Root Chilling Research Articles

Effect of Temperature and Duration of Root CHILLING on the Balance between Antioxidant Activity and Oxidative Stress in Spinach

Effect of Temperature and Duration of Root CHILLING on the Balance between Antioxidant Activity and Oxidative Stress in Spinach

Differential Transcriptional Regulation in Roots of Tomato Near-Isogenic Lines in Response to Rapid-Onset Water Stress.

Cultivated tomato (Solanum lycopersicum L.) is susceptible to abiotic stresses, including drought and chilling stress, while its wild relative (Solanum habrochaites) exhibits tolerance to many abiotic stresses. Chilling roots to 6°C induces rapid-onset water stress by impeding water movement from roots to shoots. Wild S. habrochaites responds to root chilling by closing stomata and maintaining shoot turgor, while cultivated tomato fails to close stomata and wilts. This phenotypic response (shoot turgor maintenance under root chilling) is controlled by a major QTL stm9 on chromosome 9 from S. habrochaites that was previously high-resolution mapped to a 0.32 cM region, but its effects on transcriptional regulation were unknown. Here we used paired near isogenic lines (NILs) differing only for the presence or absence of the S. habrochaites introgression containing stm9 in an otherwise S. lycopersicum background to investigate global transcriptional regulation in response to rapid-onset water stress induced by root chilling. NIL175 contains the S. habrochaites introgression and exhibits tolerance to root chilling stress, while NIL163 does not contain the introgression and is susceptible. RNA from roots of the two NILs was obtained at five time points during exposure to root chilling and mRNA-Seq performed. Differential expression analysis and hierarchical clustering of transcript levels were used to determine patterns of and changes in mRNA levels. Our results show that the transcriptional response of roots exposed to chilling stress is complex, with both overlapping and unique responses in tolerant and susceptible lines. In general, susceptible NIL 163 had a more complex transcriptional response to root chilling, while NIL175 exhibited a more targeted response to the imposed stress. Our evidence suggests that both the tolerant and susceptible NILs may be primed for response to root-chilling, with many of these response genes located on chromosome 9. Furthermore, serine/threonine kinase activity likely has an important role in the root chilling response of tolerant NIL175.

Quantitative Trait Loci for Water‐Stress Tolerance Traits Localize on Chromosome 9 of Wild Tomato

A wild tomato species (Solanum habrochaites S. Knapp & D.M. Spooner) is tolerant to many abiotic stresses, including limited water, and can serve as a valuable genetic resource for breeding cultivated tomato (S. lycopersicum L.). Previously, we developed and used a set of 18 sub‐near‐isogenic lines (sub‐NILs) containing introgressions from chromosome 9 of S. habrochaites to determine the genetic basis of tolerance to rapid‐onset water stress imposed by root chilling. In this study, we used this same set of 18 sub‐NILs to investigate the genetic basis of tolerance to slow‐onset water stress in the field imposed by restricted irrigation (with full water treatment as control) in replicated experiments for 2 yr in Davis, CA. The sub‐NILs were evaluated for C isotope discrimination (Δ13C), specific leaf area (SLA), shoot dry weight (SDW), yield, and other horticultural traits. A total of 19 QTL for Δ13C, SLA, SDW, yield, and maturity traits were mapped on chromosome 9. Significant genotype × environment interactions were detected for Δ13C, SLA, yield, and maturity. Agriculturally beneficial effects were conferred by S. habrochaites alleles at QTL for traits associated with water‐stress tolerance: low Δ13C, low SLA, and high SDW. The majority of QTL mapped to the centromeric end of the introgression, suggesting a gene‐rich region containing valuable wild alleles for breeding tomato for improved abiotic stress tolerance.

Complex Relationships among Water Use Efficiency‐Related Traits, Yield, and Maturity in Tomato Lines Subjected to Deficit Irrigation in the Field

