Shoot Temperature Research Articles

Plant Shoot Age and Temperature Effects on Essential Oil Yield and Oil Composition of Rose-Scented Geranium (Pelargonium sp.) Grown in South Africa

A field experiment was conducted during 2005/2006 at the Experimental Farm of the University of Pretoria, South Africa to investigate the changes in herbage yield, oil yield and the variation in the concentration of the major components of rose-scented geranium. The essential oil was isolated by steam-distillation and the composition was determined by gas chromatography (GC). The results revealed that herbage yield increased over time in the first four (autumn/winter) and five (spring/summer) months of the respective harvesting cycles. The plants harvested in the spring/summer cycle had higher herbage yield and oil percentage than plants harvested in the autumn/winter cycle. The decrease in oil percentage was associated with low night temperatures in the autumn/winter cycle. The geraniol content decreased with decreasing night temperatures in autumn/winter while the citronellol content showed a tendency to increase. Geraniol followed a similar trend when regressed against plant shoot age in both experimental cycles. The decrease in geraniol content with respect to plant shoot age, therefore, increased the citronellol: geraniol ratio in both experimental cycles. The spring/summer harvesting season was superior to the autumn/winter season in all commercial aspects of rose-scented geranium oil production.

Respiration of the growing shoots of Scots pine

The respiratory CO2 exchange and the growth of the annual shoots were followed in Scots pine (Pinus sylvestris L.) trees growing under extreme continental forest-steppe conditions near the lake Baikal. The temperature coefficient of dark respiration (Q10) in growing shoots dropped down from 3.2–4.0 (in the temperature range of 10–20°C) to 1.5–2.0 (in the temperature range of 20–30°C). The changes in averaged daily respiration rates correlated with the changes in shoot growth increments and temperature (with the multiple determination coefficient of 0.94). Growth respiration of the axial shoots during the phenophase reached 80% of the total respiration costs, with the coefficients of growth respiration and maintenance respiration 0.32 and 0.021. In young crown shoots, the average value of CO2 evolution in the light combined for the whole observation period (years 1976–2004) was about 1 kg/dm2, that is 9% of CO2 evolution from the trunk surface.

The effect of root and shoot temperature of 8°C or 24°C on the uptake and distribution of nitrogen in white clover (Trifolium repens L.)

Limited information is available on the factors influencing the uptake and distribution of nitrogen (N) at low temperatures. This experiment quantified the distribution of N in white clover at root and shoot temperatures of 8°C or 24°C. Stolon tip cuttings of white clover (Trifolium repens L.) were grown in silica sand. After 62 days, plants were transferred to an 8°C or 24°C controlled environment room and, to quantify the distribution of N, a pulse of 15KNO3 was applied. Thereafter, plants were supplied with a complete nutrient solution containing NH4NO3 at a concentration calculated to provide plants with 20% of their N requirement. Plants were harvested at 0, 1, 4, 8, 24, 168, or 336 h. Leaf area and dry weights plus 14N/15N distribution in all fractions and total N concentration were measured. At both temperatures, the dry weights in all fractions increased significantly (P < 0.05) with time. After 336 h the amount of labelled 15N contained in the laminae and petioles was lower at 8°C than at 24°C. The higher 15N recovery in the laminae and petioles, and the higher lamina N%, indicated more N had been transported from the roots to the laminae at 24°C. This investigation suggests that temperature does affect the movement of N around the plant, with a consequent effect on N pool sizes and, hence, growth.

