The effect of root cooling on hormone content, leaf conductance and root hydraulic conductivity of durum wheat seedlings (Triticum durum L.)

Root cooling of 7-day-old wheat seedlings decreased root hydraulic conductivity causing a gradual loss of relative water content during 45 min (RWC). Subsequently (in 60 min), RWC became partially restored due to a decrease in transpiration linked to lower stomatal conductivity. The decrease in stomatal conductivity cannot be attributed to ABA-induced stomatal closure, since no increase in ABA content in the leaves or in the concentration in xylem sap or delivery of ABA from roots was found. However, decreased stomatal conductance was associated with a sharp decline in the content of cytokinins in shoots that was registered shortly after the start of root cooling and linked to increases in the activity of cytokinin-oxidase. This decrease in shoot cytokinin content may have been responsible for closing stomata, since this hormone is known to maintain stomatal opening when applied to plants. In support of this, pre-treatment with synthetic cytokinin benzyladenine was found to increase transpiration of wheat seedlings with cooled roots and bring about visible loss of turgor and wilting.     

Stomatal Behaviours of Aspen (<Emphasis Type="Italic">Populus tremuloides</Emphasis>) Plants in Response to Low Root Temperature in Hydroponics

Hydroponic-grown seedlings of aspen (Populus tremuloides Michx.) were used to investigate how low root temperatures (5°C) affect stomatal conductance and water relations. An isohydric manner of the stomatal behaviour was found with the seedlings when their roots were subjected to the low temperature. Stomatal conductance rapidly and dramatically reduced in response to the low root temperature, while the xylem water potential did not significantly alter. Under the low root temperature, pH value of the xylem sap increased from 6.15 to 6.72 within the initial 4 h, while abscisic acid (ABA) concentration increased by the eighth hour of treatment. K⁺ concentration of the xylem sap significantly decreased within the 8th h and then reversed by the 24th h. The ion change was accompanied by a decrease and then an increase in the electrical conductivity, and an increase and then a decrease in the osmotic potential. The tempo of physiological responses to the low root temperature suggests that the rapid pH change of the xylem sap was the initial factor which triggered stomatal closure in low temperature-treated seedlings, and that the role of the more slowly accumulating ABA was likely to reinforce the stomatal closure. Xylem sap from the seedlings subjected low root temperature affected stomatal aperture on leaf discs when they were floated on the sap solution. The stomatal aperture correlated (P = 0.006) with the changed pattern of [K⁺] in the sap while the range of pH or ABA found in the xylem sap did not influence stomatal aperture of leaf discs in solution. The effect of xylem sap on stomatal aperture on leaf discs was different from on stomatal conductance in the intact seedlings. Comparison was made with previous study with the soil-grown seedlings.     

Some effects of air temperature and humidity on crop and leaf photosynthesis, transpiration and resistance to gas transfer

Measurements of carbon dioxide exchange and transpiration were made, at various air temperatures, on wheat and barley using a field enclosure system. From these were derived the stomatal and mesophyll resistances to carbon dioxide transfer. Optimum temperatures for net CO₂ uptake were about 24°C for wheat and barley. Above these optima, as temperature increased so net CO₂ uptake rates decreased, because of increasing stomatal and mesophyll resistances; transpiration rates decreased in wheat but were constant in barley. In laboratory growth cabinets, wheat plants were subjected to different regimes of temperature and humidity. Optimum temperature for net CO₂ uptake of individual leaves was 25°C. At constant humidity, a decline in net uptake rates above 25°C was associated with large increases in mesophyll resistance. At a constant 25°C, as the vapour pressure deficit (v.p.d. was increased above 1 k Pa (10 mb) v.p.d. the net uptake declined, with an increase in mesophyll resistance and a small increase in stomatal resistance. When the v.p.d. exceeded 1 k Pa at a temperature of 30°C, conditions that are experienced by field plants, then there were large increases in both mesophyll and stomatal resistances and the net uptake rates declined. Photo-respiration, as measured by CO₂ uptake in oxygen-free air, was independent of temperature, but both dark respiration and CO₂ compensation concentration increased with temperature.     

Cytokinin-producing, plant growth-promoting rhizobacteria that confer resistance to drought stress in Platycladus orientalis container seedlings

