Lettuce, a shallow-rooted crop, and Lactuca serriola, its wild progenitor, differ at QTL determining root architecture and deep soil water exploitation

Wild plant species are often adapted to more stressful environments than their cultivated relatives. Roots are critical in exploiting soil resources that enable plants to withstand environmental stresses, but they are difficult to study. Cultivated lettuce (Lactuca sativa L.) and wild L. serriola L. differ greatly in both shoot and root characteristics. Approximately 100 F_2:3 families derived from an interspecific cross were evaluated in greenhouse and field experiments. In the greenhouse, root traits (taproot length, number of laterals emerging from the taproot, and biomass) and shoot biomass were measured 4 weeks after planting. In the field, plants were grown for 9 weeks (close to harvest maturity of the cultivated parent); mild drought stress was induced by withholding water for 1 week, and gravimetric moisture of soil was then determined for five depth increments between 0–100 cm. The families were genotyped using codominantly scored AFLP markers distributed throughout the genome. Composite interval mapping was used to analyze marker-trait associations. Quantitative trait loci were identified for differences between wild and cultivated lettuce for root architectural traits and water acquisition. Thirteen QTL were detected that each accounted for 28–83% of the phenotypic variation. The loci for taproot length (i.e., cm taproot length g^–1 plant biomass) and the ability to extract water from deep in the soil profile co-localized in the genome. These coincident loci were identified in separate experiments. The wild L. serriola is therefore a potential source of agriculturally important alleles to optimize resource acquisition by cultivated lettuce, thereby minimizing water and fertilizer inputs and ultimately enhancing water quality. Received: 25 February 2000 / Accepted: 31 March 2000     

Early seedling growth of pine (Pinus densiflora) and oaks (Quercus serrata,Q. mongolica,Q. variabilis) in response to light intensity and soil moisture

In climatic chambers seed germination and seedling growth of Pinus densiflora Sieb. et Zucc., Quercus serrata Thunb., Quercus mongolica Fisch. ex Turcz. and Quercus variablilis Bl. were investigated as functions of light intensity and soil moisture. In Korea these tree species occur widely and form mixed forests with different distributions. Species clearly differed in the pattern of germination and early seedling growth between light and soil treatments. The germination of pine did not differ between the experimental treatments until the breaking of the primary buds. After that, light intensity was the deciding factor for further development. In the most moist treatment (approx. field capacity) growth of the pine seedlings was strongly inhibited. For the three oak species, differences between experimental treatments first occurred after complete formation of primary leaves. Seed development strongly correlated with the weight of the acorn. The large seeded Q. variabilis (acorns with mean weight of 4.7 g) developed faster and reached larger dimensions towards the end of the experiment than Q. mongolica (2.8 g per acorn) and Q. serrata (0.9 g per acorn). Regarding height and biomass growth, the oak species showed a higher shade tolerance than pine. The proleptic shoot growth was clearly influenced by the light intensity. Root formation was favoured by a high exposure to light. In case of the oak species reduction of soil moisture increased the length of primary roots and the number of secondary roots.     

Response of millet and sorghum to a varying water supply around the primary and nodal roots

Background and Aims

Cereals have two root systems. The primary system originates from the embryo when the seed germinates and can support the plant until it produces grain. The nodal system can emerge from stem nodes throughout the plant''s life; its value for yield is unclear and depends on the environment. The aim of this study was to test the role of nodal roots of sorghum and millet in plant growth in response to variation in soil moisture. Sorghum and millet were chosen as both are adapted to dry conditions.

Methods

Sorghum and millet were grown in a split-pot system that allowed the primary and nodal roots to be watered separately.

