Split-root Plants Research Articles

Is ABA the exogenous vector of interplant drought cuing?

ABSTRACT We have recently demonstrated that root cuing from drought-stressed plants increased the survival time of neighboring plants under drought, which came at performance costs under benign conditions. The involvement of abscisic acid (ABA) was implicated from additional experiments in which interplant drought cuing was greatly diminished in ABA-deficient plants. Here, we tested the hypothesis that ABA is the exogenous vector of interplant drought cuing. Pisum sativum plants were grown in rows of three split-root plants. One of the roots of the first plant was subjected to either drought of benign conditions in one rooting vial, while its other root shared its rooting vial with one of the roots of an unstressed neighbor, which in turn shared its other rooting vial with an additional unstressed neighbor. One hour after subjecting one of the roots of the first plant to drought, ABA concentrations were 106% and 145% higher around its other root and the roots of its unstressed neighbor, compared to their respective unstressed controls; however, the absolute concentrations of ABA found in the rooting media were substantially low. The results may indicate that despite its involvement in interplant drought and the commonly observed exchange of ABA between drought-stressed plants and their rhizospheres, ABA is not directly involved in exogenous interplant drought cuing. However, previous studies have shown that even minute concentrations of ABA in the rhizosphere can prevent ABA leakage from roots and thus to significantly increase endogenous ABA levels. In addition, under drought conditions, plants tend to accumulate ABA, which could markedly increase internal ABA concentrations over time and ABA concentrations in close proximity to the root surface might be significantly greater than estimated from entire rooting volumes. Finally, phaseic acid, an ABA degradation product, is known to activate various ABA receptors, which could enhance plant drought tolerance. It is thus feasible that while the role of ABA is limited, its more stable degradation products could play a significant role in interplant drought cuing. Our preliminary findings call for an extensive investigation into the identity and modes of operation of the exogenous vectors of interplant drought cuing.

Early Physiological Plant Response and Systemic Effects of Huanglongbing Infection in Split-Root Plants.

Huanglongbing (HLB), caused by 'Candidatus Liberibacter asiaticus' (CLas), is a devastating disease of citrus. After initial infection, CLas quickly colonizes the root system before canopy symptoms develop. There is limited understanding of CLas movement from roots to canopy and local and systemic effects on root dynamics. Using split-root rhizoboxes and late summer below-the-split bud inoculation, effects of local infection on systemic disease development were studied. Upward bacterial movement from roots is linked to seasonal flushes and CLas population in roots. CLas stayed isolated to one side of the roots for at least 8 months, until the spring flush. HLB caused differential root responses depending on tree age at infection. Systemic effects, independent of CLas movement, occur very early after infection. Stimulation of root growth occurred on noninfected roots prior to CLas detection in 1.5-year-old trees but decreased in 2.5-year-old trees. Independent of tree age, root growth was stimulated during spring root flushes after CLas population stabilized. Root dieback began simultaneously with detection of CLas in roots (6 weeks postinoculation). Infection and tree age altered root lifespan. In total, 1.5-year-old CLas-infected roots from summer and fall flushes had 3 and 6 weeks reduced lifespan. In contrast, 2.5-year-old CLas-infected plants lifespan was unaffected. Season affected root lifespan with late summer root flush lifespan was three times shorter than fall or spring root flushes. Split-root inoculation allowed study of local and systemic effects of CLas infection in roots, information crucial to prolonging the productivity of HLB-affected trees.