Cultivated tomato (Solanum lycopersicum L.) is susceptible to abiotic stresses, including water stress. In contrast, a wild tomato relative, S. habrochaites S. Knapp & D.M. Spooner, is highly tolerant to abiotic stresses. Previously, we used a set of 18 sub‐near‐isogenic tomato lines (sub‐NILs) containing chromosome 9 introgressions from S. habrochaites to investigate the genetic basis of tolerance to rapid‐onset water stress imposed by root chilling. In the present study, this same set of 18 sub‐NILs was subjected to slow‐onset water stress imposed by deficit irrigation in field experiments conducted for 2 yr. Trait data were analyzed with principal component analysis, ANOVA, and MANOVA to investigate the underlying relationships among proxy traits for water‐use efficiency (WUE) including carbon isotope discrimination (Δ13C) and specific leaf area (SLA) with horticultural traits including maturity and yield. The majority of total phenotypic variation among the sub‐NILs was accounted for by Δ13C and SLA, with minor contribution from the horticultural traits. While many trait QTL were coincident or overlapping, our analyses suggest that the genes controlling QTL for Δ13C and SLA in this chromosome 9 region are distinct from each other and also from genes controlling maturity and yield. This introgressed region contains potentially valuable wild alleles for breeding cultivated tomatoes with improved WUE while selecting for yield maintenance.

High-resolution mapping of a major effect QTL from wild tomato Solanum habrochaites that influences water relations under root chilling.

Key messageQTLstm9controlling rapid-onset water stress tolerance inS. habrochaiteswas high-resolution mapped to a chromosome 9 region that contains genes associated with abiotic stress tolerances.Wild tomato (Solanum habrochaites) exhibits tolerance to abiotic stresses, including drought and chilling. Root chilling (6 °C) induces rapid-onset water stress by impeding water movement from roots to shoots. S. habrochaites responds to such changes by closing stomata and maintaining shoot turgor, while cultivated tomato (S. lycopersicum) fails to close stomata and wilts. This response (shoot turgor maintenance under root chilling) is controlled by a major QTL (designated stm9) on chromosome 9, which was previously fine-mapped to a 2.7-cM region. Recombinant sub-near-isogenic lines for chromosome 9 were marker-selected, phenotyped for shoot turgor maintenance under root chilling in two sets of replicated experiments (Fall and Spring), and the data were used to high-resolution map QTL stm9 to a 0.32-cM region. QTL mapping revealed a single QTL that was coincident for both the Spring and Fall datasets, suggesting that the gene or genes contributing to shoot turgor maintenance under root chilling reside within the marker interval H9–T1673. In the S. lycopersicum reference genome sequence, this chromosome 9 region is gene-rich and contains representatives of gene families that have been associated with abiotic stress tolerance.Electronic supplementary materialThe online version of this article (doi:10.1007/s00122-015-2540-y) contains supplementary material, which is available to authorized users.

An Introgression of Solanum habrochaites in the Rootstock Improves Stomatal Regulation and Leaf Area Development of Grafted Tomatoes under Drought and Low Root-Zone-Temperatures

Drought and low root zone temperature (RZT) hamper plant growth and leaf area development. Introgression lines (ILs) developed from a Solanum lycopersicum × Solanum habrochaites cross with the exotic parental line as donor were shown to significantly improve cold tolerance. The goal of the present study was to study agronomical and Physiological traits improving drought and, since adaptation to cold and drought is to some extent regulated by the same genetic mechanisms, low root zone tolerance in tomatoes. The parental lines of an IL library and ILs carrying quantitative trait loci (QTL) alleles for cold tolerance introgressed from S. habrochaites were used as plant material. The recurrent parent (RP) was grafted onto the IL (IL/RP) and self-grafted (RP/RP). Grafted plants were grown under two different RZTs (10°C and 20-26°C) and two soil moisture regimes (well-watered and drought stress). Agronomical and physiological parameters as green, non-green, and total leaf area, dry matter of plant parts, root to shoot ratio, osmotic adjustment, stomatal conductance, and stress tolerance index were measured and calculated. Plants grafted onto ILs produced higher total biomass and leaf area and probably regulated stomatal opening and closure more efficiently. The stm9 introgression from S. habrochaites LA1777 renders the scion more tolerant for low root-zone temperature and drought stress by a better stomatal regulation. Results confirm that an introgression of favourable genes from S. Habrochaites can improve cold and drought tolerance in grafted tomatoes and shoot turgor maintenance under root chilling is primarily a root-based trait.