Key Results

When primary and nodal roots were watered (12 % soil water content; SWC), millet nodal roots were seven times longer than those of sorghum and six times longer than millet plants in dry treatments, mainly from an 8-fold increase in branch root length. When soil was allowed to dry in both compartments, millet nodal roots responded and grew 20 % longer branch roots than in the well-watered control. Sorghum nodal roots were unchanged. When only primary roots received water, nodal roots of both species emerged and elongated into extremely dry soil (0·6–1·5 % SWC), possibly with phloem-delivered water from the primary roots in the moist inner pot. Nodal roots were thick, short, branchless and vertical, indicating a tropism that was more pronounced in millet. Total nodal root length increased in both species when the dry soil was covered with plastic, suggesting that stubble retention or leaf mulching could facilitate nodal roots reaching deeper moist layers in dry climates. Greater nodal root length in millet than in sorghum was associated with increased shoot biomass, water uptake and water use efficiency (shoot mass per water). Millet had a more plastic response than sorghum to moisture around the nodal roots due to (1) faster growth and progression through ontogeny for earlier nodal root branch length and (2) partitioning to nodal root length from primary roots, independent of shoot size.

Conclusions

Nodal and primary roots have distinct responses to soil moisture that depend on species. They can be selected independently in a breeding programme to shape root architecture. A rapid rate of plant development and enhanced responsiveness to local moisture may be traits that favour nodal roots and water use efficiency at no cost to shoot growth.     

The effects of clipping and soil moisture on leaf and root morphology and root respiration in two temperate and two tropical grasses

Numerous studies have explored the effect of environmental conditions on a number of plant physiological and structural traits, such as photosynthetic rate, shoot versus root biomass allocation, and leaf and root morphology. In contrast, there have been a few investigations of how those conditions may influence root respiration, even though this flux can represent a major component of carbon (C) pathway in plants. In this study, we examined the response of mass-specific root respiration (μmol CO₂ g⁻¹ s⁻¹), shoot and root biomass, and leaf photosynthesis to clipping and variable soil moisture in two C₃ (Festuca idahoensis Elmer., Poa pratensis L.) and two C₄ (Andropogon greenwayi Napper, and Sporobolus kentrophyllus K. Schum.) grass species. The C₃ and C₄ grasses were collected in Yellowstone National Park, USA and the Serengeti ecosystem, Africa, respectively, where they evolved under temporally variable soil moisture conditions and were exposed to frequent, often intense grazing. We also measured the influence of clipping and soil moisture on specific leaf area (SLA), a trait associated with moisture conservation, and specific root length (SRL), a trait associated with efficiency per unit mass of soil resource uptake. Clipping did not influence any plant trait, with the exception that it reduced the root to shoot ratio (R:S) and increased SRL in P. pratensis. In contrast to the null effect of clipping on specific root respiration, reduced soil moisture lowered specific root respiration in all four species. In addition, species differed in how leaf and root structural traits responded to lower available soil moisture. P. pratensis and A. greenwayi increased SLA, by 23% and 33%, respectively, and did not alter SRL. Conversely, S. kentrophyllus increased SRL by 42% and did not alter SLA. F. idahoensis responded to lower available soil moisture by increasing both SLA and SRL by 38% and 33%, respectively. These responses were species-specific strategies that did not coincide with photosynthetic pathway (C₃/C₄) or growth form. Thus, mass-specific root respiration responded uniformly among these four grass species to clipping (no effect) and increased soil moisture stress (decline), whereas the responses of other traits (i.e., R:S ratio, SLA, SRL) to the treatments, especially moisture availability, were species-specific. Consequently, the effects of either clipping or variation in soil moisture on the C budget of these four different grasses species were driven primarily by the plasticity of R:S ratios and the structural leaf and root traits of individual species, rather than variation in the response of mass-specific root respiration.     

