Association mapping of plant height, yield, and yield stability in recombinant chromosome substitution lines (RCSLs) using Hordeum vulgare subsp. spontaneum as a source of donor alleles in a Hordeum vulgare subsp. vulgare background

Grain yield and plant height of 80 recombinant chromosome substitution lines (RCSLs) of barley were measured in six environments with contrasting available moisture profiles. Two environments were in OR, USA (Moro and Pendleton) during one growing season (2004), and four in Chile (Cauquenes and Santa Rosa) during two growing seasons (2004/2005 and 2007/2008). From the yield data obtained in the different environments, yield adaptability (Finlay–Wilkinson slope) and stability (deviations from regression) were calculated. Two commercial cultivars (Harrington and Baronesse) were used as checks in all environments. Marker-quantitative trait associations were identified using 47 simple sequence repeats (SSRs) and the general linear model (GLM) implemented in TASSEL. The mean plant height and grain yield of the 80 RCSLs differed greatly across environments, reflecting differences in water availability. In all environments, there were significant differences (P < 0.05) in grain yield among RCSLs. There was also abundant variation in yield adaptability, indicating a differential response of the RCSLs to environmental conditions across environments. Using principal component analysis, it was possible to identify genotypes with better agronomic performance than the recurrent parent cv. Harrington. The association analysis revealed 21 chromosomal regions that were highly correlated with differences in grain yield, plant height and/or yield adaptability (Finlay–Wilkinson slope). In approximately one-fourth of the cases, the H. spontaneum donor contributed favorable alleles. The associations were referenced to the quantitative trait loci (QTL) for the same traits reported in the literature.     

Grain yield responses to moisture regimes in a rice population: association among traits and genetic markers

Drought is a major constraint to rice (Oryza sativa L.) production in rainfed and poorly irrigated environments. Identifying genomic regions influencing the response of yield and its components to water deficits will aid our understanding of the genetic mechanism of drought tolerance (DT) of rice and the development of DT varieties. Grain yield (GY) and its components of a recombinant inbred population developed from a lowland rice and an upland rice were investigated under different water levels in 2003 and 2004 in a rainout DT screening facility. Correlation and path analysis indicated that spikelet fertility (SF) was particularly important for grain yield with direct effect (P=0.60) under drought stress, while spikelet number per panicle (SN) contributed the most to grain yield (P=0.41) under well-watered condition. A total of 32 quantitative trait loci (QTLs) for grain yield and its components were identified. The phenotypic variation explained by individual QTLs varied from 1.29% to 14.76%. Several main effect QTLs affecting SF, 1,000-grain weight (TGW), panicle number (PN), and SN were mapped to the same regions on chromosome 4 and 8. These QTLs were detected consistently across 2 years and under both water levels in this study. Several digenic interactions among yield components were also detected. The identification of genomic regions associated with GY and its components under stress will be useful to improve drought tolerance of rice by marker-aided approaches.G. H. Zou and H. W. Mei contribute equally to this work.     

Grain Yield and Charcoal Rot Resistance Stability in Common Beans under Terminal Drought Conditions

Charcoal rot (Macrophomina phaseolina) is a major disease of beans (Phaseolus vulgaris L.) in Mexico. The use of germplasm combining high‐yield stability with resistance to drought and charcoal rot could reduce damage from this disease. In this study, we compared the Eberhart and Russell method and the Additive Main Effect and Multiplicative Interaction (AMMI) model plus biplot analysis for measuring grain yield (GY) and charcoal rot resistance (CHRR) stabilities in 98 F_{8 : 10} recombinant inbred lines (RILs) derived from a cross between bean adapted to the tropics (BAT) 477 (resistant) × Pinto UI‐114 (susceptible). Experiments were conducted from 2007 to 2009 in Isla, Cotaxtla, Río Bravo and Díaz Ordaz, México, under irrigated or terminal drought conditions. anova detected significant differences (P ≤ 0.05) in GY and CHRR among environments, genotypes and genotype × environment interactions (GEI). Most RILs showed good responses to unfavourable environments based on GY (48) and CHRR (40). AMMI anova s for both traits showed that all sources of variation in the model accounted for approximately 49% of the total squared sum. For the first principal component (PC1), we found 13 RILs that were stable for GY, and for the second (PC2), we found 9 that were stable for GI. For CHRR, we detected 14 stable RILs (PC1) and eight (PC2). Biplot analysis showed the largest vectors for Díaz Ordaz (irrigated and drought, 2008), where the highest and most variable GYs were detected. The shortest vectors were found in Isla (drought, 2007) and Río Bravo (irrigated and drought, 2008), where the lowest and least variable GY were found. We found differential responses of RILs to locations, years and soil humidity conditions as well as significant GEI based on GY and CHRR. The two methods were complementary, and both gave us information to select stable, high‐yield germplasm associated with resistance to charcoal rot disease.     