The level of relatedness affects self/nonself discrimination inEucalyptus urophyllaseedlings

Plants can develop differentially because of their ability of self/nonself discrimination and the degree of kinship among them. Here, we evaluate the ability of self/nonself discrimination of Eucalyptus urophylla S.T. Blake roots in plant groups with different levels of relatedness. We used three plant groups: clonal, half-siblings, and population. Split-root plants were grown in pots containing either two roots of the same plant (SD) or of two different plants (NSD). The growth of root and leaves of the half-siblings and population plants was decreased in NSDin relation to SD, whereas this response was not observed in the clonal group. The multivariate analysis indicated that there was a progressive increase in plant responses likely caused by competitive interaction of roots, as the level of relatedness between individuals was lower. Our results suggest that the group of clonal plants minimized the competitive interaction among them, indicating low ability to discriminate from each other. However, half-sibling and populational plants reduced growth as a result of root competition, showing high capacity of self discrimination. Thus, a minimum degree of genetic variation between plants seems necessary for kin recognition to be expressed.

Self and Non-self Discrimination Affects the Seedlings Development of Different Kinship Degrees Under Water Deficit

The strategy adopted by plants for exploit environments with low moisture may depend on the neighbor's identity. Roots of the plants have the ability to discriminate self (roots of the same plant) of non-self (roots of other plants), even without physical contact and while other plants are genetically identical. We investigated if there is any change in the plant behavior that self (Sd) and non-self (NSd) discriminate in situations of water limitation, and whether such behavior can be varied in individuals more or less kin. It was used two Eucalyptus urophylla groups: clones (high kinship-Hk); and population (low kinship-Lk). Split-root plants were planted so that each pot contained either two roots of the same plant (Sd) or of two different plants (NSd). The plants were subjected to full irrigation (Irr) or water deficit (WD) for thirty days, and biomass parameters of were evaluated. Plants of both groups and both discrimination conditions reduced their total dry mass (DMt) under WD. In Hk group, the WD increased the root/shoot ratio (23% compared to irrigated plants) by NSd, indicating that even when plants show a reduction in growth mediated by lack of water, there is an increase in biomass allocation to the roots when the plants interact each plants. In Hk group, the NSd reduced the dry mass (DMr) and volume (Vr) of roots only in Irr. In Lk group, DMt, DMr, Vr, and leaf area were reduced in NSd under Irr condition, whereas under WD, the NSd reduced just the specific leaf area (21% compared to Sd), indicating a change in biomass allocation to leaves. The results suggest that the discrimination is more evident in population group and in non-stressful environments, and although not so pronounced in situations of WD, these responses also occur as observed in Lk plants.

Shoot-derived signals other than auxin are involved in systemic regulation of strigolactone production in roots.

Nitrogen and phosphorus fertilization in one side of split-root sorghum plants systemically reduced root contents of strigolactones in both sides of the split roots. Shoot-derived signals other than auxin appeared to be involved in this process. Strigolactones (SLs) are a novel class of plant hormones regulating both shoot and root architectures and suggested to be functioning downstream of auxin. The levels of SLs in plant tissues and root exudates are regulated by nutrients, especially phosphorus (P) and nitrogen (N); however, the underlying mechanism remains elusive. We examined the effects of N and P fertilization on root contents of two SLs, sorgomol and 5-deoxystrigol, in sorghum plants pre-incubated under N and P free conditions using a split-root system. N and P fertilization to one side of the split-root plants systemically reduced root contents of SLs in both sides of the split roots. The shoot N and P levels increased when one side of the split-root plants was fertilized, while N and P levels in the non-fertilized split roots were unaffected. N fertilization decreased shoot and root IAA (indole-3-acetic acid) levels, while P fertilization did not affect them. IAA applied to the shoot apices increased root contents of 5-deoxystrigol but not that of sorgomol only when the plants were grown under P free conditions. Shoot (leaf) removal dramatically decreased the root contents of SLs but did not affect root IAA levels, and IAA applied to the stumps of leaves could not restore root contents of SLs. Consequently, shoot-derived signals other than auxin are suggested to be involved in the regulation of SL production in roots.

Increased root oxygen uptake in pea plants responding to non-self neighbors.