Plant growth and nutrient uptake characteristics of Fe-deficiency chlorosis susceptible and resistant subclovers

The objective of this study was to evaluate the growth and nutrient-uptake characteristics of Fe-deficiency resistant and susceptible subclover (Trifolium subterraneum L., T. yanninicum Katzn. and Morley, T. brachcalycinum Katzn. and Morley) cultivars on a calcareous soil. Ten subclover cultivars showing varying susceptibilities to Fe-deficiency chlorosis (Karridale, Nangeela, Geraldton, Mt. Barker, Woogenellup, Larisa, Trikkala, Rosedale, Koala and Clare) were grown on a low-Fe, calcareous soil (Petrocalcic Paleustoll) under moist (18% water content, 85% of water holding capacity) and water-saturated conditions using a Cone-tainer^® culture system. Chlorosis and its correlation with growth traits and mineral nutrition of the 10 cultivars were examined. The Fe-deficiency susceptibilities of the 10 cultivars decreased in the above order under the moist condition, but in slightly different order under the saturated condition. Shoot and root dry weights, total dry weight, and root-to-shoot ratio were each negatively correlated with chlorosis under both soil-moisture conditions, as was total shoot content of P, Ca, Fe, Mn and Zn. Shoot P and Fe concentrations were each positively correlated with chlorosis under the moist soil condition. Iron and Cu utilization efficiencies (biomass per unit weight of nutrient) in the shoot were each negatively correlated with chlorosis under the moist soil condition. These results suggest that there may be several characteristics of Fe-deficiency chlorosis resistance in subclovers, such as a more effective soil-Fe mobilizing mechanism(s), more balanced nutrition, lower required Fe concentration in the shoot, higher shoot-Fe utilization efficiency, and higher root/shoot ratio under Fe-deficiency stress conditions.     

Root vertical distributions of two Artemisia species and their relationships with soil resources in the Hunshandake desert,China

1. Plant root variations and their relations with soil moisture and nutrient supply have been well documented for many species, while effects of drought, combined with extreme poor soil nutrients, on plant roots remain unclear.
2. Herein, we addressed root vertical distributions of two typical xerophyte semishrub species, Artemisia sphaerocephala and A. intramongolica, and their relations with soil moisture, total soil nitrogen and carbon contents in arid Hunshandake desert, China. The two species experienced similar light regimes and precipitation, but differed in soil moisture and soil nutrients.
3. Root vertical distribution patterns (e.g., coarse root diameter, root depth and root biomass) differed considerable for the two species due to high heterogeneity of soil environments. Coarse and fine root biomasses for A. intramongolica, distributed in relatively moist fixed dunes, mainly focused on surface layers (94%); but those for A. sphaerocephala dropped gradually from the surface to 140 cm depth. Relations between root traits (e.g., diameter, root biomass) and soil moisture were positive for A. intramongolica, but those for A. sphaerocephala were negative.
4. In general, the root traits for both species positively correlated with total soil nitrogen and carbon contents. These findings suggest that both soil moisture and poor soil nutrients were the limiting resources for growth and settlement of these two species.

     

Different distribution patterns of woody species on a slope in relation to vertical root distribution and dynamics of soil moisture profiles

Distribution patterns along a slope and vertical root distribution were compared among seven major woody species in a secondary forest of the warm-temperate zone in central Japan in relation to differences in soil moisture profiles through a growing season among different positions along the slope. Pinus densiflora, Juniperus rigida, Ilex pedunculosa and Lyonia ovalifolia, growing mostly on the upper part of the slope with shallow soil depth had shallower roots. Quercus serrata and Quercus glauca, occurring mostly on the lower slope with deep soil showed deeper rooting. Styrax japonica, mainly restricted to the foot slope, had shallower roots in spite of growing on the deepest soil. These relations can be explained by the soil moisture profile under drought at each position on the slope. On the upper part of the slope and the foot slope, deep rooting brings little advantage in water uptake from the soil due to the total drying of the soil and no period of drying even in the shallow soil, respectively. However, deep rooting is useful on the lower slope where only the deep soil layer keeps moist. This was supported by better diameter growth of a deep-rooting species on deeper soil sites than on shallower soil sites, although a shallow-rooting species showed little difference between them.     