An Introgression from Wild Tomato (Solanum habrochaites) Affects Tomato Photosynthesis and Water Relations

ABSTRACTA quantitative trait locus (QTL) stm9 (shoot‐turgor maintenance; located on chromosome 9) from chilling‐tolerant Solanum habrochaites S. Knapp & D.M. Spooner maintains plant water status via stomatal closure in response to root chilling, which induces a rapid onset form of root water stress. Here we examined the effect of an introgressed S. habrochaites chromosome 9 region, which contains QTL stm9, on water relations, photosynthesis, and yield in the field. Three near‐isogenic lines (NILs) of cultivated tomato (Solanum lycopersicum L.) with and without S. habrochaites chromosome 9 introgressions, and their parent cultivar T5, were evaluated in 2 yr of field experiments in Davis, CA. The NILs with a S. habrochaites introgression exhibited predawn and midday leaf conductances that were 23 and 10% smaller, respectively, than those of the NIL without the S. habrochaites introgression and the parent cultivar. This resulted in the NILs with the S. habrochaites region having 23% higher predawn leaf water potential, 13% higher midday leaf water potential, and 13% lower net photosynthesis than the NIL without the S. habrochaites introgression and the parent cultivar. Yield in the NILs containing small (NIL175) and large (NIL1322) introgressions of S. habrochaites decreased an average of 42 and 71%, respectively, compared with the NIL without the introgression and the parent cultivar. The effect of the introgression from S. habrochaites under nonchilling conditions may prove useful for understanding tradeoffs between productivity and transpiration.

Ultrastructure and functional activity of chloroplasts in wheat leaves under root chilling

The effects of root chilling (2 °C; during 1, 5 h, 1, 2, 4 and 7 days) on the ultrastructure, functional activity of chloroplasts and cold tolerance of leaf cells of wheat (Triticum aestivum L.) were studied. Results indicated that the area of the chloroplasts increased and the number of grana in the chloroplast decreased already within first hours of the experiment. On the 2nd–7th day of the cold treatment, the length of photosynthetic membranes in the chloroplasts increased owing to the membranes of thylakoids in grana. The number of chloroplasts per cell was increased by the end of the experiment. Reduction of electron transport rate and intensification of non-photochemical quenching of chlorophyll fluorescence were observed in the first hours of root chilling. The growth of the leaves slowed in the first day of the treatment and resumed on the second day. Leaf area in the root-chilled plants by the end of the experiment exceeded the initial values by 60 %. The significant rise in cold tolerance of leaf cells was detected after 24 h of root chilling. After 48 h of the treatment, the cold tolerance reached a maximum, and did not change thereafter. It is assumed that most of the observed structural and functional changes are adaptive, and meant to support the photosynthetic function and promote the cold tolerance of the plants.

Chilling‐induced water stress: Variation in shoot turgor maintenance among wild tomato species from diverse habitats

Cultivated tomato, Solanum lycopersicum, suffers chilling induced wilting because water movement through its roots decreases with declining soil temperatures. Certain wild tomato species exhibit resistance to chilling-induced wilting, but the extent of this chilling tolerance in wild tomatoes is not known. • We measured shoot wilting during root chilling in wild Solanum accessions from habitats differing in elevation, temperature, and precipitation. We also measured shoot wilting during root chilling in introgression lines (ILs) with chromosome 9 segments collinear to the shoot turgor maintenance QTL stm9 region from chilling-tolerant S. habrochaites, chilling and drought-tolerant S. lycopersicoides, or drought-tolerant S. pennellii. • Wild tomato species, which experience chilling temperatures (<10°C) in their native habitat, maintain shoot turgor under root chilling. Among accessions of S. lycopersicum var. cerasiforme, a typically chilling sensitive species, shoot turgor maintenance during root chilling was correlated with the precipitation of the native habitat. By contrast, S. pennellii, a species that is typically drought adapted, did not maintain turgor under root chilling. Grafted plants with roots containing S. habrochaites and S. lycopersicoides introgressions improved shoot turgor maintenance under root chilling. • Resistance to chilling-induced water stress is an important adaptation to chilling temperatures in wild tomatoes. There is some overlap in adaptation to drought and chilling stress in some tomato species. Root-based resistance to chilling-induced water stress in wild tomatoes may involve orthologous gene(s).

The influence of signals from chilled roots on the proteome of shoot tissues in rice seedlings

Low root temperature causes a decrease in water uptake, which leads to mineral and nutrient deficiencies with potentially decreased root and shoot growth. Differential temperature effects in plants have been studied extensively, however, the effect of root chilling on the global protein expression in shoots has not been explored. In this study, we imposed chilling temperatures on roots of rice plants while maintaining shoots at optimum atmospheric temperature. Shoot materials (growing zones and leaves) were harvested at five points over a time course of four days, including a two-day recovery period. Proteins were quantified by tandem mass tags and triple stage MS, using a method developed to overcome ratio compression in isobaric-labelled quantitation. Over 3000 proteins in each of the tissues were quantified by multiple peptides. Proteins significantly differentially expressed as compared with the control included abscisic acid-responsive and drought-associated proteins. The data also contained evidence of a possible induction of a sugar signalling pathway.

