Photosynthesis, photochemistry and antioxidative defence in response to two drought severities and with re-watering in Allocasuarina luehmannii

Gas exchange, chlorophyll fluorescence and water potentials, together with ascorbate and glutathione concentrations, were studied during moderate and severe drought stress and in response to re-watering in Allocasuarina luehmannii seedlings. Moderate drought stress (MS) decreased stomatal conductance (g_s) and net CO₂ assimilation rates (A) to ∼40% and ∼60% of control values, respectively, and caused decreases in internal CO₂ concentration (C_i) and maximum light use efficiency of light-acclimated photosystem II (PSII) centres (Fv'/Fm'). Severe drought stress (SS) decreased g_s and A to ∼5% and ∼15% of the control values, respectively, and caused increases in C_i and PSII excitation pressure (1 − qP), as well as decreases in water potentials, effective quantum yield of PSII (ΦPSII), maximum efficiency of PSII (Fv/Fm) and Fv'/Fm'. Ascorbate and glutathione concentrations remained unaffected by drought treatments, but ascorbate became more oxidised under severe stress. MS seedlings recovered within 1 day (C_i, Fv'/Fm') to 1 week (A, g_s) of re-watering. In comparison, SS seedlings had longer-lasting after-stress effects, with recovery of many variables (g_s, water potentials, Fv/Fm, ΦPSII, Fv'/Fm') taking between 1 and 3 weeks from re-watering. We found no indication that interaction with antioxidants played a significant role in recovery. In conclusion, A. luehmannii seedlings appear to function normally under moderate drought, but do not seem to have particular metabolic tolerance mechanisms to endure severe drought, which may have implications for its persistence under climate change at the drier margins of its distribution.     

Interaction of enriched CO2 and water stress on the physiology of and biomass production in sweet potato grown in open-top chambers

Abstract. The objective of this study was to investigate the effects of water stress in sweet potato ( Ipomoea batatas L. [Lam] 'Georgia Jet') on biomass production and plant-water relationships in an enriched CO₂ atmosphere. Plants were grown in pots containing sandy loam soil (Typic Paleudult) at two concentrations of elevated CO₂ and two water regimes in open-top field chambers. During the first 12 d of water stress, leaf xylem potentials were higher in plants grown in a CO₂ concentration of 438 and 666 μmol mol⁻¹ than in plants grown at 364 μmol mol⁻¹. The 364 μmol mol⁻¹ CO₂ grown plants had to be rewatered 2 d earlier than the high CO₂-grown plants in response to water stress. For plants grown under water stress, the yield of storage roots and root: shoot ratio were greater at high CO₂ than at 364 μmol mol⁻¹; the increase, however, was not linear with increasing CO₂ concentrations. In well-watered plants, biomass production and storage root yield increased at elevated CO₂, and these were greater as compared to water-stressed plants grown at the same CO₂ concentration.     

Ontogeny affects response of northern red oak seedlings to elevated CO2 and water stress: II. Recent photosynthate distribution and growth

Northern red oak in the western Lake States area of the USA exists on the most xeric edge of its distribution range. Future climate-change scenarios for this area predict decreased water availability along with increased atmospheric CO₂. We examined recent photosynthate distribution and growth in seedlings as a function of CO₂ mole fraction (400, 530 and 700 μmol mol⁻¹ CO₂), water regime (well watered and water-stressed), and ontogenic stage. Water stress effects on growth were largely offset by elevated CO₂.
Water stress increased root mass ratio without concurrently increasing allocation of recent photosynthate to the roots. However, apparent sink strength of water-stressed seedlings at the completion of the third growth stage tended to be greater than that of well watered seedlings, as shown by continued high export, which may contribute carbon reserves to support preferential root growth under water-stressed conditions.
Elevated CO₂ decreased apparent shoot sink strength associated with the rapid expansion of the third flush. Carbon resources for the observed enhanced growth under elevated CO₂ could be provided by enhanced photosynthetic rate over an increased leaf area (Anderson & Tomlinson, 1998, this volume).
Increased sink strength of LG seedlings under water-stressed conditions, together with decreased apparent shoot sink strength associated with growth in elevated CO₂ provide mechanisms for offsetting water stress effects by growth in elevated CO₂.
Careful control of ontogeny was necessary to discern these changes and provides further evidence of the need for such careful control in mechanistic studies.     