Recent studies have demonstrated that plants alter root growth and decrease competition with roots of the same individual (self); however, the physiological traits accompanying this response are still widely unknown. In this study, we investigated the effect of root identity on gas exchange in the model species pea (Pisum sativum L.). Split-root plants were planted so that each pot contained either two roots of the same plant (self) or of two different plants (non-self), and the responses of biomass, photosynthesis, and respiration were measured. The photosynthetic rate was not affected by the identity of the root neighbor. We found a reduction of leaf dark respiration by half, accompanied by an increase in nocturnal root respiration by 29% in plants neighboring with non-self. The activity of the alternative oxidase (AOX) pathway increased when plants responded to non-self neighbors. The increased activity of AOX in plants responding to non-self indicates carbon imbalances in roots, possibly as a consequence of increased root exudation and communication between individuals. If such an effect occurs more widely, it may change the assumptions made for the quantity of respiration as used in carbon budget models.

Systemic Feedback Regulation of K + Uptake in Cotton at Seedling Stage

This hydroponic split-root experiment was conducted to elucidate the long-distance systemic regulation of K+uptake in cotton(Gossypium hirsutum L.)seedlings with two cotton cultivars CCRI41 and Liaomian17 as materials.Inverted Y grafting(one scion/two rootstocks,hypocotyl-to-hypocotyl)was used to establish split-root plants.The K+-uptake characteristics of cotton seedlings were measured by depletion method and described by kinetic parameters,Imax(the maximum rate of uptake),Km(the external concentration at which the uptake rate is 0.5 Imax)and Cmin(the external concentration at which no net flux occurs).In relative to the sufficient K+control(both root halves supplied with 2.50 mmol L–1K+),the root half supplied with 2.50 mmol L–1 K+for six days showed 79% to 92% more Imax owing to the induction of K+-demand signal derived from the root half starved with K+.Compared with the K+starvation control(both root halves supplied with 0.01 mmol L–1K+),Imax of root half supplied with 0.01 mmol L–1K+for six days decreased by 27% to 40% due to the suppression of K+-supply signal initiated by the root half grown in sufficient K+.When the root half starved with K+in CCRI41 was removed after three days of split-root treatment,the remained root half incessantly supplied with 2.50 mmol L–1K+for six days had less Imax relative to the whole split-root plants,which is likely attributed to the loss of K+-demand signal from the other root half during last three days.This systemic feedback regulation of K+uptake in cotton seedlings was independent of K+content of either root or shoot.

The effect of steepness of temporal resource gradients on spatial root allocation

Plants are able to discriminately allocate greater biomass to organs that grow under higher resource levels. Recent evidence demonstrates that split-root plants also discriminately allocate more resources to roots that grow under dynamically improving nutrient levels, even when their other roots grow in richer patches. Here, we further tested whether, besides their responsiveness to the direction of resource gradients, plants are also sensitive to the steepness of environmental trajectories. Split-root Pisum sativum plants were grown so that one of their roots developed under constantly-high nutrient levels and the other root was subjected to dynamically improving nutrient levels of variable steepness. As expected, plants usually allocated a greater proportion of their biomass to roots that developed under constantly high resource availability; however, when given a choice, they allocated greater biomass to roots that initially experienced relatively low but steeply improving nutrient availabilities than to roots that developed under continuously-high nutrient availability. Such discrimination was not observed when the roots in the poor patch experienced only gentler improvements in nutrient availability. The results are compatible with the notion that responsiveness to the direction and steepness of environmental gradients could assist annual plants to increase their performance by anticipating resource availabilities foreseeable before the end of their growing season. The results exemplify the ability of plants to integrate and utilize environmental information and execute adaptive behaviours which, until recently, were attributed only to animals with central nervous systems.