Root Temperature and Aeration Effects on the Protein Profile of Canola Leaves

Canola (Brassica napus L.) is planted in early spring and must survive both low soil temperatures and periods of wet weather. Shoot effects result from both low root temperature and low aeration, but little is known about the interaction between these two environmental factors, particularly with respect to changes in gene products. In this study, canola plants (‘46A65’) were treated in solution culture. Shoot temperatures were maintained at day/night temperatures of 24/18°C, while roots were maintained at an ambient temperature of 24/18°C, or cooled to 10°C. Roots were either aerated, or not aerated to create hypoxic treatments. Plants with roots in cold and hypoxic solution accumulated starch in the root, and had greater reductions in fresh weight and leaf area than those in either cold or hypoxic treatments alone. Twenty‐one changes in protein expression were also found in the cold hypoxic treatment, 17 of which were not found in either cold or hypoxic treatments alone. Gene products up‐regulated in leaves included cytochrome oxidase Subunit I (COX1) in response to hypoxia, elongation factor eEF1 γ chain in plants with cooled roots, and chaperonin 10 when roots were cooled without aeration. Results demonstrate the interaction between multiple stresses on a molecular level, and suggest that flooding under cool soil temperatures will be more detrimental to canola than that which occurs at warmer temperatures.

Effects of different cultivation temperatures on plasma membrane ATPase activity and lipid composition of sugar beet roots

Sugar beet seedlings ( Beta vulgaris L. cv. Monohill) were cultivated for 3 weeks at different root and shoot temperatures and the plasma membranes (PM) from roots were purified by aqueous two-phase partitioning and analyzed for lipid composition and ATPase activities. Lipid analyses, undertaken immediately after PM purification from the roots, showed that a low root zone temperature (10 °C) decreased the ratio between the major lipids phosphatidylcholine (PC) and phosphatidylethanolamine (PE). A low temperature in the root environment increased the mol% of PE and decreased the mol% of phosphatidic acid (PA), independent on the shoot growth temperature. A low temperature also decreased the mol% of linoleic acid (18:2) and increased mol% of linolenic acid (18:3) in the analyzed lipid classes, especially in PC and PE. The ratio between acyl chain lipids and protein generally increased in PM from roots grown at 10 °C, compared with higher temperature. The changes in lipid composition correlated with changes in ATPase activities, detected as hydrolyses of MgATP. The kinetic parameters, K m and V of the PM H +ATPase in roots increased at a low cultivation temperature, independent on shoot temperature. Moreover, Arrhenius analyses showed that the transition temperature was independent of both root or shoot growth temperature at 10–24 °C, whereas the activation energy of the ATPase was dependent on the growth temperature of the root, and independent on shoot temperature. Thus, acclimation processes can take place in roots, irrespective of the shoot temperature.

Ecophysiological analysis of two arctic sedges under reduced root temperatures.

Shoot physiological activity in arctic vascular plants may be controlled by low soil temperatures. While leaves may be exposed to moderate temperatures during the growing season, root temperatures often remain near freezing. In this study, two tundra sedges, Eriophorum vaginatum and Carex bigellowii, were subjected to reduced soil temperatures, and photosynthetic parameters (light saturated photosynthesis A(max), variable to maximal fluorescence and F(v)/F(m) stomatal conductance) and abscisic acid concentrations were determined. Stomatal conductance and A(max) for both E. vaginatum and C. bigellowii strongly decreased with declining soil temperatures. Decreasing soil temperature, however, impacted F(v)/F(m) to a much lesser degree. Root and leaf ABA concentrations increased with decreasing root temperature. These observations support the contention that soil temperature is a significant photosynthetic driving factor in arctic sedges exposed to variable root and shoot temperatures. Because these two species comprise approximately 30% of the vascular ground cover of wet tussock tundra, the soil temperature responses of these sedges potentially scale up to significant effects on ecosystem carbon exchange.

Impact of root and shoot temperature on bud dormancy and floral induction in lychee ( Litchi chinensis Sonn.)