Distribution and metabolism of root-applied cytokinins in Pisum sativum

When N ⁶ [8–¹⁴C] furfuryladenine was applied to the intact root system of Pisum sativum L. cv. Meteor seedlings it was almost completely metabolised to other compounds within 24 h. Of the total activity recovered from the plants 94.5% was retained in the root system itself. ¹⁴C was recovered in a number of ethanol-soluble compounds and in ribonucleic acid, deoxyribonucleic acid and protein fractions of roots, stems, leaves and axillary buds. In rapidly growing axillary buds released from apical dominance by removal of the shoot apex the combined nucleic acid fractions accounted for 63.3% of the total ¹⁴C recovered from these organs. Xylem exudate collected from decapitated plants 0 to 12 h after supplying N ⁵[8–¹⁴C]furfuryladenine to the roots consistently contained a single major ¹⁴C-labelled compound which, in three different solvent systems, had the same Rf values as a major endogenous cytokinin isolated from the xylem of unlabelled plants. The content of N ⁶ [8–¹⁴C] furfuryladenine itself in the xylem exudate was always low and in some experiments it could not be detected.
It is suggested that part of the label from N ⁶ [8- ¹⁴CJfurfuryladenine taken up by the intact root system may have become incorporated in an endogenous cylokinin before export to the shoot.     

Does soil nitrogen influence growth,water transport and survival of snow gum (Eucalyptus pauciflora Sieber ex Sprengel.) under CO2 enrichment?

Eucalyptus pauciflora Sieber ex Sprengel. (snow gum) was grown under ambient (370  µ L L⁻¹) and elevated (700  µ L L⁻¹) atmospheric [CO₂] in open-top chambers (OTCs) in the field and temperature-controlled glasshouses. Nitrogen applications to the soil ranged from 0.1 to 2.75 g N per plant. Trees in the field at high N levels grew rapidly during summer, particularly in CO₂-enriched atmosphere, but suffered high mortality during summer heatwaves. Generally, wider and more numerous secondary xylem vessels at the root–shoot junction in CO₂-enriched trees conferred fourfold higher below-ground hydraulic conductance. Enhanced hydraulic capacity was typical of plants at elevated [CO₂] (in which root and shoot growth was accelerated), but did not result from high N supply. However, because high rates of N application consistently made trees prone to dehydration during heatwaves, glasshouse studies were required to identify the effect of N nutrition on root development and hydraulics. While the effects of elevated [CO₂] were again predominantly on hydraulic conductivity, N nutrition acted specifically by constraining deep root penetration into soil. Specifically, 15–40% shallower root systems supported marginally larger shoot canopies. Independent changes to hydraulics and root penetration have implications for survival of fertilized trees under elevated atmospheric [CO₂], particularly during water stress.     

Fluoride inhibits root water transport and affects leaf expansion and gas exchange in aspen (Populus tremuloides) seedlings

The effects of sodium fluoride (0.3, 5 and 10 m M NaF) on root hydraulic conductivity, and gas exchange processes were examined in aspen ( Populus tremuloides Michx.) seedlings grown in solution culture. A long-term exposure of roots to NaF significantly decreased root hydraulic conductivity ( L _p) and stomatal conductance ( g _s). Root absorbed NaF significantly affected electrolyte leakage in leaf tissues and substantially restricted leaf expansion. NaF did not significantly affect leaf chlorophyll contents but decreased net photosynthesis ( P _n). A short-term exposure of excised roots to 5 m M NaF and KF significantly decreased root water flow ( Q _v) with a concomitant decline in root respiration and reduced g _s when applied through intact roots or excised stems. The same molar concentration of NaCl also decreased Q _v and g _s in intact seedlings, but to a lesser extent than NaF or KF, and did not significantly affect root respiration. The results suggest that fluoride metabolically inhibited Q _v or L _p, probably by affecting water channel activity. We suggest that the metabolic inhibition of L _p by root-absorbed fluoride affected gas exchange and leaf expansion in aspen seedlings.     