One of the proposed mechanisms through which plant growth-promoting rhizobacteria (PGPR) enhance plant growth is the production of plant growth regulators, especially cytokinin. However, little information is available regarding cytokinin-producing PGPR inoculation on growth and water stress consistence of forest container seedlings under drought condition. This study determined the effects of Bacillus subtilis on hormone concentration, drought resistance, and plant growth under water-stressed conditions. Although no significant difference was observed under well-watered conditions, leaves of inoculated Platycladus orientalis (oriental thuja) seedlings under drought stress had higher relative water content and leaf water potential compared with those of noninoculated ones. Regardless of water supply levels, the root exudates, namely sugars, amino acids and organic acids, significantly increased because of B. subtilis inoculation. Water stress reduced shoot cytokinins by 39.14 %. However, inoculation decreased this deficit to only 10.22 %. The elevated levels of cytokinins in P. orientalis shoot were associated with higher concentration of abscisic acid (ABA). Stomatal conductance was significantly increased by B. subtilis inoculation in well-watered seedlings. However, the promoting effect of cytokinins on stomatal conductance was hampered, possibly by the combined action of elevated cytokinins and ABA. B. subtilis inoculation increased the shoot dry weight of well-watered and drought seedlings by 34.85 and 19.23 %, as well as the root by 15.445 and 13.99 %, respectively. Consequently, the root/shoot ratio significantly decreased, indicative of the greater benefits of PGPR on shoot growth than root. Thus, inoculation of cytokinin-producing PGPR in container seedlings can alleviate the drought stress and interfere with the suppression of shoot growth, showing a real potential to perform as a drought stress inhibitor in arid environments.     

Photosynthesis of plant régénérants. Specificity ofin vitro conditions and plantlet response

Regenerants from tobacco(Nicotiana tabacum L. cv. White Burley) leaf segments cultivatedin vitro in vessels with solid agar medium under usual conditions (plantlets) grew under very low irradiance (I = 40 μxmol m?2 s?1), very high relative humidity (more than 90%) and decreased CO₂ concentration (c_a) during light period. In comparison with seedlings of a similar number of leaves and similar total leaf area grown in sand and nutrient solution, the plantlets had lower dry mass of shoots and roots per plant and thinner leaves almost without trichomes and epicuticular waxes. Due to a low transpiration rate under high relative humidity the water potential of plantlet leaves was higher than that of seedling leaves and the difference in water potential between leaves and medium was lowei. The rate of water loss from leaves detached from plantlets was considerably faster than that from seedlings under the same conditions (I = 110 μrnol m^?2; s^?1, temperature 30 °C, relative humidity 50 %). Net photosynthetie rates (P_n) of leaves of plantlets and seedlings measured under saturating I, natural ca and the leaf temperature 20 °C were similar, nevertheless the shape of curves relating Pn to c» indicated some differences in photosynthetie parameters(e.g. saturation of Pn under lower ca> higher CO2 compensation concentration in plantlets than in seedlings). Similarly compensation and saturating I were lower in plantlets than in seedlings. The shape of transpiration curves as well as the expressive linear phases of P_N(ca) and P_N(I) curves of plantlet leaves indicated ineffective stomatal control of gas exchance. These results were confirmed by microscopic observations of stomatal movementsin situ     

Effect of ethylene perception inhibitor on growth, water relations, and abscisic acid content in wheat plants under water deficit

The effects of treatment with 1-methylcyclopropene (1-MCP, the inhibitor of ethylene receptors) of 7-day-old wheat (Triticum durum Desf., cv. Bezenchukskaya 139) seedlings on growth characteristics, water relations, and the content of phytohormones during three days after watering cessation were studied. In treated seedlings, a decrease in the water content in the substrate resulted in a decrease in stomatal conductance in the leaves by one day earlier than in untreated seedlings. This could be related to the more rapid and substantial accumulation of ABA in treated plants. There was no clear relationship between changes in the content of cytokinins and water relations under the influence of 1-MCP under drought conditions. The role of ethylene and ABA in the regulation of growth and water relations in plants suffering from water deficit is discussed.     

Changes in cytokinin levels of Phalaenopsis leaves at high temperature

The effect of high temperatures on cytokinin levels in Phalaenopsis hybrida leaves was investigated. Endogenous cytokinins were identified and quantified in Phalaenopsis leaves grown under high temperature conditions (30/25 °C day/night) using high performance liquid chromatography, bioassay and gas chromatography-selected ion monitoring-mass spectrometry. After 5 and 20 d of low temperature (25/20 °C day/night), zeatin, zeatin riboside and dihydrozeatin levels in the leaves were higher than that in leaves subjected to high temperature treatments. When Phalaenopsis leaves were exposed to low temperatures, about 76 % of the free cytokinins detected were of the zeatin-type. Glucoside cytokinins in the leaves increased significantly 5 d following high temperatures, and the rate of increase in glucoside cytokinins corresponded to the duration of high temperatures. At the same time, zeatin riboside and dihydrozeatin declined significantly following high temperature application. A significant accumulation of glucoside cytokinins, zeatin-9-glucoside, zeatin-O-glucoside, zeatin riboside-O-glucoside, and dihydrozeatin-O-glucoside was observed 20 d following high temperatures. These results suggest that high temperatures lead to an accumulation of glucoside cytokinins and a reduction of free base and riboside cytokinins.     