Acclimation to root chilling increases sugar concentrations in tomato (Solanum lycopersicum L.) fruits

Sugar concentrations in tomato (Solanum lycopersicum L.) fruits increase under water or salt stress. Water stress can be induced by root chilling, but the relationship with fruit sugars has not been reported. To elucidate this relationship, we compared fruit sugar concentrations and other characteristics between tomato plants cultivated at two root temperatures (20 and 12°C) using a nutrient-film technique during the spring-to-summer season (SS) and during two summer-to-winter seasons (SW) in a greenhouse in Morioka, Japan. The soluble solids, glucose, and fructose concentrations in tomato fruits increased similarly under root chilling at 12°C without a significant effect on acidity in all experiments. However, the increase was greater in the SW treatment, where water stress was smaller than in the SS treatment based on the vapour-pressure deficit. Root chilling also increased the starch, sucrose, glucose, and fructose concentrations in the leaves. The photosynthetic CO2 exchange rate and stomatal conductance per unit leaf area were not significantly affected by root chilling, despite a marked decrease in leaf area. Root chilling increased fruit dry weight but decreased the dry weight of vegetative organs, especially leaves and roots. The root chilling-induced increase in soluble sugar concentrations was likely caused by sink limitations in the vegetative organs. Water stress did not contribute directly to this result, although it might have contributed to the decreased dry weights of leaves and roots. Root chilling appears to be an effective way to improve tomato fruit quality and avoids the disadvantages of other methods of increasing fruit sugar content.

Trocas gasosas e balanço de carboidratos em plantas de cana-de-açúcar sob condições de estresses radiculares

Embora a resposta da fotossíntese de plantas de cana-de-açúcar a estresses ambientais seja conhecida, o acúmulo de fitomassa e a dinâmica de carboidratos de reserva diante da exposição simultânea ao frio e à seca são pouco conhecidos. Este trabalho objetiva investigar o efeito do déficit hídrico e da baixa temperatura radicular, isolados e simultaneamente, no genótipo de cana-de-açúcar IACSP94-2094, considerado tolerante à seca. Como hipótese, consideramos que este genótipo também é tolerante à baixa temperatura radicular, já que baixas temperaturas e déficit hídrico ocorrem simultaneamente no campo. A imposição da restrição hídrica de forma isolada ou simultaneamente à baixa temperatura radicular causou redução do potencial da água na folha e da assimilação de CO2, o que não foi observado nas plantas submetidas apenas à baixa temperatura do substrato. Os teores foliares de carboidratos não estruturais, de sacarose e de amido aumentaram nas plantas sob frio radicular. Nos tratamentos com déficit hídrico, apenas o teor de amido foliar diminuiu. Os estresses radiculares causaram aumento nos teores de açúcares solúveis totais e diminuição no teor de amido nas raízes. Como o acúmulo de fitomassa das plantas não foi afetado, mesmo com a restrição no crescimento radicular nos tratamentos com baixa temperatura do substrato, conclui-se que o genótipo de cana-de-açúcar IACSP94-2094 contém indícios de tolerância à baixa temperatura radicular. A manutenção do crescimento da planta deve estar associada à degradação das reservas de amido foliares e radiculares.

A cytosolic vegetative storage protein (TrVSP) from white clover is encoded by a cold‐inducible gene

The stolons of white clover (Trifolium repens L.) contain a predominant 17.3‐kDa protein, previously characterized as a vegetative storage protein (VSP), which accumulates under autumn and winter conditions. Its full‐length complementary DNA, TrVsp, was obtained and its 157 amino acid sequence deduced. This VSP has common characteristics to stress‐responsive proteins (high neutral amino acid content and potential alpha helices in its secondary structure) and shows high homologies to abscisic acid‐responsive and pathogenesis‐related‐10 proteins. The lack of any common amino acid sequence domains with known dehydrins or late embryogenesis abundant proteins suggests that clover VSP is not related to these proteins. Antibodies raised against the protein were produced and used in light and electron microscopic studies to show that it is localized to the cytosol of cortical parenchyma cells. This is in agreement with the VSP sequence, which does not contain any transit peptide signal. The accumulation of the transcript and the protein in roots is quickly induced by root chilling, suggesting a direct transcriptional regulation of TrVsp in response to low temperatures. Altogether, these results suggest that the 17.3‐kDa protein may have an additional or alternative function to its role in nitrogen storage and may confer putative tolerance to chilling in white clover.