Hybrid rye performance under natural drought stress in Europe

Several rye growing regions of Central Europe suffered from severe drought stress in the last decade. Rye is typically grown on sandy soils with low water-holding capacity in areas with low rainfall, thus drought-tolerant varieties are urgently needed. The main objective of our study was to evaluate the drought stress tolerance of rye hybrids using large-scaled field experiments. Two biparental populations (Pop-A, Pop-B) each consisting of 220 F_2:4 lines from the Petkus gene pool and their parents were evaluated for grain yield testcross performance under irrigated (I) and rainfed (R) regime in six environments. We observed for most environments severe drought stress leading to an average grain yield reduction of 23.8 % for rainfed compared to irrigated regime in drought stress environments. A decomposition of the variance revealed significant (P < 0.01) genotypic and genotype × environment interaction variances but only a minor effect of drought stress on the ranking of the genotypes with regard to grain yield. In conclusion, separate breeding programs for drought-tolerant genotypes are not superior to the currently practiced selection under rainfed conditions without irrigation in hybrid rye breeding in Central Europe.     

QTLs for Tolerance of Drought and Breeding for Tolerance of Abiotic and Biotic Stress: An Integrated Approach

Background

The coupling of biotic and abiotic stresses leads to high yield losses in rainfed rice (Oryza sativa L.) growing areas. While several studies target these stresses independently, breeding strategies to combat multiple stresses seldom exist. This study reports an integrated strategy that combines QTL mapping and phenotypic selection to develop rice lines with high grain yield (GY) under drought stress and non-stress conditions, and tolerance of rice blast.

Methodology

A blast-tolerant BC₂F₃-derived population was developed from the cross of tropical japonica cultivar Moroberekan (blast- and drought-tolerant) and high-yielding indica variety Swarna (blast- and drought-susceptible) through phenotypic selection for blast tolerance at the BC₂F₂ generation. The population was studied for segregation distortion patterns and QTLs for GY under drought were identified along with study of epistatic interactions for the trait.

Results

Segregation distortion, in favour of Moroberekan, was observed at 50 of the 59 loci. Majority of these marker loci co-localized with known QTLs for blast tolerance or NBS-LRR disease resistance genes. Despite the presence of segregation distortion, high variation for DTF, PH and GY was observed and several QTLs were identified under drought stress and non-stress conditions for the three traits. Epistatic interactions were also detected for GY which explained a large proportion of phenotypic variance observed in the population.

Conclusions

This strategy allowed us to identify QTLs for GY along with rapid development of high-yielding purelines tolerant to blast and drought with considerably reduced efforts. Apart from this, it also allowed us to study the effects of the selection cycle for blast tolerance. The developed lines were screened at IRRI and in the target environment, and drought and blast tolerant lines with high yield were identified. With tolerance to two major stresses and high yield potential, these lines may provide yield stability in rainfed rice areas.     

Leaf allometry and prediction of specific leaf area (SLA) in a sugar beet (<Emphasis Type="Italic">Beta vulgaris</Emphasis> L.) cultivar

Sugar beet cv. Rizor was grown for five growing seasons (2002–2006) in field conditions in Thessaly, central Greece. A total of 55 samplings took place during the growing seasons and allometric growth of the leaves was monitored. Highly significant (p<0.001) quadratic relationships were found between individual leaf mass (LM), individual leaf area (LA), aboveground dry biomass (ADB), and leaf area index (LAI). Only the LM-LA relationship (LA = 43.444 LM² − 10.693 LM + 118.34) showed a relatively high r ² (0.63) and thus could be used for prediction of LA. Specific leaf area (SLA) was significantly related with leaf water content (LWC) (SLA = 26 279 LWC² − 44 498 LWC + 18 951, r ² = 0.91, p<0.001) and thus LWC could be a good indirect predictor of SLA in this cultivar.     