Cadmium immobilization in the rhizosphere of arbuscular mycorrhizal plants by the fungal extraradical mycelium

The aim of the study was to assess how the extraradical mycelium (ERM) of arbuscular mycorrhizal (AM) fungi contributes to Cd immobilization in the rhizosphere. Substrates prepared by cultivation of AM and non-mycorrhizal tobacco (Nicotiana tabacum L.) in quartz sand in two experiments were amended with Cd in a range of concentrations and Cd immobilization was assessed as Cd toxicity using root growth tests. Split-root plants, inoculated at one part of the root system, and hyphal compartments colonized by ERM only were used to separate the effects of ERM from plant-mediated effects of mycorrhiza and from the effects of roots. AM decreased Cd toxicity in the substrates obtained by 12 weeks of cultivation (Experiment 1), while the effect was less clear after 8 weeks (Experiment 2). No indication was found for an involvement of plant-mediated effects; in contrast, the effect of ERM could be clearly demonstrated. Lower Cd toxicity in the substrates colonized by ERM was related to ERM-induced alkalinization, but not directly to ERM density. It is concluded that the ERM of AM fungi may enhance Cd immobilisation in soil not only due to its high Cd sorption capacity but also by its activity.

An experimental and modelling exploration of the host-sanction hypothesis in legume–rhizobia mutualism

Despite the importance of mutualism as a key ecological process, its persistence in nature is difficult to explain since the existence of exploitative, “cheating” partners that could erode the interaction is common. By analogy with the proposed policing strategy stabilizing intraspecific cooperation, host sanctions against non-N 2 fixing, cheating symbionts have been proposed as a force stabilizing mutualism in legume– Rhizobium symbiosis. Following this proposal, penalizations would include decreased nodular rhizobial viability and/or early nodule senescence in nodules occupied by cheating rhizobia. In this work, we analyse the stability of Rhizobium–legume symbiosis when non-fixing, cheating strains are present, using an experimental and modelling approach. We used split-root experiments with soybean plants inoculated with two rhizobial strains, a cooperative, normal N 2 fixing strain and an isogenic non-fixing, “perfect” cheating mutant derivative that lacks nitrogenase activity but has the same nodulation abilities inoculated to split-root plants. We found no experimental evidence of functioning plant host sanctions to cheater rhizobia based on nodular rhizobia viability and nodule senescence and maturity molecular markers. Based on these experiments, we developed a population dynamic model with and without the inclusion of plant host sanctions. We show that plant populations persist in spite of the presence of cheating rhizobia without the need of incorporating any sanction against the cheater populations in the model, under the realistic assumption that plants can at least get some amount of fixed N 2 from the effectively mutualistic rhizobia occupying some nodules. Inclusion of plant sanctions leads to the unrealistic effect of ultimate extinction of cheater strains in soil. Our simulation results are in agreement with increasing experimental evidence and theoretical work showing that mutualisms can persist in presence of cheating partners.

The influence of water stress on biomass and N accumulation, N partitioning between above and below ground parts and on N rhizodeposition during reproductive growth of pea ( Pisum sativum L.)

In the next few years, grain legumes should be used as a mean of N acquisition in cropping systems due to the depletion of non-renewable sources of energy. However, this requires improvements in the accuracy with which biological N 2 fixation, N balances and the N benefit for following crops are estimated. Moreover, grain legume crops are largely influenced by water stress while the world area exposed to drought periods may increase in the coming years due to global warming. This work aims to quantify biomass and N accumulation, N partitioning between above and below ground parts and N rhizodeposition by a pea ( Pisum sativum L.) when influenced by water stress. In a controlled environment, pea plants were exposed to a severe drought or not stressed, either at flowering or during pod filling. N rhizodeposition was measured using the split root method and plants were harvested at the end of flowering (59 days after sowing, DAS 59), at the end of the drought period applied during pod filling (DAS 74) and at maturity (DAS 101). Water stress strongly affected pea dry weight and N accumulation. In both stressed treatments, nodule biomass and N content were reduced by about 65% in the absence of stress. Regardless of the treatment, total below ground plant N (root N + N rhizodeposition; BGN) and N rhizodeposition were correlated with total plant N content and the proportion of BGN to total plant N was similar among treatments at each sampling date. At DAS 59 and 74, the N contained in rhizodeposits represented around 30% of the total BGN and increased to around 60% at maturity though BGN decreased from around 20 to 13% of the total plant N between DAS 74 and maturity. The results suggest that water stress has no specific effect on N partitioning between above and below ground parts.