Nitrogen assimilation and transport in carob plants

Most of the nitrate reductase activity (80%;) in carob ( Ceratonia siliqua L. cv. Mulata) is localised in the roots. The nitrate concentration in the leaves is relatively low compared to that in the roots, suggesting that nitrate influx into the leaf may be a major factor limiting the levels of nitrate reductase in the shoot. Transport of nitrate from root to shoot appears limited by the entrance of nitrate into the xylem. In order to study this problem, we determined the nitrate concentrations and nitrate reductase activities along the roots of nitrate-grown plants, as well as the composition of the xylem sap and the nitrate levels in the leaves. Some of the the bypocotyl, in order to bypass the loading of nitrate into the xylem of the roots. The results show that the loading of nitrate into the xylem is a limiting step.
The cation and anion concentrations of nitrate- and ammonium-fed plants were similar, showing almost no production of organic anions. In both nitrate- and ammonium-fed plants, the transport of nitrogen from root to shoot was in the form of organic nitrogen compounds. The nitrate reductase activity in the roots was more than sufficient to explain all the efflux of OH⁻ into the root medium of nitrate-fed plants. In carob plants the K-shuttle may thus be operative to a limited extent only, corresponding to between 11 and 27%; of the nitrate taken up. Potassium seems to be the cation accompanying stored nitrate in the roots of carob seedlings, since they accumulate nearly stoichiometric amounts of K⁺ and NO⁻₃.     

Colloidal suspensions of clay or titanium dioxide nanoparticles can inhibit leaf growth and transpiration via physical effects on root water transport

A laboratory investigation was conducted to determine whether colloidal suspensions of inorganic nanoparticulate materials of natural or industrial origin in the external water supplied to the primary root of maize seedlings ( Zea mays L.) could interfere with water transport and induce associated leaf responses. Water flow through excised roots was reduced, together with root hydraulic conductivity, within minutes of exposure to colloidal suspensions of naturally derived bentonite clay or industrially produced TiO₂ nanoparticles. Similar nanoparticle additions to the hydroponic solution surrounding the primary root of intact seedlings rapidly inhibited leaf growth and transpiration. The reduced water availability caused by external nanoparticles and the associated leaf responses appeared to involve a rapid physical inhibition of apoplastic flow through nanosized root cell wall pores rather than toxic effects. Thus: (1) bentonite and TiO₂ treatments also reduced the hydraulic conductivity of cell wall ghosts of killed roots left after hot alcohol disruption of the cell membranes; and (2) the average particle exclusion diameter of root cell wall pores was reduced from 6.6 to 3.0 nm by prior nanoparticle treatments. Irrigation of soil-grown plants with nanoparticle suspensions had mostly insignificant inhibitory effects on long-term shoot production, and a possible developmental adaptation is suggested.     

Effect of elevated CO2 on carbon partitioning and exudate release from Plantago lanceolata seedlings

Plantago lanceolata L. seedlings were grown in sand microcosm units over a 43‐day experimental period under two CO₂ regimes (800 or 400 µmol mol⁻¹) to investigate the effect of elevated atmospheric CO₂ concentration on carbon partitioning and exudate release. Total organic carbon (TOC) content of the collected exudate material was measured throughout the experimental period. After 42 days growth the seedlings were labelled with [¹⁴C]‐CO₂ and the fate of the label within the plant and its release by the roots monitored. Elevated CO₂ significantly (P ≤ 0.001) enhanced shoot, root and total dry matter production although the R:S ratio was unaltered, suggesting no alteration in gross carbon partitioning. The cumulative release of TOC (in mg C) over 0‐42 days was unaltered by CO₂ treatment however, when expressed as a percentage of net assimilated C, ambient‐grown plants released a significantly (P≤ 0.001) higher percentage from their roots compared to elevated CO₂‐grown plants (i.e. 8 vs 3%). The distribution of ¹⁴C‐label was markedly altered by CO₂ treatment with significantly (P≤ 0.001) greater per cent label partitioned to the roots under elevated CO₂. This indicates increased partitioning of recent assimilate below‐ground under elevated CO₂ treatment although there was no significant difference in the percentage of ¹⁴C‐label released by the roots. Comparison of plant C budgets based on ¹⁴C‐pulse‐chase methodology and TOC measurements is discussed.     