Modification of root architecture in woody plants is possible for the presence of two different mechanisms of lateral root production: The effect of slope in Spartium junceum L. seedlings

Abstract

This paper investigates the modification of root architecture of Spartium junceum L. seedlings grown in slope condition. It is reported that 50% of the total number of lateral roots are concentrated in few centimetres of the taproot near the collar. The anatomical analysis of transverse sections along the taproot axis reveals that this taproot zone is characterised by two types of lateral roots: one with a trace extending to the centre of the vascular cylinder by following the path of a medullar ray; one with a trace which ends in the vascular cambium. The first type may be lateral roots originated from the taproot primary structure; the second type seems to be lateral roots developing later when a secondary structure has completely substituted the primary structure. The emission of this second type of lateral roots seems to be strongly controlled by environmental conditions with considerable consequences upon the overall root architecture. In the example reported in this paper, young plants growing under mechanical stress due to a slope develop asymmetric root architecture with lateral roots elongating in two prevalent directions: up-slope or down-slope. This asymmetric architecture is produced in the zone of the taproot where a secondary structure is present and represents the plant response to the need of increasing its anchorage strength.     

Effects of root nodules and taproots on survival and abundance of Sitona hispidulus (Coleoptera: Curculionidae) on Medicago sativa

ABSTRACT.

• 1 The effects of food resources on populations of larvae of Sitona hispidulus (F.), a root and nodule herbivore of Medicago sativa L., were investigated in 1983 and 1984.
• 2 Density-dependent mortality among first-instar larvae indicated that a lack of available root nodules, the primary resource for first instars, contributed to the 66.2±5.2% and 80.7±4.1% mortalities of first-instar larvae in 1983 and 1984, respectively. Initial densities of larvae entering the soil were 31.7±2.4 and 15.6±1.3 per soil sample (10.5 cm diam. ×15 cm deep) in 1983 and 1984, respectively (±SEM).
• 3 Survivorship and number of first-instar larvae per soil core sample were significantly related to number and biomass of nodules per soil sample. Percentage soil moisture was not consistently correlated with survivorship and number of larvae per soil sample.
• 4 Numbers of second- and third-instar larvae were not consistently correlated with either nodule or taproot biomass.
• 5 Numbers of fourth-instar larvae were associated with taproot biomass, suggesting that taproot surface area may also limit population levels of S.hispidulus. No correlations were found between taproot and nodule biomass on any sampling date indicating that high numbers of fourth-instar larvae associated with larger taproots were not merely due to a greater survival of first instars associated with larger taproots.
• 6 The study suggests that population levels of nodule-herbivores are controlled by the availability of root nodules. A sparse distribution of nodules results in high levels of mortality among nodule-herbivores of Medicago sativa.

     

Air humidity as key determinant of morphogenesis and productivity of the rare temperate woodland fern Polystichum braunii

(1) Most ferns are restricted to moist and shady habitats, but it is not known whether soil moisture or atmospheric water status are decisive limiting factors, or if both are equally important. (2) Using the rare temperate woodland fern Polystichum braunii, we conducted a three‐factorial climate chamber experiment (soil moisture (SM) × air humidity (RH) × air temperature (T)) to test the hypotheses that: (i) atmospheric water status (RH) exerts a similarly large influence on the fern's biology as soil moisture, and (ii) both a reduction in RH and an increase in air temperature reduce vigour and growth. (3) Nine of 11 morphological, physiological and growth‐related traits were significantly influenced by an increase in RH from 65% to 95%, leading to higher leaf conductance, increased above‐ and belowground productivity, higher fertility, more epidermal trichomes and fewer leaf deformities under high air humidity. In contrast, soil moisture variation (from 66% to 70% in the moist to ca. 42% in the dry treatment) influenced only one trait (specific leaf area), and temperature variation (15 °C versus 19 °C during daytime) only three traits (leaf conductance, root/shoot ratio, specific leaf area); RH was the only factor affecting productivity. (4) This study is the first experimental proof for a soil moisture‐independent air humidity effect on the growth of terrestrial woodland ferns. P. braunii appears to be an air humidity hygrophyte that, whithin the range of realistic environmental conditions set in this study, suffers more from a reduction in RH than in soil moisture. A climate warming‐related increase in summer temperatures, however, seems not to directly threaten this endangered species.     