Potted lychee trees (cv. Tai so) with mature vegetative flushes were grown under three day/night temperature regimes known to induce floral (18/13 °C), intermediate (23/18 °C) and vegetative (28/23 °C) shoot structures. Heating roots respective to shoots accelerated bud-break and shoot emergence, but reduced the level of floral initiation in emergent shoots. At 18/13 °C, root temperatures of 20 and 25 °C decreased the period of shoot dormancy from 9 weeks to 5 and 3 weeks, respectively. A root temperature of 20 °C also increased the proportion of both leafy and stunted panicles to normal leafless panicles, and reduced the number of axillary panicles accompanying each terminal panicle. A root temperature of 25 °C produced only vegetative shoots. At 23/18 °C, heating roots increased the proportion of vegetative shoots and partially emerged buds to leafy and stunted panicles as well as accelerating bud-break. Cooling of roots in relation to the shoot resulted in non-emergence of buds at both 28/23 and 23/18 °C. Bud-break did not occur until root cooling was terminated and root temperature returned to that of the shoot. At 23/18 °C, subsequent emergent shoots had a greater proportion of leafy panicles relative to control trees. At 28/23 °C, all emergent shoots remained vegetative. Lychee floral initiation is influenced by both root and shoot temperature. Root temperature has a direct effect on the length of the shoot dormancy period, with high temperatures reducing this period and the subsequent level of floral initiation. However, an extended period of dormancy in itself is not sufficient for floral initiation, with low shoot temperatures also a necessary prerequisite.

Composition and Quality of Wheat Grown Under Different Shoot and Root Temperatures During Maturation

ABSTRACTDiminished quality of wheat (Triticum aestivum L.) from high temperature during maturation is usually attributed to direct effects of the stress on the shoots or grain. However, the upper soil temperature approaches the air temperature, and roots are highly sensitive and interact profoundly with other plant parts. The objective of this study was to determine the effect of differential shoot and root temperatures on quality of hard red spring wheat (cv. Len). Plants were grown in hydroponic containers at 15/10°C day/night until 10 days after anthesis, when shoot/root treatments of 15/15°C, 15/30°C, 30/15°C, and 30/30°C were imposed until the grain ripened. Both high shoot and high root temperature affected quality of the grain. Kernel size and weight were diminished more by high root than by high shoot temperature, but flour yield was decreased significantly only by the 30/30°C treatment. The percentage of starch in B granules was reduced by high shoot temperature, and the diameter of A granules was decreased by all heat treatments. Amylose concentration was increased by high temperatures of both shoot and root, resulting in decreased pasting characteristics. Flour protein increased after all heat treatments, but high shoot temperature decreased the polymer‐to‐monomer ratio and unextractable polymeric protein and it affected dough mixing. We concluded that stress on roots directly affects properties of the grain that are important for milling and baking.

Accumulation of starch in wheat grain under different shoot/root temperatures during maturation.

The impact of high temperatures on accumulation of starch in the grain of wheat (Triticum aestivum L.) is usually attributed to direct effects of the stress on the enzymes involved. However, roots are extremely sensitive to temperatures that can be as high as those experienced by the shoots, and their role in whole-plant responses should be considered. Wheat (cv. Len) was grown at 15/15, 30/15, 15/30, and 30/30˚C shoot/root temperatures during maturation, and accumulation of dry matter and N, contents of sucrose and starch, and activities of enzymes in the pathway of starch assimilation in the endosperm, were measured weekly. Dry matter and N accumulation were affected more by root than by shoot temperatures. High whole-plant temperatures (30/30˚C) accelerated linear grain growth but diminished the duration of assimilation, the contents of sucrose and starch, and the activities of the enzymes involved. The effects of high root temperature (15/30˚C) resembled those of high whole-plant temperature, whereas low root temperature (30/15˚C) tended to ameliorate them. Sucrose synthase and soluble starch synthase were affected more than the other enzymes by high shoot and/or root temperature. However, treatments that caused the lowest activities resulted in the fastest, but briefest, linear rates of grain growth. We concluded that shoots and roots interact in the response of wheat to high temperature, and that stress on both organs affects accumulation of starch in grain.