An experimental and modelling exploration of the host-sanction hypothesis in legume-rhizobia mutualism

AbstractDespite the importance of mutualism as a key ecological process, its persistence in nature is difficult to explain since the existence of exploitative, 'cheating' partners that could erode the interaction is common. By analogy with the proposed policing strategy stabilizing intraspecific cooperation, host sanctions against non N2 fixing, cheating symbionts have been proposed as a force stabilizing mutualism in legume-Rhizobium symbiosis. Following this proposal, penalizations would include decreased nodular rhizobial viability and/or early nodule senescence in nodules occupied by cheating rhizobia. In this work, we analyze the stability of Rhizobium-legume symbiosis when "cheating" strains are present, using an experimental and modelling approach. We used split-root experiments with soybean plants inoculated with two rhizobial strains, a cooperative, normal N2 fixing strain and an isogenic non-fixing, “perfect” cheating mutant derivative that lacks nitrogenase activity but has the same nodulation abilities inoculated to split-root plants. We found no experimental evidence of functioning plant host sanctions to cheater rhizobia based on nodular rhizobia viability and nodule senescence and maturity molecular markers. Based on these experiments, we developed a population dynamic model with and without the inclusion of plant host sanctions. We show that plant populations persist in spite of the presence of cheating rhizobia without the need of incorporating any sanction against the cheater populations in the model, under the realistic assumption that plants can at least get some amount of fixed N2 from the effectively mutualistic rhizobia occupying some nodules. Inclusion of plant sanctions merely reduces the time needed for reaching plant population equilibrium and leads to the unrealistic effect of ultimate extinction of cheater strains in soil. Our simulation results are in agreement with increasing experimental evidence and theoretical work showing that mutualisms can persist or even improve in presence of cheating partners.

Induced resistance by the mutualistic endophyte, Fusarium oxysporum strain 162, toward Meloidogyne incognita on tomato

The non-pathogenic endophytic fungus, Fusarium oxysporum strain 162, originally isolated from the endorhiza of tomato roots, reduces damage caused by Meloidogyne incognita, by inhibiting juvenile penetration of and development in the root. However, little is known about the mode of action of this endophyte fungus against the nematode. This study aimed at investigating how the endophyte affects nematode motility and survival and if induced resistance plays a role in the relationship. In a previous study, F. oxysporum strain 162 decreased nematode penetration of tomato up to 60%. In experiments using a split-root chamber to test for induced resistance, nematode penetration, number of galls, and number of egg masses were investigated 2 and 5 weeks after nematode inoculation. Split-root plants treated with F. oxysporum strain 162 showed 26–45% less nematode penetration, 21–36% less galls and a 22–26% reduction in the number of egg masses in the roots not directly inoculated with the fungus when compared to untreated control plants in repeated tests. In conclusion, inoculation of tomato plants with the non-pathogenic fungal endophyte F. oxysporum strain 162 resulted in a signficant reduction of nematode infection, which was in part due to induced resistance in the first 2–3 weeks after fungal inoculation.

Nitrogen Fixation Control under Drought Stress. Localized or Systemic?

Legume-Rhizobium nitrogen fixation is dramatically affected under drought and other environmental constraints. However, it has yet to be established as to whether such regulation of nitrogen fixation is only exerted at the whole-plant level (e.g. by a systemic nitrogen feedback mechanism) or can also occur at a local nodule level. To address this question, nodulated pea (Pisum sativum) plants were grown in a split-root system, which allowed for half of the root system to be irrigated at field capacity, while the other half was water deprived, thus provoking changes in the nodule water potential. Nitrogen fixation only declined in the water-deprived, half-root system and this result was correlated with modifications in the activities of key nodule's enzymes such as sucrose synthase and isocitrate dehydrogenase and in nodular malate content. Furthermore, the decline in nodule water potential resulted in a cell redox imbalance. The results also indicate that systemic nitrogen feedback signaling was not operating in these water-stressed plants, since nitrogen fixation activity was maintained at control values in the watered half of the split-root plants. Thus, the use of a partially droughted split-root system provides evidence that nitrogen fixation activity under drought stress is mainly controlled at the local level rather than by a systemic nitrogen signal.