Patterns of root architecture adaptation of a phreatophytic perennial desert plant in a hyperarid desert

Plant root systems can respond to nutrient availability and distribution by changing the three-dimensional deployment of their root architecture. The year after year variation of root architecture was investigated in a perennial phreatophyte in the controlled condition vegetation situated in the oasis in the Chinese Taklamakan desert with the goal to elucidate their adaptation to hyperarid environment. The whole plants of an indigenous perennial legume Alhagi sparsifolia Shap. with intact root systems were excavated at the end of each growing season from 2007 to 2009 and analyzed for architecture, above and belowground biomass, root/shoot ratio, root depth, seed yields and ramet. Changes in water availability were found to have stupendous effects on taproot depth, lateral root length and ramet quantity. Relative to shoot dry weight, taproot depth decreased with increasing water availability. In contrast, lateral root elongation was promoted by higher water availability.We tested the hypothesis that (1) irrigation increases root biomass and the quantity of ramets, and (2) A. sparsifolia Shap. develops an efficient root architecture that could absorb soil water and nutrition.     

Growth rates,biomass and distribution of selected woody plant roots in Burkea africana-Ochna pulchra savanna

Woody plants in an African Burkea africana-Ochna pulchra savanna on deep sandy soil were found to have characteristically bimorphic root systems. The shallow lateral root component was often well developed and roots extended up to seven times the extent of the plant canopy in several species. Exponential tapering of lateral roots was found in Terminalia sericea. The wide-ranging roots, together with the high degree of multispecies root system interpenetration, result in the so-called, open grassy areas in the savanna mosaic often containing a competitively significant woody plant component. Root systems of Ochna pulchra were found to be relatively specialized and included: negatively geotropic, superficial roots; sinker roots to bedrock; high suckering response to damage in roots; belowground lignotuber-type organs; and sustained subterranean interconnections between some aboveground stems. These features are likely to contribute substantially to the resilience of this plant species to various climatic and veld management stress factors. Root/shoot mass ratios averaged unity but depended on plant size and aboveground growth form in Ochna pulchra. The dependence of these ratios on sizes of plant also applied to plant clones. Initiation of root tip growth occurred in early summer in one year and late spring in another. Main root tip growth occurred in late summer and early autumn, well after completion of most growth of leafy shoots in spring. It is suggested that some active uptake of water and nutrients by non-extending roots allows this form of phased growth in the plant. In an analysis of the seasonal growth of individual root tip systems, it was clear that transitory states of rest occur in fine root development but that these are far more frequent in the branching (and hence proliferation) of roots than in the continuing development of any root axis.Nomenclature follows the present system of the Botanical Research Institute, Pretoria, and the Flora of Southern Africa.I thank M.D. Panagos, P.S. Carr and J. Steyn for assistance at various stages of this work.     

Aluminum inhibition of shoot lateral branches ofGlycine max and reversal by exogenous cytokinin