Effect of whole-tree harvesting on microclimate during establishment of second rotation forestry

The effect of whole-tree harvesting on microclimate during the restocking of three upland forest sites in the UK was studied from January 1994 to November 1999. Mean soil temperatures at 0.1 m depth were greater during summer months and reduced during winter months at all three sites in whole-tree harvested treatments compared to those where harvest residues were retained on site. Whole-tree harvesting also affected mean shoot temperatures (0.3 m above ground level) at each of the three sites and, for much of the time, the effects on shoot temperatures were the reverse of those observed for the soil. In comparison to conventional harvesting, shoots on whole-tree harvested plots tended to be cooler in spring and summer whereas the same shoots were warmer during winter. Whole-tree harvesting increased mean wind speeds measured at 0.3 m by around 40% at each of the three sites during the first year after replanting. This effect diminished to 26, 20 and 5% over the following 3 years at the one site studied for the longest period, located in Kielder Forest, Northumberland. The biomass of 4-year-old Sitka spruce trees was greater on whole-tree harvested plots at two sites where growth was increased by fertiliser applications but was not affected by harvesting treatment at the more fertile third site in which no response to fertiliser was observed.

Mineral Nutrition of Tomato under Diurnal Temperature Variation of Root and Shoot

Should root and shoot temperature vary in synchrony to optimize nutrient uptake, particularly when there is a large difference in temperature from day to night (DIF) of air and soil? To answer this question, tomato (Lycopersicon esculentum Mill.) seedlings were grown in greenhouses with the air heated to give either a +14°C DIF or a +5°C DIF in air temperature with a 16°C mean. The root medium was either unheated except by the air, or heated to 21°C constantly, only in the day, or only in the night. Experiments were repeated in early March and April in two years. Overall, growth was faster and there were higher concentrations of elements in leaves under +5°C compared with +14°C air DIF. Root‐zone heating significantly increased growth and nutrition, compared with no heating. There was a trend in growth and nutrient concentration with timing of root heating: constant > day > night. These differences in growth and nutrition were similar under a +5°C or +14°C air DIF, and they were slight compared with no root‐zone heating. For most nutrients, coordination of root and shoot activity related to uptake and metabolism did not require synchronous variation of air and soil temperature. Uptake and transport of nitrate was an exception. Heating roots in the day resulted in the highest nitrate concentration in leaves under a +14°C air DIF, whereas heating constantly was optimal under a +5°C DIF. Our results indicate nitrate metabolism did benefit from synchronous variation in air and root temperature.

Root growth and nutrient element status of creeping bentgrass cultivars differing in heat tolerance as influenced by supraoptimal shoot and root temperatures

This study was designed to determine and compare root growth and nutritional responses of creeping bentgrass cultivars that differ in heat tolerance to differential, supraoptimal, shoot and root temperatures. Shoots and roots of ‘Penncross’ (heat sensitive) and ‘L‐93’ (heat tolerant) were exposed to four air/soil temperature regimes (20/20°C‐control, 20/35°C, 35/20°C, and 35/35°C) in water baths and growth chambers. Exposing roots to supraoptimal root temperature (35°C) while maintaining shoots at normal temperature (20°C) or particularly at 35°C reduced root fresh weight, root number, and contents of nitrogen (N), phosphorus (P), and potassium (K) in shoots and roots and accelerated root death for both cultivars. High root temperature had greater detrimental effects on root growth and nutrient element accumulation than high shoot temperature for both cultivars. A low root temperature at supraoptimal shoot temperature improved root growth, reduced root mortality; and increased N, P, and K contents in shoots and roots. Among the three nutrient elements, K was the most sensitive to changes in root temperature. L‐93 generally maintained higher fresh weight and number of roots and higher N, P, and K contents in shoots and roots, particularly K in roots, under high root (20/35°C) or shoot/root (35/35°C) temperatures. The results indicated that root growth and nutrient element accumulation, particularly of K, played an important role in creeping bentgrass tolerance to heat stress imposed on shoots by high air temperature or to roots by high soil temperatures. The enhanced root growth and nutrient element relations with a low root temperature at supraoptimal ambient temperatures could lead to the improved shoot growth in cool‐season grasses observed under these conditions.