Partial rootzone drying does not affect fruit yield of raspberries

SummaryPot experiments with split-root plants and two years of ®eld experiments with manipulation of soil water content have shown that partial rootzone drying (PRD) does not affect yield in raspberries, Rubus idaeus L. `Glen Ample' and `Glen Prosen'. In some cases PRD can reduce stomatal conductance, thus reducing water loss. This results in a higher water use ef®ciency. Partial rootzone drying may be an ef®cient irrigation strategy where raspberries are produced under shelter.

Intra-plant versus Inter-plant Root Competition in Beans: avoidance, resource matching or tragedy of the commons

Root competition inhibits root proliferation. All else equal, a plant should invest roots in a nutrient patch devoid of roots rather than one already occupied by roots. Less clear is how a plant should respond to intra-plant versus inter-plant root competition. We consider three responses for how a plant may select habitats based on intra-versus inter-plant root competition: inter-plant avoidance, resource matching, or intra-plant avoidance. The first assumes that plants prefer to have their own space and preferentially proliferate roots away from neighboring plants. The second response, based on the ideal free distribution, assumes that plants invest so as to equalize average returns from roots, regardless of the identity of the neighboring roots. The third, based on game theory, assumes that the plant proliferates roots so as to maximize whole-plant fitness, in which case it is better to proliferate plants among a neighbor's roots than to continue proliferating amongst one's own roots. To test among these models we grew beans (Phaseolus varigaris, var. Kenya) in a greenhouse under two planting scenarios. Both scenario were tested under 0.5 and 0.1 strength of nutrient solution. Under scenario A (fence-sitters), two split-root plants each shared two patches by virtue of having roots in each. Under scenario B (owners) two plants each had their own patch. The results supported the game theory model of intra-plant avoidance (whole plant habitat selection). Fence-sitters produced 150% more root mass per individual than owners. Owners produced 90% more yield (dry mass of pods) than fence-sitters. Furthermore, owners had significantly higher shoot-root ratios than fence-sitters. These effects did not vary with high or low nutrient levels. The over-proliferation of roots under inter-plant competition (fence-sitters) was manifest by the tenth day after planting. In short, the fence-sitters engaged in a tragedy of the commons in which they competed with each other through root proliferation. At the ESS, the fitness maximizing strategy of the individual is to sacrifice collective yield in a quest to `steal' nutrients from its neighbor. The research has three implications. First, plants may be able to assess and respond to local opportunities in a manner that maximizes the good of the whole plant. Second, nutrient foraging as a game may provide a fresh perceptive for viewing root competition either intra-specifically or inter-specifically. Third, it may be possible to increase the yield of certain crop species by breeding more `docile' cultivars that do not overproduce roots in response to inter-plant competition.

Iron stress-induced changes in root epidermal cell fate are regulated independently from physiological responses to low iron availability.

Iron-overaccumulating mutants were investigated with respect to changes in epidermal cell patterning and root reductase activity in response to iron starvation. In all mutants under investigation, ferric chelate reductase activity was up-regulated both in the presence and absence of iron in the growth medium. The induction of transfer cells in the rhizodermis appeared to be iron regulated in the pea (Pisum sativum L. cv Dippes Gelbe Viktoria and cv Sparkle) mutants bronze and degenerated leaflets, but not in roots of the tomato (Lycopersicon esculentum Mill. cv Bonner Beste) mutant chloronerva, suggesting that in chloronerva iron cannot be recognized by putative sensor proteins. Experiments with split-root plants supports the hypothesis that Fe(III) chelate reductase is regulated by a shoot-borne signal molecule, communicating the iron status of the shoot to the roots. In contrast, the formation of transfer cells was dependent on the local concentration of iron, implying that this shoot signal does not affect their formation. Different repression curves of the two responses imply that the induction of transfer cells occurs after the enhancement of electron transfer across the plasma membrane rather than being causally linked. Similar to transfer cells, the formation of extra root hairs in the Arabidopsis mutant man1 was regulated by the iron concentration of the growth medium and was unaffected by interorgan signaling.