Structural and physiological changes in the roots of tomato plants over-expressing a basic peroxidase

Previous studies on the tomato ( Lycopersicon esculentum Mill.) peroxidase TPX1, including the development of transgenic tomato over-expressing this gene, supported an involvement of this peroxidase in the synthesis of lignin and suberin. The transgenic plants showed a wilty phenotype at flowering, but the relationship between this role in ligno-suberization and this phenotype was not clear. In the present study a histological approach and the measurement of water-related parameters have been performed in order to obtain an insight into the origin of this phenotype. Clear differences between transgenic and non-transgenic roots were observed in the cross-sections of the basal root zones where secondary growth was evident. The diameter of the xylem vessel was diminished in the transgenic plants. Total area corresponding to xylem in the basal cross-sections decreased 3.9 fold in the transgenic roots. In addition, the radial and outer tangential walls of the exodermis cells were more ligno-suberized in transgenic than in non-transgenic plants. After fruit set, predawn and midday water potentials were lower in transgenic than in-non-transgenic plants. At midday, the stomatal conductance was also lower in the transgenic plants, 494±69 versus 594±60 mmol m⁻² s⁻¹. Root hydraulic conductances of the transgenic and non-transgenic plants were 1.4±0.38 and 3.47±0.19 g water min⁻¹ MPa⁻¹, respectively. The results obtained support that the phenotype is caused by the anatomical differences found in the transgenic roots. These differences would be the cause of a increased resistance to water flow in the roots that would negatively affect the water supply to the shoot and, as a consequence, resulted in a decreased water potential in the leaves.     

Adaptation of potassium translocation into the shoot of maize (Zea mays) to shoot demand: Evidence for xylem loading as a regulating step

In order to manipulate the shoot demand for mineral nutrients per unit root weight, maize ( Zea mays L.) seedlings were grown in nutrient solution with different temperatures in the root zone and at the shoot base. The aerial temperature was kept uniform at 24/20°C day/night. At a root zone temperature (RZT) of 24°C, shoot growth was reduced by decreasing the shoot base temperature (SBT) to 12°C; at a RZT of 12°C, shoot growth was increased by raising the SBT to 24°C. At both RZT root growth was not affected by the SBT. Thus, the shoot demand for nutrients per unit root was either increased by raising, or decreased by lowering the SBT. The net uptake rate of potassium (K), as determined from accumulation rates between sequential harvests, was not affected within the first 3 days after lowering the SBT, whereas net translocation rates of K into the shoot and translocation rates in the xylem exudate of decapitated plants were markedly reduced. Obviously, translocation of K into the shoot seems to be regulated independently from K uptake into the root cells. Translocation rates of K in the xylem exudate of decapitated plants were markedly reduced when the nutrient solution was replaced by CaCl₂ solution during exudation. But, depending on the SBT before decapitation, significant differences remained in the translocation rates of K even when K uptake from the nutrient solution was prevented.
From the results it is suggested that xylem loading of K is regulated separately from K uptake from the external solution and that the adaptation of K translocation to shoot demand is coupled with an altered capacity of the root for xylem loading.     