Aluminum effects on the morphological development of soybean (Glycine max (L.) Merr.) were characterized in greenhouse and growth chamber experiments. An Al-sensitive cultivar, ‘Ransom’, was grown in an acid soil (Aeric Paleudult) adjusted to 3 levels of exchangeable Al. Lateral shoot development at the nodes of the main stem was extensive in the limed soil containing 0.06 cmol(+) Alkg⁻¹. However, lateral shoot length and weight were severely inhibited in the unlimed soil containing 2.19 cmol(+) Alkg⁻¹, and in the unlimed soil amended to 2.63 cmol(+) Alkg⁻¹ with AlCl₃. This inhibition by the high Al/low pH condition was reversed by the exogenous application of a synthetic cytokinin 6-benzylaminopurine (BA). The daily application of 20 μg mL⁻¹ BA applied locally to the lateral meristems of plants grown in the unlimed soil stimulated lateral shoot growth substantially, such that it was either comparable to or greater than that observed in the limed treatment without BA. Accumulation of K, Ca, and Mg in lateral shoot branches was also stimulated by the local application of BA. The inhibitory effects of Al on lateral shoot development were confirmed in solution culture. In addition, differential sensitivity to Al was evident among the primary root, first order lateral roots, and second order lateral roots. The length of the primary root was only slightly decreased by increasing concentrations of Al up to 30 μM. In contrast, the length of basipetally located first order lateral roots was restricted to greater extent; up to 50% by 30 μM Al. Second order lateral lengths were inhibited even more severely; up to 86% by 30 μM Al. Substantial evidence in the literature indicates that the root apex is a major site for the biosynthesis of cytokinin that is supplied to shoots, and cellular function and development in this region of the root are impaired during Al toxic conditions. This suggests that one mode of action by which Al may affect shoot growth is by inhibiting the synthesis and subsequent translocation of cytokinin to the meristematic regions of the shoot. The present observation of a reversal of Al-inhibited lateral shoot development by exogenously applied cytokinin supports this hypothesis. However, the inability of applied cytokinin to counter the restriction imposed by Al on total shoot dry matter production implies the impairment by Al toxicity of other root functions, such as ion and water transport, also played an important role in altering shoot morphology.     

Growth dynamics of oak seedlings (Quercus macrocarpa Michx. and Quercus muhlenbergii Engelm.) from gallery forests: implications for forest expansion into grasslands

Seedling growth dynamics of Quercus macrocarpa Michx. and Quercus muhlenbergii Engelm. were compared over a 3-month period under optimal growth conditions. These two species are the dominant trees at the western limit of the eastern deciduous forest, and are typically confined to gallery forests along stream beds in tallgrass prairie. Since tallgrass prairie is characterized by a highly variable climate and is prone to periodic drought, we hypothesized that these oaks would have rapid root growth and produce deep taproots as seedlings, enabling them to avoid drought stress and persist in this region. These traits may also facilitate forest expansion into the more xeric tallgrass prairie if fires are suppressed. Taproots of Q. macrocarpa and Q. muhlenbergii grew to approximately 140 cm and 100 cm in length, respectively, after 104 days. In both species, 65% or more of seedling biomass was allocated below ground, and root/total biomass was significantly greater in Q. muhlenbergii at 0-20 and 21-40 days after germination. The seedling taproot elongation rates reported here are much greater than rates reported in other eastern deciduous forest trees. Long-term precipitation data and soil moisture patterns from tallgrass prairie, when combined with rapid taproot elongation rates, suggest that soil moisture may not limit oak establishment or growth in tallgrass prairie in most years, although water uptake by roots was not measured in this study. Other factors, such as fire, herbivory, and seed predation and dispersal may be equally important in constraining the distribution of these species to gallery forests.     

The feeding behaviour of a sit-and wait-predator,Ranatra dispar (Heteroptera: Nepidae): optimal foraging and feeding dynamics

Summary Nitrogen and phosphorus flow in litterfall and throughfall were studied in two California Quercus species (the evergreen Q. agrifolia and deciduous Q. lobata) before, during, and after an outbreak of the California oak moth, Phryganidia californica. All of the foliage of both oak species was removed by the herbivore during the course of this outbreak. During the outbreak, total N and P flow to the ground more than doubled from Q. agrifolia and increased to a lesser extent from Q. lobata over the previous year. The composition of the litter during the outbreak year shifted so that in Q. agrifolia, almost 70% of the total N and P flow to the ground moved through frass and insect remains, while in Q. lobata, approximately 60% of the N and 40% of the P moved through frass and insect remains. Short-term leaching experiments showed that nitrogen was far more rapidly lost from Phryganidia frass than from leaf litter of either species. These results and the relative frequency of Phryganidia outbreaks suggest that this herbivore has significant effects on the nutrient cycling beneath these trees.     