Effects of nitrogen rates on grain yield and nitrogen agronomic efficiency of durum wheat genotypes under different environments

Durum wheat is an important staple food crop in Tunisia and other Mediterranean countries and is grown in various climatic conditions. Production and yield are however severely limited not only by drought events but also by reduced levels of nitrogen fertilisation. A study was carried out at two locations in the sub‐humid area of Tunisia: Mateur in 2009–10 and 2010–11 and Beja in 2011–12 and 2012–13 under rainfed conditions. Four durum wheat genotypes (landraces: Bidi, Azizi; improved: Om Rabia, Khiar) were evaluated for nitrogen agronomic efficiency and related agronomic traits under various nitrogen rates: 0, 50, 100, 150, 200 and 250 kg N ha^?1, with three replications. There was a significant interaction effect (P ≤ 0.001) environments × genotypes × N treatments for grain yield (GY), biomass yield (BY), harvest index (HI), partial factor productivity of applied nitrogen (PFP_N) and nitrogen agronomic use efficiencies (NAE). GY was the most affected trait by nitrogen applied showing an increase of 94% under high N treatment (250 kg N ha^?1) compared to control plots without N treatments. A significant linear regression exists between GY (0 N) and GY for the different N rates (r = 0.70; P < 0.001). This effect was more pronounced for improved genotypes than landraces for all parameters excepting BY and NAE_BY. BY showed +11% increase in landraces than improved genotypes. PFP_N showed an average decrease of 65% under high‐N fertilisation with 10% prevalence for improved genotypes. Landraces tend to promote vegetative growth while grain filling efficiency was higher for improved genotypes.     

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

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

The Effect of Drought and Vapour Pressure Deficit on Gas Exchange of Young Kiwifruit (Actinidia deliciosavar.deliciosa) Vines

To evaluate the effect of drought and vapour pressure deficit (VPD) on stomatal behaviour and gas exchange parameters, young kiwifruit vines (Actinidia deliciosavar.deliciosacv. Hayward) were exposed to alternating periods of drought and drought-relief over two growing seasons. Vines were grown either in the field or in containers. Stomatal conductance of fully-expanded leaves rapidly decreased as pre-dawn leaf water potential was reduced below a threshold value of -0.3MPa. Stomatal conductance reached minimum values of 10–20mmol m^-2s^-1. Transpiration rate was similarly sensitive to changes in leaf water status, whereas more severe drought levels were necessary to affect photosynthesis significantly. Net daily carbon gains were estimated at 4.7 and 2.7gm^-2for irrigated and droughted vines, respectively. Gas exchange parameters recovered to values of irrigated vines within a few hours after relief of stress. Rate of recovery depended on the level of stress reached during the previous drought period. There was a steady decline in stomatal conductance when VPD was increased from 0.8 to 2.5kPa in both irrigated and droughted vines. The VPD at which stomatal conductance reached 50% of maximum values was 2.1–2.2kPa for both treatments. We conclude that stomata were highly sensitive to changes in soil water status and that midday depression of photosynthesis measured in kiwifruit vines was related to water deficits arising in the leaf because of both transpirational losses and to the direct effect of increasing VPD.     

Biomass production and water use efficiency in perennial grasses during and after drought stress

Drought is a great challenge to agricultural production, and cultivation of drought‐tolerant or water use‐efficient cultivars is important to ensure high biomass yields for bio‐refining and bioenergy. Here, we evaluated drought tolerance of four C₃ species, Dactylis glomerata cvs. Sevenop and Amba, Festuca arundinacea cvs. Jordane and Kora, Phalaris arundinacea cvs. Bamse and Chieftain and Festulolium pabulare cv. Hykor, and two C₄ species Miscanthus × giganteus and M. lutarioriparius. Control (irrigated) and drought‐treated plants were grown on coarse and loamy sand in 1 m² lysimeter plots where rain was excluded. Drought periods started after harvest and lasted until 80% of available soil water had been used. Drought caused a decrease in dry matter yield (DM; P < 0.001) for all species and cultivars during the drought period. Cultivars Sevenop, Kora and Jordane produced DM at equal levels and higher than the other C₃ cultivars in control and drought‐treated plots both during and after the drought period. Negative correlations were observed between stomatal conductance (g_s) and leaf water potential (P < 0.01) and positive correlations between g_s and DM (P < 0.05) indicating that g_s might be suitable for assessment of drought stress. There were indications of positive associations between plants carbon isotope composition and water use efficiency (WUE) as well as DM under well‐watered conditions. Compared to control, drought‐treated plots showed increased growth in the period after drought stress. Thus, the drought events did not affect total biomass production (DM_total) of the whole growing season. During drought stress and the whole growing season, WUE was higher in drought‐treated compared to control plots, so it seems possible to save water without loss of biomass. Across soil types, M. lutarioriparius had the highest DM_total (15.0 t ha^?1), WUE_total (3.6 g L^?1) and radiation use efficiency (2.3 g MJ^?1) of the evaluated grasses.     