Electrical signalling and cytokinins mediate effects of light and root cutting on ion uptake in intact plants

Nutrient acquisition in the mature root zone is under systemic control by the shoot and the root tip. In maize, exposure of the shoot to light induces short-term (within 1–2 min) effects on net K⁺ and H⁺ transport at the root surface. H⁺ efflux decreased (from −18 to −12 nmol m⁻² s⁻¹) and K⁺ uptake (∼2 nmol m⁻² s⁻¹) reverted to efflux (∼−3 nmol m⁻² s⁻¹). Xylem probing revealed that the trans-root (electrical) potential drop between xylem vessels and an external electrode responded within seconds to a stepwise increase in light intensity; xylem pressure started to decrease after a ∼3 min delay, favouring electrical as opposed to hydraulic signalling. Cutting of maize and barley roots at the base reduced H⁺ efflux and stopped K⁺ influx in low-salt medium; xylem pressure rapidly increased to atmospheric levels. With 100 m m NaCl added to the bath, the pressure jump upon cutting was more dramatic, but fluxes remained unaffected, providing further evidence against hydraulic regulation of ion uptake. Following excision of the apical part of barley roots, influx changed to large efflux (−50 nmol m⁻² s⁻¹). Kinetin (2–4  µ m ), a synthetic cytokinin, reversed this effect. Regulation of ion transport by root-tip-synthesized cytokinins is discussed.     

Ontogenetic changes in potassium transport in xylem of tomato

The influence of plant ontogeny on xylem exudate K⁺ concentrations and K⁺ transport to the shoot was studied in both nutrient-solution and field-grown tomato plants ( Lycopersicon esculentum ).
K⁺ concentrations in xylem exudate from decapitated plants decreased during tomato plant development from a high of 12 m M to a low of 5 m M . In the nutrient-solution plants, the most rapid decline occurred during the vegetative growth phase, while in field-grown plants, the xylem K⁺ concentrations remained high during an-thesis and then subsequently declined. The rapid decline in nutrient-solution plants might be related to a decrease in the absorptive efficiency of the root system. In field-grown plants, a reduction in the availability of assimilates to the root might account in part for the decrease in xylem exudate K⁺ concentrations. The volume (ml h⁻¹ plant⁻¹) and the net rates of K⁺ exudation (mmol h⁻¹ plant⁻¹) decreased dramatically as the fruits approached maturity. Since only a small reduction in xylem exudate K⁺ concentrations occurred during fruiting, the hydraulic conductivity of the root system decreased as the tomato plants aged. It is proposed that the ontogenetic changes in xylem transport of K⁺ contribute to a reduction in leaf free space K⁺ concentration which would explain the decline in tomato leaf K⁺ concentrations.     

Leaf xylem embolism, detected acoustically and by cryo-SEM, corresponds to decreases in leaf hydraulic conductance in four evergreen species

Hydraulic conductance of leaves ( K _leaf) typically decreases with increasing water stress. However, the extent to which the decrease in K _leaf is due to xylem cavitation, conduit deformation or changes in the extra-xylary pathway is unclear. We measured K _leaf concurrently with ultrasonic acoustic emission (UAE) in dehydrating leaves of two vessel-bearing and two tracheid-bearing species to determine whether declining K _leaf was associated with an accumulation of cavitation events. In addition, images of leaf internal structure were captured using cryo-scanning electron microscopy, which allowed detection of empty versus full and also deformed conduits. Overall, K _leaf decreased as leaf water potentials ( Ψ _L) became more negative. Values of K _leaf corresponding to bulk leaf turgor loss points ranged from 13 to 45% of their maximum. Additionally, Ψ _L corresponding to a 50% loss in conductivity and 50% accumulated UAE ranged from −1.5 to −2.4 MPa and from −1.1 to −2.8 MPa, respectively, across species. Decreases in K _leaf were closely associated with accumulated UAE and the percentage of empty conduits. The mean amplitude of UAEs was tightly correlated with mean conduit diameter ( R ² = 0.94, P  = 0.018). These results suggest that water stress-induced decreases in K _leaf in these species are directly related to xylem embolism.     