The role of nodal roots in prostrate clonal herbs: ‘phalanx’ versus ‘guerrilla’

The influence of nodal rooting on branching was studied in three evolutionarily and morphologically diverse species of prostrate clonal herbs: Tradescantia fluminensis (a monocotyledonous extreme ‘phalanx’ species), Calystegia silvatica (a dicotyledonous extreme ‘guerrilla’ species) and Trifolium repens (a dicotyledonous intermediate species). In all three, branch development from axillary buds is regulated by a positive signal produced by roots together with inhibitory influences from both pre-existing branches and shoot apical buds (apical dominance). Responses to nodal roots are cumulative and increased root activity leads to more vigorous bud outgrowth. In the absence of nodal roots, a single basal root system is unable to maintain continued extension growth of the shoot. We suggest that as individual nodal roots and stem internodes are both short-lived in these nodally-rooting clonal species, the plants’ investment in them is minimal. Thus, in contrast to perennial species lacking nodal roots, individual root systems in prostrate clonal herbs are small and stems have little secondary thickening and development of long-distance transport tissues. Hence the decline in extension growth of the shoot in the absence of nodal roots could be linked to the weak development of long-distance transport tissues in their relatively thin horizontal stems and to resource sharing between primary stems and lateral branches (as suggested by the greater retardation of primary stem growth in the more profusely branched ‘phalanx’ species (Trifolium and Tradescantia) than in the weakly branched ‘guerrilla’ species, Calystegia). These findings are consistent with the view that the long-term persistence of genotypes of nodally-rooting prostrate species is dependent upon them encountering the moist conditions required to facilitate the continual development of new young nodal root systems.     

Control of soil physical properties and response ofBrassica rapa L. seedling roots

A system was designed, constructed, tested, and used to growBrassica rapa L. seedling roots which were exposed to O₂ concentrations from 0 to 0.21 mol mol⁻¹, water potentials from 0 to −80 kPa, temperatures from 10 to 34°C, and mechanical impedance from 0 to 20.8 kPa. The experimental design was a central composite rotatable design with seven replications of the center point. Measurements were taken of taproot length, taproot diameter at the point of initiation of root hairs (diameter 1), and one cm above the first measurement (diameter 2), and total length and number of first-order laterals. Temperature had the greatest effect on seedling root growth, with linear and quadratic temperature effects significant for all root measurements except taproot diameter 2 which just had a significant linear effect. Water potential had a significant linear effect on lateral length and number of laterals and a significant quadratic effect on taproot diameter 1. Mechanical impedance had a significant effect only on taproot diameter 2. Oxygen was not significant for any root measurement. The mechanical impedance by water potential interaction was significant for taproot length and taproot diameter 1. A temperature optimum was found for taproot length, taproot diameter 1, lateral length, and lateral number, at 26.0, 42.5, 26.5, and 26.4°C, respectively. Taproot diameter 1 had a water potential optimum at −36.5 kPa, whereas taproot diameter 2 had a mechanical impedance optimum at 12.5 kPa. A growth cell designed for this study allows independent control of soil strength, water potential, oxygen concentration, and temperature. Thus, the cell provides the capability which was demonstrated forBrassica rapa L. to grow seedling roots under complete control of the soil physical properties.     