142 Root Mortality and Nutrient Uptake of Creeping Bentgrass in Response to Differential Root and Shoot Temperatures

This study was designed to compare and determine root growth and nutritional responses of creeping bentgrass cultivars that differ in heat tolerance to deferential, supraoptimal shoot and root temperatures. Shoots and roots of `Penncross' (heat sensitive) and `L-93' (heat tolerant) were exposed to four differential air/soil temperature regimes (20/20-control, 20/35, 35/20, and 35/35 °C) in water baths and growth chambers. Exposing roots to supraoptimal root temperature (35 °C) while maintaining shoots at normal temperature (20 °C), or at 35 °C in particular, reduced root fresh weight, root number, the content of N, P, and K in shoots and roots, and accelerated root death for both cultivars. High root temperature had a greater detrimental effects on root growth and nutrient accumulation than high shoot temperature for both cultivars. Reducing root temperature at supraoptimal shoot temperature improved root growth, reduced root mortality, and increased N, P, and K content in shoots and roots. Among the three nutrient elements, K was the most sensitive to changes in root temperature. L-93 generally maintained higher root fresh weight and number, and N, P, K content in shoots and roots, particularly K in roots, under high root (20/35 °C) or shoot/root (35/35 °C) temperatures. The results indicated that root growth and nutrient accumulation, particularly K, played an important role in creeping bentgrass tolerance to heat stress imposed to shoots by high air temperature or to roots by high soil temperatures. Reducing root temperature under supraoptimal ambient temperatures enhanced root growth and nutrient relations, and thus could lead to the improved shoot growth in cool-season grasses as reported previously.

Interaction Effects of Nitrogen and Phosphorus on Nodulation in Red Clover (Trifolium pratense L.)

Red clover (Trifolium pratense L.) is one of the most important plants in forage production, especially in northern areas. Fertilisation practices are focused on high yield and forage quality but effects of nutrients on nodulation and N2 fixation are poorly understood. The aim of this work was to study how nitrogen (N) and phosphorus (P) separately as well as in combination affected nodulation. Red clover plants were grown in pots with gravel in a greenhouse for 11 weeks. To resemble field conditions the root temperature was kept lower than the shoot temperature. Plants were given five different combinations of N and P concentrations during growth. The result showed that at high N concentrations P had a counteracting effect on the N inhibition. The N2-fixation parameters, nodule number, nodule dry matter and specific nitrogenase activity, were six times higher in plants grown with high N and high P than in plants with high N and low P. When the N2-fixation parameters and the dry matter of roots and shoots were related to total plant dry matter, there was a stronger effect of P on nodulation parameters than on roots and shoots. This indicates that P has a direct effect on the N2-fixation parameters, rather than an indirect effect via increased plant growth. These results demonstrate the importance to studying the effects of more than one nutrient at a time.

Modeling of Shoot Elongation and Leaf Appearance in Potted Mulberry

SummaryTo predict mulberry growth, modeling of shoot elongation and leaf appearance of potted mulberries was attempted. Potted mulberries were grown in temperature-controlled phytotrons. Changes in the rates of mulberry shoot elongation and leaf appearance, and the relationship between the rates and temperature were investigated. The rate of shoot elongation increased until the shoot length reached about 100cm, and became stable thereafter. The rates of both shoot elongation and leaf appearance increased with the increase in temperature from 15°C to 27°C, but not at higher temperatures. However, the rate of leaf appearance increased with the rise in temperature up to 31°C. On the basis of these results, the shoot elongation and leaf appearance of mulberry in phytotron were simulated in non -linear regression models. The relationship between the rate of shoot elongation and temperature, and the rate of leaf appearance and temperature in the model fitted to measured values very well, and that between the rate of shoot elongation and shoot length in the models tended to be in accordance with the measured values. Shoot length and leaf number predicted by the model were in concurrence with the measured values.