Ion supply capacity of roots in relation to rejuvenation of primary leaves in vivo

Removal of the shoot above the primary node (detopping) of 3-week-old bean plants ( Phaseolus vulgaris L. cv. Contender) altered the metabolism and development of the remaining leaves. An increase in levels of chlorophyll, protein, stomatal opening, photosynthesis and growth, i.e. rejuvenation of primary leaves, was established within 7 days of detopping. These levels were maintained while the primary leaves of equivalent intact plants senesced.
The flux of xylem solution (mineral ions, cytokinins and water) into leaves is related to the leaf area to be supplied and root supply capacity; it has been suggested that detopping leads to an increased availability of root-supplied solutes and hence rejuvenation of the remaining leaves. This assumes however that root output of solutes is not decreased by the defoliation treatment.
We found that root output of ions (electrical conductivity of passive xylem exudate) in detopped plants was 30% lower than in intact plants after 24 h and 60% lower after 7 days. The output of Ca²⁺, Mg²⁺ and K⁺ were similarly reduced 7 and 14 days after detopping as were fresh and dry weights of roots. Furthermore, neither the calculated xylem flux of ions nor directly measured levels of Ca²⁺, Mg²⁺ and K⁺ were significantly increased in leaves of detopped plants during their rejuvenation. We therefore conclude that root output is tightly coupled to shoot demand and that the apparent rejuvenation of primary leaves caused by detopping bean plants is not a consequence of increased xylem flux of mineral ions into the leaves.     

Effect of gibberellic acid on K+ (Rb+) uptake and transport in sunflower roots

Four-week-old sunflower plants ( Helianthus annuus L. cv. Halcón), grown in different nutrient solutions, were used to study the effects of gibberellic acid (GA₃) on K⁺ (Rb⁺) uptake by roots or transport to the shoot. Gibberellic acid application to the nutrient solution did not affect the exudation process of excised roots. When GA₃ was sprayed on leaves 2 to 6 days before excising the roots, the rate of exudation and the K⁺ flux increased. When the exudation study was done keeping the roots in a nutrient solution in which Rb⁺ replaced K⁺, the GA₃ effects were evident also on Rb⁺ uptake and transport. In intact plants, GA₃ increased the Rb⁺ transported to the shoot but did not affect Rb⁺ accumulation in the root. It is suggested that these GA₃ effects can be explained if it is assumed that GA₃ acts on the transport of ions to the xylem vessels.     

Does calcium ameliorate the negative effect of NaCl on melon root water transport by regulating aquaporin activity?

The hydraulic conductance ( L ₀) of detached, exuding root systems from melon ( Cucumis melo cv. Amarillo oro) was measured. All plants received a half-strength Hoagland nutrient solution, and plants stressed either solely with NaCl (50 mM) or with NaCl (50 mM) following treatment (2 d) with CaCl₂ (10 mM) were compared with controls and CaCl₂-treated (10 mM) plants. The L ₀ of NaCl-treated plants was markedly decreased when compared to control and CaCl₂-treated plants, but the decrease was smaller when NaCl was added to plants previously treated with CaCl₂. A similar effect was observed when the flux of Ca²⁺ into the xylem and the Ca²⁺ concentration in the plasma membrane of the root cells were determined. In control, CaCl₂- and NaCl + CaCl₂-treated plants, HgCl₂ treatment (50 μM) caused a sharp decline in L ₀ to values similar to those of NaCl-stressed roots, but L ₀ was restored by treatment with 5 mM DTT. However, in NaCl roots only a slight effect of Hg²⁺ and DTT were observed. The effect of all treatments on L ₀ was similar to that on osmotic water permeability ( P _f) of individual protoplasts isolated from roots. The results suggest that NaCl decreased the passage of water through the membrane and roots by reducing the activity of Hg-sensitive water channels. The ameliorative effect of Ca²⁺ on NaCl stress could be related to water-channel function.     

