remained almost constant in the long term, while stand density decreased. The study demonstrates that the loss of living trees was not regulated by competitive interactions between trees, but by disturbances caused by climatic events. Dry years caused losses of minor and younger trees without affecting biomass. In contrast, periodic storms resulted in a loss of biomass without affecting density, except for extreme events, where the whole stand may fall. Dry years followed by wet years enhance the effect on stand density. Since mainly younger trees were lost, the apparent average age of the stand increased more than real time (20% for Picea). Average mortality was 2.8 ± 0.5% yr−1 for spruce. Thus, the forest is turned over once every 160–180 yr by disturbances. The demography of dead trees shows that the rate of decay depends on the way the tree died. Storm causes uprooting and stem breakage, where living trees fall to the forest floor and decay with a mean residence time (t1/2) of about16 yr (decomposition rate constant kd = 0.042 yr−1). This contrasts with trees that die by drought or insect damage, and which remain as standing dead trees with a mean residence time of 3–13 yr until they are brought to ground, mainly by wind. These standing dead trees require an additional mean residence time of about 22 yr for decay on the ground (kd = 0.031). In conclusion, we demonstrate that, rather than competitive interactions, it is climate extremes, namely drought, rapid changes of dry years followed by wet years, and storm that determine stand structure, biomass and density, which then affect the net exchange with the atmosphere. The climatic effects are difficult to predict, because the sensitivity of a stand to climate extremes depends on the past history. This may range from no effect, if the stand was recovering from an earlier drought and exhibited a relatively low density, to a total collapse of canopies, if drought reduces stand density to an extent that other climatic extremes (especially wind) may cause further damage.

Climatic control of stand thinning in unmanaged spruce forests of the southern taiga in European Russia

The demography of Picea abies trees was studied over a period of about 30 yr on permanent plots in six forest types of an unmanaged forest located in a forest reserve of the Southern Taiga, NW of Moscow. This study encompassed a broad range of conditions that are typical for old growth spruce forests in the boreal region, including sites with a high water table and well drained sites, podzolic soils, acidic soils and organic soils. At all sites stand density, tree height, breast height diameter and age has been periodically recorded since 1968. Tree density ranged between 178 and 1035 trees ha−1 for spruce and between 232 and 1168 trees ha−1 for the whole stand, including mainly Betula and Populus. Biomass ranged between 5.4 and 170 tdw ha−1 for spruce and between 33 to 198 tdw ha−1 for the whole stand. Averaged over a long period of time, biomass did not change with stand density according to the self-thinning rule. In fact, on most sites biomass remained almost constant in the long term, while stand density decreased. The study demonstrates that the loss of living trees was not regulated by competitive interactions between trees, but by disturbances caused by climatic events. Dry years caused losses of minor and younger trees without affecting biomass. In contrast, periodic storms resulted in a loss of biomass without affecting density, except for extreme events, where the whole stand may fall. Dry years followed by wet years enhance the effect on stand density. Since mainly younger trees were lost, the apparent average age of the stand increased more than real time (20% for Picea). Average mortality was 2.8 ± 0.5% yr−1 for spruce. Thus, the forest is turned over once every 160–180 yr by disturbances. The demography of dead trees shows that the rate of decay depends on the way the tree died. Storm causes uprooting and stem breakage, where living trees fall to the forest floor and decay with a mean residence time (t1/2)of about16 yr (decomposition rate constant kd= 0.042 yr−1). This contrasts with trees that die by drought or insect damage, and which remain as standing dead trees with a mean residence time of 3–13 yr until they are brought to ground, mainly by wind. These standing dead trees require an additional mean residence time of about 22 yr for decay on the ground (kd= 0.031). In conclusion, we demonstrate that, rather than competitive interactions, it is climate extremes, namely drought, rapid changes of dry years followed by wet years, and storm that determine stand structure, biomass and density, which then affect the net exchange with the atmosphere. The climatic effects are difficult to predict, because the sensitivity of a stand to climate extremes depends on the past history. This may range from no effect, if the stand was recovering from an earlier drought and exhibited a relatively low density, to a total collapse of canopies, if drought reduces stand density to an extent that other climatic extremes (especially wind) may cause further damage.

Yield response of rice ( Oryza sativa L.) genotypes to drought under rainfed lowlands : 2. Selection of drought resistant genotypes

Drought frequently reduces grain yield of rainfed lowland rice. A series of experiments were conducted in drought-prone northeast Thailand to study the magnitude and consistency of yield responses of diverse, rainfed lowland rice genotypes to drought stress environments and to examine ways to identify genotypes that confer drought resistance. One hundred and twenty-eight genotypes were grown under non-stress and four different types of drought stress conditions. The relationship of genotypic variation in yield under drought conditions to genetic yield potential, flowering time and flowering delay, and to a drought response index (DRI) that removed the effect of potential yield and flowering time on yield under stress was examined. Drought stress that developed prior to flowering generally delayed the time of flowering of genotypes, and the delay in flowering was negatively associated with grain yield, fertile panicle percentage and filled grain percentage. Genotypes with a longer delay in flowering time had extracted more water during the early drought period, and as a consequence, had higher water deficits. They were consistently associated with a larger yield reduction under drought and in one experiment with a smaller DRI. Genotypes, however, responded differently to the different drought stress conditions and there was no consistency in the DRI estimates for the different genotypes across the drought stress experiments. The results indicate that with the use of irrigated-control and drought test environments, genotypes with drought resistance can be identified by using DRI or delay in flowering. However, selections will differ depending on the type of drought condition. The inconsistency of the estimates in DRI and flowering delay across different drought conditions reflects the nature of the large genotype-by-environment interactions observed for grain yield under various types of drought in rainfed lowland conditions.

On-farm assessment of constraints to wheat yield in the south-eastern Pampas

The aim of this study was to identify environmental constraints to wheat yield in the south-eastern wheat belt of the Pampas. We were concerned with the relevance of known physiological mechanisms in commercial crops. Our approach involved three steps. First, we used data from 103 large (>20 ha) grower managed fields in two locations (annual rain: 990 mm) to develop a parsimonious model based on knowledge of the system, i.e. a model that uses the minimum number of variables and parameters without losing predictive power. Second, the model was tested with independent data from 49 crops in two drier locations (annual rain: 790 mm). Third, yield of crops grown under a range of soil depths and management conditions (e.g. varying fertiliser rate) were compared with predicted yield. Actual, attainable and potential yields were also estimated and analysed in the light of environmental and management constraints. For a period including six growing seasons, potential yield ranged from 7.5 to 8.1 t ha −1, attainable yield from 4.6 to 6.6 t ha −1 and average yield from 3.5 to 5.2 t ha −1. Water deficit in the period from 30 days before to 10 days after flowering accounted for 65% of the variation in yield. Extending the critical period to account for water stress during earlier growth stages increased r 2 to 0.71. Actual yield was also negatively associated with air temperature during grain filling. An empirical model using rainfall and grain filling temperature as independent variables accounted for 55–69% of the variation in yield in an independent data set. This model together with long-term rainfall records indicated that closing the gap between attainable and potential yield would require improved management to attenuate water deficit, particularly late in the season. Efforts to alleviate the effect of early drought, which had a dramatic effect on yield not reversed by good water supply later in the season, would be less profitable given the low frequency (10%) of this type of yield restriction. Strategies to cope with late season drought are particularly important in farms where the detrimental effect of low rain is compounded by shallow soil.