The influence of supra-optimal root-zone temperatures on growth and stomatal conductance in Capsicum annuum L.

Pepper (Capsicum annuum L.) plants were grown aeroponically in a Singapore greenhouse under natural diurnally fluctuating ambient shoot temperatures, but at two different root-zone temperatures (RZTs): a constant 20 +/- 2 degrees C RZT and a diurnally fluctuating ambient (A) (25-40 degrees C) RZT. Plants grown at 20-RZT had more leaves, greater leaf area and dry weight than A-RZT plants. Reciprocal transfer experiments were conducted between RZTs to investigate the effect on plant growth, stomatal conductance (gs) and water relations. Transfer of plants from A-RZT to 20-RZT increased plant dry weight, leaf area, number of leaves, shoot water potential (psi shoot), and gs; while transfer of plants from 20-RZT to A-RZT decreased these parameters. Root hydraulic conductivity was measured in the latter transfer and decreased by 80% after 23 d at A-RZT. Transfer of plants from 20-RZT to A-RZT had no effect on xylem ABA concentration or xylem nitrate concentration, but reduced xylem sap pH by 0.2 units. At both RZTs, gs measured in the youngest fully expanded leaves increased with plant development. In plants with the same number of leaves, A-RZT plants had a higher gs than 20-RZT plants, but only under high atmospheric vapour pressure deficit. The roles of chemical signals and hydraulic factors in controlling gs of aeroponically grown Capsicum plants at different RZTs are discussed.

Development of Frost Tolerance in Winter Wheat as Modulated by Differential Root and Shoot Temperature

Abstract: Winter wheat plants (Triticum aestivum L. cv. Urban), grown in nutrient solution, were exposed to differential shoot/root temperatures (i.e., 4/4, 4/20, 20/4 and 20/20 d̀C) for six weeks. Leaves grown at 4d̀C showed an increase in frost tolerance from ‐ 4d̀C down to ‐ 11 d̀C, irrespective of root temperature. In 4/20 d̀C plants, high root temperature decreased the rate of hardening of the leaves, but did not influence the final level of frost tolerance. In roots grown at 4d̀C frost tolerance increased from ‐ 3 d̀C down to ‐ 4 d̀C, independently of shoot temperature. An accumulation of soluble sugars in the leaves was only observed when both shoot and root were grown at 4d̀C and was not correlated with final frost tolerance achieved. However, the rate of hardening was correlated with the soluble sugar concentration. An increase in root soluble sugar concentration was exclusively observed in roots exposed to a temperature of 4d̀C, irrespective of shoot temperature. Proline concentration only increased in plant parts exposed to a temperature of 4d̀C. The present results indicate that the importance of root temperature in low‐temperature hardening of winter wheat is limited, even though exposure to differential root and shoot temperatures brought about pronounced changes in growth, soluble sugar concentration, insoluble sugar concentration and proline concentration in roots and leaves.

Development of Frost Tolerance in Winter Wheat as Modulated by Differential Root and Shoot Temperature

Abstract: Winter wheat plants (Triticum aestivum L. cv. Urban), grown in nutrient solution, were exposed to differential shoot/root temperatures (i.e., 4/4, 4/20, 20/4 and 20/20 d̀C) for six weeks. Leaves grown at 4d̀C showed an increase in frost tolerance from - 4d̀C down to - 11 d̀C, irrespective of root temperature. In 4/20 d̀C plants, high root temperature decreased the rate of hardening of the leaves, but did not influence the final level of frost tolerance. In roots grown at 4d̀C frost tolerance increased from - 3 d̀C down to - 4 d̀C, independently of shoot temperature. An accumulation of soluble sugars in the leaves was only observed when both shoot and root were grown at 4d̀C and was not correlated with final frost tolerance achieved. However, the rate of hardening was correlated with the soluble sugar concentration. An increase in root soluble sugar concentration was exclusively observed in roots exposed to a temperature of 4d̀C, irrespective of shoot temperature. Proline concentration only increased in plant parts exposed to a temperature of 4d̀C. The present results indicate that the importance of root temperature in low-temperature hardening of winter wheat is limited, even though exposure to differential root and shoot temperatures brought about pronounced changes in growth, soluble sugar concentration, insoluble sugar concentration and proline concentration in roots and leaves.