Contribution of current carbon assimilation in supplying root exudates of Lolium perenne measured using steady-state C labelling

Coupling growth of Lolium perenne L. in sterile solution culture with steady-state ¹³CO₂ labelling allowed quantification of the contribution of C, assimilated either before or after a specific time point, both to plant compartments and root exudates. Plants were grown for 27 days in atmospheres containing CO₂ with δ ¹³C signatures of either −13.5 or −36.1‰. Air supplies to plants were then reciprocally switched to the opposing signature (day 0), plants were destructively harvested and root exudates collected over the next 8 days. Following the switch, C assimilated after day 0 and transported to the roots initially only appeared in root tips, later appearing in both tip and non-tip material. The δ ¹³C signature of the root exudate changed exponentially with time. Assimilation pre- and post-day 0 contributed equally to exudate C at 4.5 days. Beyond day 8, assimilation pre-day 0 still contributed 41.7% of exudate C. Over all 8 days, a linear relationship existed between the δ ¹³C signatures of root tips and exudate, suggesting that all newly assimilated C in the exudate was from root tips. Results imply pulse-labelling approaches to study root exudates are discriminative in the sources of exudates labelled and in the sites from which exudation occurs.     

Kinetin application to roots and its effect on uptake, translocation and distribution of nitrogen in wheat (Triticum aestivum) grown with a split root system

The root systems of wheat seedlings ( Triticum aestivum L. cv. SUN 9E) were pruned to two seminal roots. One of the roots was supplied with a suboptimal level of NO₃, the other was deprived of N. Different levels of kinetin were supplied to the NO₃-deprived roots. Root respiration and the increment of C and N in the roots were measured to determine the C/N ratio of the phloem sap feeding the NO₃-deprived roots. Thus, it was possible to determine retranslocation of N from the shoots to the roots, as affected by the rate of kinetin application. It was calculated that the C/N ratio of phloem sap feeding roots growing without kinetin was ca 61. Kinetin application increased this ratio to ca 75, partly due to decreased translocation of N from the shoots back to the roots. Kinetin application decreased the proportion of N that was retranslocated to the roots after translocation to the shoots. Kinetin increased the rate of NO₃ uptake per root and the rate of N incorporation in both roots and shoots by ca 60%, but had no effect on shoot dry matter production. In control plants at most 70% of the N incorporated in the NO₃-fed roots could have been imported from the shoots, whilst kinetin application reduced this value to ca 40%. Thus root growth was not fully dependent on a supply of N via the phloem.
It is concluded that cytokinins affect the pattern of N-translocation in wheat plants by increasing incorporation of N in dry matter of the shoot, thus leaving less for export. Cytokinins did not play a major role in the regulation of shoot growth and the shoot to root ratio of the present plants.     

Physiological function of water channels as affected by salinity in roots of paprika pepper

The sap flow (Jv) and the osmotic pressure-dependent hydraulic conductance (L₀) of detached exuding root systems from paprika pepper plants (cv. Albar) were measured. Plants stressed with NaCl (30 m M ) and with six times the macronutrients of the Hoagland nutrient solution (6×HNS) were compared with controls grown in complete Hoagland nutrient solution. Jv of +NaCl and +6×HNS plants decreased markedly, but recovered to values similar to those of controls after removal of the treatments. Hydraulic conductance L₀ was always less in NaCl plants than in controls and 6×HNS. A total increase in the ion concentration of the xylem (except Na⁺ and Cl⁻) was observed with both treatments. In control and 6×HNS plants, HgCl₂ treatment (50 μ M ) caused a sharp decline in L₀ to values similar to those of NaCl-stressed roots, but were restored by treating with 5 m M dithiothreitol (DTT). However, in NaCl roots only a slight effect of Hg²⁺ and DTT was observed. In each treatment, there was no difference in the flux of K⁺ into the xylem after HgCl₂ and DTT application. The results suggest that NaCl decreased L₀ of the roots by reducing either the activity or abundance of Hg-sensitive water channels. The putative reduction in water-channel function of NaCl-treated plants did not seem to be due to the osmotic effect.