Root System Dynamics of Miscanthus × giganteus and Panicum virgatum in Response to Rainfed and Irrigated Conditions in California

Miscanthus (Miscanthus × giganteus) and switchgrass (Panicum virgatum) are large perennial grass bioenergy crops in the USA and Europe. Despite much research into their agronomic potential, few studies have examined in situ root growth dynamics under irrigation and soil water deficits, particularly as they relate to shoot performance. We grew miscanthus and switchgrass in outdoor mesocosms under irrigated and rainfed conditions and assessed the spatial distribution and abundance of roots using minirhizotron images and whole root system sampling. Despite surviving an extended period of drought, shoot and root biomass, root length density, numbers of culms, and culm height were reduced in both species under rainfed (dry) conditions. However, rainfed switchgrass far outperformed rainfed miscanthus in all shoot and root growth metrics. The rainfed (drought) treatment reduced switchgrass and miscanthus whole plant biomass by 83 and 98 %, culm production by 67 and 90 %, and root length density by 67 and 94 % compared to irrigated plants, respectively. Root nitrogen concentration was higher for miscanthus (3-fold) and switchgrass (4-fold) in the rainfed treatment compared to irrigated plants and did not significantly differ between species. Unlike miscanthus, switchgrass grew roots continuously into regions of available soil moisture as surface soil layers grew increasingly dry, indicating a drought avoidance strategy. Our study suggests that switchgrass is more likely to tolerate drought by mining deep wet soils, while miscanthus relies on shallow rhizome production to tolerate dry soils.     

Evaluation of some physiological traits as indicators of drought tolerance in bread wheat genotypes

The objective of this work was to evaluate the ability of some physiological traits to identify drought-tolerant bread wheat genotypes. To this end, twenty bread wheat genotypes were assessed under post anthesis drought stress (rain-fed) and irrigated conditions. The Stress Tolerance Index (STI) was used as a measure of drought tolerance. Relationships between STI and Cell Membrane Stability (CMS), Proline Concentration (PC), Relative Water Content (RWC), Chlorophyll a/b Ratio (Chl a/b), Relative Chlorophyll Content (RCC), Excised Leaf Water Retention (ELWR), and Relative Water Loss (RWL) were determined in order to find out whether these physiological traits could be used as the indicators of drought tolerance. The results showed that ELWR, RWL, and CMS could be considered as reliable indicators in screening wheat genotypes for drought tolerance.     

The relationship between seedling growth and grain yield under drought conditions in maize and triticale genotypes

The effects of drought stress on seedlings?? growth and grain yield of 13 single cross maize hybrids and 11 breeding lines and cultivars of spring triticale were studied in greenhouse and field experiments. In the field experiment, the drought susceptibility index (DSI_GY) was calculated by determining the change in grain yield (GY) in conditions with two soil moisture levels (IR, irrigated; D, drought). In the greenhouse experiment the response to soil drought was evaluated using DSI_DW, by determining changes in the dry weight (DW) of vegetative plant parts. Marked variations in GY and DW were observed among the studied genotypes. In control conditions, the GY and DW in drought-sensitive genotypes were higher compared to the drought-resistant ones; but in drought conditions, the decreases in GY and DW in resistant genotypes were smaller than in drought-sensitive ones. DSI_GY and DSI_DW revealed variations in the degree of drought tolerance among the examined maize and triticale genotypes. The values of DSI_GY in the field experiment and DSI_DW in the greenhouse experiment enabled a division of the studied genotypes into drought-resistant or -sensitive groups. A close correlation between DSI_GY and DSI_DW was found. The positive linear correlation and determination coefficients between DSI_GY and DSI_DW were statistically significant (P?=?0.05), being equal to R ²?=?0.614 (maize) and R ²?=?0.535 (triticale). The ranking of the studied genotypes based on DSI_GY was in most cases consistent with the ranking based on DSI_DW, which indicates that genetically conditioned drought tolerance is similar for plants in the seedling and reproductive growth stages or may at least partly have a common genetic background.     