A major QTL introgressed from wild Lycopersicon hirsutum confers chilling tolerance to cultivated tomato (Lycopersicon esculentum)

Many plants of tropical or subtropical origin, such as tomato, suffer damage under chilling temperatures (under 10 degrees C but above 0 degrees C). An earlier study identified several quantitative trait loci (QTLs) for shoot turgor maintenance (stm) under root chilling in an interspecific backcross population derived from crossing chilling-susceptible cultivated tomato (Lycopersicon esculentum) and chilling-tolerant wild L. hirsutum. The QTL with the greatest phenotypic effect on stm was located in a 28 cM region on chromosome 9 (designated stm 9), and enhanced chilling-tolerance was conferred by the presence of the Lycopersicon hirsutum allele at this QTL. Here, near-isogenic lines (NILs) were used to verify the effect of stm 9, and recombinant sub-NILs were used to fine map its position. Replicated experiments were performed with NILs and sub-NILs in a refrigerated hydroponic tank in the greenhouse. Sub-NIL data was analyzed using least square means separations, marker-genotype mean t-tests, and composite interval mapping. A dominant QTL controlling shoot turgor maintenance under root chilling was confirmed on chromosome 9 using both NILs and sub-NILs. Furthermore, sub-NILs permitted localization of stm 9 to a 2.7 cM interval within the original 28 cM QTL region. If the presence of the L. hirsutum allele at stm 9 also confers chilling-tolerance in L. esculentum plants grown under field conditions, it has the potential to expand the geographic areas in which cultivated tomato can be grown for commercial production.

Boron nutrition and chilling tolerance of warm climate crop species.

Field observations and glasshouse studies have suggested links between boron (B)-deficiency and leaf damage induced by low temperature in crop plants, but causal relationships between these two stresses at physiological, biochemical and molecular levels have yet to be explored. Limited evidence at the whole-plant level suggests that chilling temperature in the root zone restricts B uptake capacity and/or B distribution/utilization efficiency in the shoot, but the nature of this interaction depends on chilling tolerance of species concerned, the mode of low temperature treatment (abrupt versus gradual temperature decline) and growth conditions (e.g. photon flux density and relative humidity) that may exacerbate chilling stress. This review explores roles of B nutrition in chilling tolerance of continual root or transient shoot chills in crop species adapted to warm season conditions. It reviews current research on combined effects of chilling temperature (ranging from >0 to 20 degrees C) and B deficiency on growth and B nutrition responses in crop species differing in chilling tolerance. For subtropical/tropical species (e.g. cucumber, cassava, sunflower), root chilling at 10-17 degrees C decreases B uptake efficiency and B utilization in the shoot and increases the shoot : root ratio, but chilling-tolerant temperate species (e.g. oilseed rape, wheat) require much lower root chill temperatures (2-5 degrees C) to achieve the same responses. Boron deficiency exacerbates chilling injuries in leaf tissues, particularly under high photon flux density. Suggested mechanisms for B x chilling interactions in plants are: (a) chilling-induced reduction in plasmalemma hydraulic conductivity, membrane fluidity, water channel activity and root pressure, which contribute to the decrease in root hydraulic conductance, water uptake and associated B uptake; (b) chilling-induced stomatal dysfunction affecting B transport from root to shoot and B partitioning in the shoot; and (c) B deficiency induced sensitivity to photo-oxidative damage in leaf cells. However, specific evidence for each of the mechanisms is still lacking. Impacts of B status on chilling tolerance in crop species have important implications for the management of B supply during sensitive stages of growth, such as early growth after planting and early reproductive development, both of which can coincide with the occurrence of chilling temperatures in the field.

Chilling responses of maize (Zea mays L.) seedlings: root hydraulic conductance, abscisic acid, and stomatal conductance.