596 Landscape Design and History Affect Urban Plant Gas Exchange Parameters

Effects of landscape design and land use history on gas exchange parameters were evaluated for woody plants in a factorial site matrix of formerly desert or agricultural land uses and xeric or mesic residential landscape designs within the metropolitan area of Phoenix, Ariz. Remnant Sonoran Desert sites and an alfalfa agricultural field functioned as controls. Residential landscapes and the alfalfa field were irrigated regularly. Monthly instantaneous measurements of maximum leaf and stem carbon assimilation (A), conductance (gs), and transpiration (E) were made within three replicates of each site type during 1998 and 1999. Measurements were repeated monthly on three woody plant life forms: trees, shrubs, and ground covers. Assimilation fluxes were not related to former land use, but were lower for plants in xeric compared with those in mesic landscapes. Transpiration fluxes were higher for plants in formerly agricultural sites than in formerly desert sites, and were lower in xeric than in mesic landscape design. Compared with plants in residential landscapes, A and E fluxes were generally higher for plants in the agricultural control sites and were lower for plants at the desert control sites. Plant instantaneous transpiration efficiency (ITE = A/E) was higher in formerly agricultural sites than in formerly desert sites but was not affected by landscape design. Patterns of A, gs, and shoot temperature at irrigated sites suggest that maximum plant carbon assimilation was not limited by shoot conductance but was more responsive to shoot temperature. Similarities in patterns of ITE between plants in the different landscape design types suggest that xeric and mesic landscape plants do not differ in terms of water use efficiency.

The effect of rhizosphere dissolved inorganic carbon on gas exchange characteristics and growth rates of tomato seedlings

The possibility that an enhanced supply of dissolved inorganic carbon (DIC = CO 2 + HCO 3 - ) to the root solution could increase the growth of Lycopersicon esculentum (L.) Mill, cv. F144 was investigated under both saline and non-saline root medium conditions. Tomato seedlings were grown in hydroponic culture with and without NaCl and the root solution was aerated with CO 2 concentrations in the range between 0 and 5000 μmol mol -1 . The biomass of both control and salinity-stressed plants grown at high temperatures (daily maximum of 37°C) and an irradiance of 1500 μmol m -2 s -1 was increased by up to 200% by enriched rhizosphere DIC. The growth rates of plants grown with irradiances of less than 1000 μmol m -2 s -1 were increased by elevated rhizosphere DIC concentrations only when grown at high shoot temperatures (35 C) or with salinity (28°C). At high light intensities, the photosynthetic rate, the CO 2 and light-saturated photosynthetic rate (J max ) and the stomatal conductance of plants grown at high light intensity were lower in plants supplied with enriched compared to ambient DIC. This was interpreted as 'down-regulation' of the photosynthetic system in plants supplied with elevated DIC. Labelled organic carbon in the xylem sap derived from root DI 14 C incorporation was found to be sufficient to deliver carbon to the shoot at rates equivalent to 1% and 10% of the photosynthetic rate of the plants supplied with ambient- and enriched-DIC, respectively. It was concluded that organic carbon derived from DIC incorporation and translocated in the xylem from the root to the shoot may provide a source of carbon for the shoots, especially under conditions where low stomatal conductance may be advantageous, such as salinity stress, high shoot temperatures and high light intensities.