QTLs linked to leaf epicuticular wax,physio-morphological and plant production traits under drought stress in rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

Drought stress is the major constraint to rice (Oryza sativa L.) production and yield stability in rainfed ecosystems. Identifying genomic regions contributing to drought resistance will help to develop rice cultivars suitable for rainfed regions through marker-assisted breeding. Quantitative trait loci (QTLs) linked to leaf epicuticular wax, physio-morphological and plant production traits under water stress and irrigated conditions were mapped in a doubled haploid (DH) line population from the cross CT9993-5-10-1-M/IR62266-42-6-2. The DH lines were subjected to water stress during anthesis. The DH lines showed significant variation for epicuticular wax (EW), physio-morphological and plant production traits under stress and irrigated conditions. A total of 19 QTLs were identified for the various traits under drought stress and irrigated conditions in the field, which individually explained 9.6%–65.6% of the phenotypic variation. A region EM15_10-ME8_4-R1394A-G2132 on chromosome 8 was identified for leaf EW and rate of water loss i.e., time taken to reach 70% RWC from excised leaves in rice lines subjected to drought stress. A large effect QTL (65.6%) was detected on chromosome 2 for harvest index under stress. QTLs identified for EW, rate of water loss from excised leaves and harvest index under stress in this study co-located with QTLs linked to shoot and root-related drought resistance traits in these rice lines and might be useful for rainfed rice improvement.     

小麦抗旱种质资源的遗传多样性

在雨养和灌水条件下,田间栽培小麦抗旱材料。根据结实器官建成与物候期、生育期的对应关系,通过供试材料产量构成因素的旱、水表现,分析在各因素形成时期的抗旱性。分别以抗旱系数和抗旱指数作为评价抗旱性的指标,通过聚类分析供试材料的遗传多样性。结果发现有些材料表现为全生育期抗旱,而有些材料只在苗期、拔节期、开花期和灌浆期等一个或几个生育时期表现抗旱;有的材料表现为抗旱高产,但有的材料产量水平较低;同时还发现部分抗旱种质资源在灌水条件下有较大的增产潜力。     

Tree-ring wood anatomy and stable isotopes show structural and functional adjustments in olive trees under different water availability

Background and Aims

Olive tree (Olea europaea L.) is a drought-tolerant tree species cultivated in Mediterranean-type environments. Although it is tolerant to drought, dry conditions decrease its productivity. A thorough analysis of the hydraulic architecture and wood anatomical plasticity, as well as of their physiological significance, is needed to understand how olive trees will adapt to the predicted increase in frequency and severity of drought in the Mediterranean region.

Methods

Dendrochronological, stable isotopic (δ¹³C, δ¹⁸O) and wood anatomical analyses were applied to understand how different water availability can affect wood stem structure and function, in rainfed and irrigated at 100 % of crop evapotranspiration (ETc) olive trees in an experimental orchard close to Benevento (Italy) from 1992 to 2009.

Results

Dendrochronological data indicate that cross-dating and synchronization of ring-width time series in olive tree is possible. After the start of irrigation, significantly more negative δ¹³C and lower δ¹⁸O values were recorded in irrigated trees indicating higher stomatal conductance and transpiration rates. Increased water balance induced the formation of a higher number of vessels with higher diameter.

Conclusions

Water balance variations affected wood anatomy and isotopic composition. Anatomical analyses detected structural and functional adjustments in rainfed trees that produced more vessels with lower diameter to prevent cavitation. Isotopic analyses confirmed that irrigated trees continuously showed enhanced transpiration rates.     

Yield, carbon isotope discrimination, canopy reflectance and cuticular conductance of barley isolines of differing glaucousness