Transfer of water from roots into dry soil and the effect on wheat water relations and growth

This investigation was performed to study the effect on plant water relations and growth when some of roots grow into dry soil. Common spring water (Triticum aestivum) plants were grown from seed in soil in 1.2 m long PVC (polyvinyl chloride) tubes. Some of the tubes had a PVC partition along their center so that plants developed a split root system (SPR). Part of the roots grew in fully irrigated soil on one side of the partition while the rest of the roots grew into a very dry (-4.1 MPa) soil on the other side of the partition. Split root plants were compared with plants grown from emergence on stored soil moisture (STOR) and with plants that were fully irrigated as needed (IRR). The experiment was duplicated over two temperature regimes (10°/20°C and 15°/25°C, night/day temperatures) in growth chambers. Data were collected on root dry matter distribution, soil moisture status, midday leaf water potential (LWP), leaf relative water content (RWC) and parameters of plant growth and yield.

14Carbon-labeled Photosynthate Partitioning in Ilex crenata `Rotundifolia' at Supraoptimal Root-zone Temperatures

Ilex crenata Thunb. `Rotundifolia' split-root plants were grown for 3 weeks with root zones at 30/30, 30/34, 30/38, 30/42, 34/34, 38/38, and 42/42C. The 38C root-zone treatment was the upper threshold for several growth and physiological characteristics. A portion of the root system grown at or near the optimum temperature could compensate, in terms of shoot growth, for part of the root system exposed to supraoptimal root-zone temperatures up to 38C. Higher root-zone temperatures did not affect short-term photosynthetic rates or root : shoot ratios, but altered photosynthate partitioning to various stem and root sinks. Although no differences were found for total 14C partitioned to the roots, partitioning of 14C into soluble and insoluble fractions and the magnitude of root respiration and exudation were influenced by treatment. Heating half of a root system at 38C increased the amount of 14C respired from the heated side and increased the total CO2respired from the nonheated (30C) half. Exposure of both root halves to 42C resulted in membrane damage that increased the loss of 14C-labeled photosynthates through leakage into the medium.

THE INFLUENCE OF SUPRAOPTIMAL ROOT-ZONE TEMPERATURES ON 14C PHOTOSYNTHATE PARTITIONING IN ILEX CRENATA THUNB. `ROTUNDIFOLIA'

Ilex crenata Thunb. `Rotundifolia' split-root plants were grown for 3 weeks at root-zone temperatures of 30/30, 30/34, 30/38, 30/42, 34/34, 38/38 and 42/42. The 38 C root-zone temperature treatment was the upper threshold for a number of growth and physiological parameters. A portion of the root system grown at near optimum temperatures could compensate in terms of shoot growth for part of the root system exposed to supraoptimal root-zone temperatures up to the 38 C critical threshold. Higher root-zone temperatures did not affect photosynthetic rates or root:shoot ratios, but altered photosynthate partitioning to different stem and root sinks. Although no differences were found for total 14C partitioned to the roots, partitioning of the 14C into soluble and insoluble fractions and the magnitude of root respiration and exudation were influenced by treatment. Heating half of a root system at 38 C increased the amount of 14C respired from the heated side and increased the total CO2 respired from the non-heated (30 C) half. Exposure of both root halves to 42 C resulted in membrane damage which increased the leakage of 14C photosynthates into the medium.