Influence of drought on plant performance through changes in belowground tritrophic interactions

Climate change is predicted to increase the risk of drought in many temperate agroecosystems. While the impact of drought on aboveground plant‐herbivore‐natural enemy interactions has been studied, little is known about its effects on belowground tritrophic interactions and root defense chemistry. We investigated the effects of low soil moisture on the interaction between maize, the western corn rootworm (WCR, Diabrotica virgifera), and soil‐borne natural enemies of WCR. In a manipulative field experiment, reduced soil moisture and WCR attack reduced plant performance and increased benzoxazinoid levels. The negative effects of WCR on cob dry weight and silk emergence were strongest at low moisture levels. Inoculation with entomopathogenic nematodes (EPNs, Heterorhabditis bacteriophora) was ineffective in controlling WCR, and the EPNs died rapidly in the warm and dry soil. However, ants of the species Solenopsis molesta invaded the experiment, were more abundant in WCR‐infested pots and predated WCR independently of soil moisture. Ant presence increased root and shoot biomass and was associated with attenuated moisture‐dependent effects of WCR on maize cob weight. Our study suggests that apart from directly reducing plant performance, drought can also increase the negative effects of root herbivores such as WCR. It furthermore identifies S. molesta as a natural enemy of WCR that can protect maize plants from the negative impact of herbivory under drought stress. Robust herbivore natural enemies may play an important role in buffering the impact of climate change on plant‐herbivore interactions.     

Shifts in plant nutrient use strategies under secondary forest succession

In evergreen broad-leaved forests (EBLFs) in Tiantong National Forest Park, Eastern China, we studied the soil chemistry and plant leaf nutrient concentration along a chronosequence of secondary forest succession. Soil total N, P and leaf N, P concentration of the most abundant plant species increased with forest succession. We further examined leaf lifespan, leaf nutrient characteristics and root–shoot attributes of Pinus massoniana Lamb, the early-successional species, Schima superba Gardn. et Champ, the mid-successional species, and Castanopsis fargesii Franch, the late-successional species. These species showed both intraspecific and interspecific variability along succession. Leaf N concentration of the three dominant species increased while N resorption tended to decrease with succession; leaf P and P resorption didn’t show a consistent trend along forest succession. Compared with the other two species, C. fargesii had the shortest leaf lifespan, largest decay rate and the highest taproot diameter to shoot base diameter ratio while P. massoniana had the highest root–shoot biomass ratio and taproot length to shoot height ratio. Overall, P. massoniana used ‘conservative consumption’ nutrient use strategy in the infertile soil conditions while C. fargesii took up nutrients in the way of ‘resource spending’ when nutrient supply increased. The attributes of S. superba were intermediate between the other two species, which may contribute to its coexistence with other species in a wide range of soil conditions.     

Response of liquid flow resistance to soil drying in seedlings of four deciduous angiosperms

Summary Soil-leaf resistance to liquid water flow (R) in moist and drying soil was compared in three-month-old seedlings of two drought tolerant (white [Quercus alba L.], post oak [Q. stellata Wangenh.]) and two drought sensitive forest species (sugar maple [Acer saccharum Marsh.], black walnut [Juglans nigra L.]). At high soil moisture (_s–0.3 MPa), R was higher in J. nigra than in the other species, and as soil water was depleted R increased most in this species. In contrast, the lowest resistance at all levels of soil moisture was observed in Q. stellata. At _s of –1.5 MPa, R of drought-sensitive J. nigra and A. saccharum was about twice as high as that of the two drought-tolerant Quercus species. The difference in R between the two Quercus species was much smaller than that between this pair and the other two species. These differences among species in flow resistance may be attributable to: 1) variation in the balance between root surface area and leaf area, 2) variation in the inherent absorption capacity of the root systems and in xylem water conducting systems or 3) differences in root permeability, shrinkage and mortality in severely stressed seedlings. As the soil dried, seedlings of all species exhibited pronounced reductions in transpiration rate, which prevented development of large water potential gradients between leaves and the soil. Reduction in transpiration in J. nigra was especially pronounced, resulting in a decrease in the soil-to-leaf water potential gradient in dry soil despite high flow resistance. The observed differences among species in flow resistance are correlated with natural distribution patterns.