The roles of wax bloom (Glaucousness) in discrimination against stable isotope ¹³C () and yield were studied for 2 years on a pair of near-isogenic lines of two-rowed barley (Hordeum vulgare L.) differing in their degree of glaucousness of the spike and the upper vegetative parts of the plant other than leaf blades. The effects of glaucousness on canopy reflectance and cuticular conductance were also assessed. Plants were grown under fully irrigated and rainfed Mediterranean conditions for 2 years. Grain yield for both years and total straw biomass measured in the second year were higher in glaucous than non-glaucous lines under both irrigated and rainfed conditions. followed the same pattern in both grain and straw with the highest values in the glaucous line under irrigation and the lowest in the non-glaucous line under rainfed conditions. Differences in yield and between the lines were greater under irrigated than rainfed conditions. The higher of the glaucous line suggests that these plants had lower transpiration efficiency (net photosynthesis to transpiration ratio) than the non-glaucous line in the long term. Canopy reflectance at some of the wavelengths of the PAR region (400-700 nm) was significantly higher in the glaucous than in the non-glaucous line under both irrigated and rainfed conditions. Rainfed conditions also increased canopy reflectance in the PAR region. No significant effect of either glaucousness or water regime was observed in cuticular conductance or amount of cuticular waxes of the flag leaf blades or the ear.     

Carbon isotope discrimination function analysis and drought tolerance of stylo species grown under rain-fed environment

Limited germplasm and narrow genetic base in the past has hinders wider applicability of Stylosanthes in India. However, build up in Stylosanthes germplasm in recent years provided an opportunity to evaluate and use them under different agro-ecological situations preferably targeting drought tolerance. Rate reducing resistance (RRR) allo-tetraploid lines of S. scabra and diploid S. seabrana, a newly introduced species, observed well suited in hard and cracking soils under complete rainfed condition. Genotypes of S. scabra were more tolerant to drought over lines of other species as evidenced by high leaf thickness, more proline accumulation, contents of malondialdehyde (MDA), sugars, starch and chlorophyll and low carbon isotope discrimination (CID) values. Both in control and stress conditions, a positive relationship (r = 0.465 and 0.328) was observed between specific leaf area (SLA) and CID. Earlier reports have emphasized measurement of CID as an indirect way of measuring the transpiration efficiency (TE) in Stylosanthes (Thumma et al. Aust J Agr Res 49:1039–1045, 1998). The negative relationship observed between TE and CID suggested low CID bearing lines would have high TE, useful traits to select lines growing better in dry environments. Thus, selected lines having low CID provided better scope for the introduction of this important range legume for semi- arid regions of India.     

Seasonal variations on water relations ofAmygdalus communis L. under drip irrigated and non irrigated conditions

Almond plants (Amygdalus communis L.) of the Garrigues variety were grown in the field drip irrigated and rainfed. Leaf water potential (Ψ) and leaf conductance (g₁) were determined throughout one growing season. Pre-dawn measurement for Ψ in the irrigated treatment was consistent through the growing season, whereas in the rainfed treatment it decreased gradually. Ψ values at midday (Ψ minimum) was closely dependent on atmospheric evaporative demand, and their recovery was quicker in the wet treatment than in the dry. The g₁ values were higher in the wet than dry treatments, decreasing in both cases by leaf ageing. Maximum values for g₁ were reached when evaporative demand was highest in the day. The relationship between Ψ and g₁ revealed a decrease in the hysteresis throughout the growing season, being most marked in the dry treatment. The results highlight the close dependence of Ψ and g₁ on evaporative demand, leaf ageing and irrigtion treatment during the growing season.     

The influence of drought on the relationship between leaf and conducting sapwood area in Eucalyptus globulus and Eucalyptus nitens

 Development of the relationship between leaf area (A _l ) and sapwood area (A _s ) was investigated in two important hardwoods, Eucalyptus globulus (Labill) and E. nitens (Deane and Maiden) Maiden, growing in an experimental plantation established in a low rainfall zone (approx. 515 mm year^–1) of Tasmania. The experiment compared irrigated controls and a rainfed treatment which was subjected to cyclical summer droughts from age 1 to 6 years old. Leaf area and sapwood area were determined by destructive sampling at ages 2, 3 and 6 years old. There was no effect of stand age on A _l :A _s when sapwood area was measured at crown break. At age 3 years old A _l :A _s was significantly greater in the rainfed than the irrigated trees. It was concluded that this difference was due to earlier canopy closure in the irrigated trees. When the plantation was 6 years old A _l :A _s was significantly greater in the irrigated than the rainfed treatment. An analysis based on an equation which links A _l :A _s with transpiration and volumetric flow rate (Whitehead et al. 1984) was used to infer a positive correlation between stem hydraulic conductivity (k _h ) and water availability. Independent of water availability E. globulus maintained a higher A _l :A _s than E. nitens at all ages. Received: 20 March 1997 / Accepted: 30 December 1997