Maize seedling water relations and abscisic acid (ABA) levels were measured over 24 h of root chilling (5.5 degrees C). At 2.5 h into chilling, leaf ABA levels increased by 40x and stomatal conductance (g(s)) decreased to 20% compared with prechill levels. Despite a rapid g(s) response to root chilling, leaf water potential (Psi(L)) of chilled seedlings decreased to -2.2 MPa resulting in a complete loss of turgor potential (psi(p)). Ineffective g(s) control early in chilling resulted from decreased root hydraulic conductance (L(r)) due to increased water viscosity and factor(s) intrinsic to the roots. After 24 h chilling, Psi(L) and psi(p) of chilled seedlings recovered to control levels due to stomatal control of transpiration and increased L(r). The impact of the temporal changes in g(s) and L(r) on maize seedling water relations during chilling was analysed using a simple, quantitative hydraulic model. It was determined that g(s) is critical to stabilizing Psi(L) at non-lethal levels in chilled seedlings at 2.5 h and 24 h chilling. However, there was also a significant contribution due to increased L(r) at 24 h chilling so that psi(p) increased to control levels. As a first step in determining the factor(s) responsible for the increase in L(r), cDNA microarrays were used to quantify the transcript levels of eight aquaporins obtained from mature root tissue at 24 h chilling. None of these were significantly up-regulated, suggesting that the increase in L(r) was not due to regulation of these aquaporins at the transcriptional level.

Temperate Fuits Production in the Tropics: A Review on Apples in Kenya

Temperate zone fruit crops undergo bud dormancy which can be described as a mechanism for avoiding the exposure of tender flowers and leaves to low winter temperatures. In Kenya, apple growing is mostly hampered by inadequate chilling that causes the plants to have prolonged dormancy leading to poor flowering and consequently low yield. Although the chilling requirements are obligatory, under subtropical and especially tropical conditions avoidance is possible. To achieve this, it is necessary to select cultivars with low chilling requirements. This has proven effective in Zimbabwe with cultivar Matsu which is grown without a need for artificial breaking of dormancy. In Kenya like Zimbabwe, low chilling requiring cultivars such as Anna have been grown successfully. However, for cultivars with high chilling requirements, there is need to apply artificial techniques/methods to enhance bud break. Some of the cultural techniques used are: defoliation after harvesting and bending of the shoots holizontally. Defoliation after harvesting has particularly been used successfully in the island of Java in Indonesia and it enables two crops to be grown per year. Root chilling of rootstock has also been found to enhance bud break of the shoot. In addition, chemicals like KNO3, mineral oil and thiourea (TU) have been found to be effective in breaking bud dormancy in Kenya. This paper is reviewing the challenges encountered in growing apples in the tropics and Kenya in particular and the progress that has made in addressing them.

Water relations under root chilling in a sensitive and tolerant tomato species

ABSTRACTThe shoots of cultivated tomato (Lycopersicon esculentum cv. T5) wilt if their roots are exposed to chilling temperatures of around 5 °C. Under the same treatment, a chilling‐tolerant congener (Lycopersicon hirsutum LA 1778) maintains shoot turgor. To determine the physiological basis of this differential response, the effect of chilling on both excised roots and roots of intact plants in pressure chambers were investigated. In excised roots and intact plants, root hydraulic conductance declined with temperature to nearly twice the extent expected from the temperature dependence of the viscosity of water, but the response was similar in both species. The species differed markedly, however, in stomatal behaviour: in L. hirsutum, stomatal conductance declined as root temperatures were lowered, whereas the stomata of L. esculentum remained open until the roots reached 5 °C, and the plants became flaccid and suffered damage. Grafted plants with the shoots of one genotype and roots of another indicated that the differential stomatal behaviour during root chilling has distinct shoot and root components.

Increases in Water Potential Gradient Reduce Xylem Conductivity in Whole Plants. Evidence from a Low-Pressure Conductivity Method

A new method using hydrostatic suctions (less than 0.02 MPa) was used to measure whole-root conductivity (K(r)) in saplings of two angiosperm pioneer trees (Eucalyptus regnans and Toona australis) and two rainforest conifers (Dacrycarpus dacrydioides and Nageia fleurii). The resultant K(r) was combined with measurements of stem and leaf hydraulic conductivity to calculate whole-plant conductivity and to predict leaf water potential (Psi(l)) during transpiration. At normal soil temperatures there was good agreement between measured and predicted Psi(l) during transpiration in all species. Changes in the soil-to-leaf water potential gradient were produced by root chilling, and in three of the four species, changes in Psi(l) corresponded to those expected by the effect of increased water viscosity on K(r). In one species, however, root chilling produced severe plant wilting and a decline in Psi(l) significantly below the predicted value. In this species Psi(l) decreased to a value close to, or below, the Psi(l) at 50% xylem cavitation. It is concluded that decreased whole-plant conductivity in T. australis resulted from a decrease in xylem conductivity due to stress-induced cavitation.

Influence of Root Chilling on Winter and Spring Wheat Growth and Carbon Dioxide Assimilation

Growth and carbon dioxide (CO2) assimilation responses of winter wheat (Triticum aestivum