Effects of Four Temperature Regimens on the Gibberellin Content of Winter Wheat cv. Kharkov

Kharkov 22 M.C. winter wheat was grown continuously from seed under four temperature regimens; 2°C; 2°C then transferred to 20°C; 6°C, and 20°C. Plants were harvested at the 4-leaf stage (except the 2°C plants which were harvested at the 3-leaf stage) and the levels of endogenous gibberellins (GAs) were measured and their cold hardiness determined. The GA content of 20°C and the 6°C seedlings were about equal and greater (by 4- to 30-fold) than the GA content of 2°C seedlings that had been transferred to 20°C (i.e. vernalized). The GA content of the 2°C seedlings was least of all, being 28- to 320-fold less than the 20°C seedlings. At harvest the leaves of the 20°C seedlings were somewhat larger than those of the seedlings grown at 2°C and transferred to 20°C. The leaves of the 6°C seedlings were very much smaller and the leaves of the 2°C seedlings were smallest of all. It is concluded that while GAs may play a role in controlling leaf size and level of cold hardiness of wheat, they are not the most important factor.     

The effect of cytokinins on nitrate reductase activity

Cytokinins in addition to nitrate induce nitrate reductase activity (NRA) in some plants. Effects of cytokinins onNRA was investigated in stem pith parenchyma of kale, intact wheat and barley seedlings and isolated cucumber cotyledons. The most profound effect onNRA was found in barley and wheat seedlings.NRA in seedlings sprayed with 100 μM 6-benzylaminopurine (BAP) for three subsequent days was increased in leaves and decreased in roots. These changes were further enhanced in seedlings grown in nutrient solution lacking nitrate:NRA in wheat and barley leaves was increased by 57% and 202%, respectively, in plants supplied with nitrate theNRA increase was not significant: in wheat and barley leaves by 22% and 9%, respectively. Similar effect of BAP and kinetin was found in kale stem parenchyma and cucumber cotyledons. The cytokinin kinetin or BAP alone increasedNRA about twice in kale and three times in cucumber. Addition of nitrate to the medium enhanced the effect of kinetin in kale discs, but the two effects were not additive. Additive effect of nitrate and BAP onNRA was found in cucumber cotyledons in light. In general NRA was more affected by cytokinins in intact seedlings of wheat and barley as compared to explanted tissue of kale and cucumber, and lack of nitrogen made their effect more expressive.     

Acclimation effects of heat waves and elevated [CO2] on gas exchange and chlorophyll fluorescence of northern red oak (Quercus rubra L.) seedlings

Heat wave frequency and intensity are predicted to increase. We investigated whether repeated exposure to heat waves would induce acclimation in Quercus rubra seedlings and considered [CO₂] as an interacting factor. We measured gas exchange and chlorophyll fluorescence of seedlings grown in 380 (C _A) or 700 (C _E) μmol CO₂ mol^?1, and three temperature treatments (ambient, ambient +3 °C, and an ambient +12 °C heat wave every fourth week). Measurements were performed during the third and fourth +12 °C heat waves (July and August 2010) at Whitehall Forest, GA, USA. Additionally, previously unexposed seedlings were subjected to the August heat wave to serve as a control to determine acclimation of seedlings which were previously exposed. Seedlings with a history of heat wave exposure showed lower net photosynthesis (A _net) and stomatal conductance (on average ?47 and ?38 %, respectively) than seedlings with no such history, when both were subjected to the same +12 °C heat wave. During both heat waves, A _net significantly declined in the +12 °C treatment compared with the other treatments. Additionally, the A _net decline during the August compared with the July heat wave was stronger in C _E than in C _A, suggesting that elevated [CO₂] might have had a negative effect on acclimation capacity. We conclude that seedlings subjected to consecutive heat waves will moderate stomatal conductance outside the heat wave, to reduce water usage at lower temperatures, increasing survival at the expense of carbon assimilation.     

Abscisic Acid may Mediate the Rapid Thermal Acclimatization of Photosynthesis in Wheat

Exposing cold-grown (6°C) plants of a winter cultivar (Avalon)and a spring cultivar (Highbury) of wheat to a single warm night(18 h and 25°C) significantly increased the photosyntheticrate and stomatal conductance of fully-expanded leaves withoutaffecting the calculated, substomatal CO₂ pressure. Prior to the warm-night treatment net photosynthesis and stomatalconductance in Avalon were very small at high leaf temperaturesduring measurement (25–30°C). Such high temperatureinhibitions were not observed in Highbury. Exposure to the warm-nighttreatment alleviated these inhibitions in Avalon. Consequently,the temperature response of photosynthesis was similar in theleaves of the treated cultivars and showed a relatively broadoptimum. The application of exogenous abscisic acid in the transpirationstream to the leaves of treated plants caused a reduction ofsimilar proportions in the steady-state rate of photosynthesisand in stomatal conductance and essentially reversed the effectsof the warm-night treatment in a manner which depended on thecultivar. The endogenous abscisic acid content of leaves declinedby 50% during the warm-night treatment. In conjunction, thesedata suggest that changes in the content of abscisic acid inthe leaf may regulate the observed temperature acclimatizationof apparent photosynthesis in the fully-expanded leaves of cold-grownwheat. Key words: Abscisic acid, photosynthesis, stomatal conductance, temperature, acclimatizatio     

Effects of low nocturnal temperature on photosynthetic characteristics and chloroplast ultrastructure of winter rapeseed

We investigated the effects of low nocturnal temperature on photosynthetic apparatus of winter rapeseed (Brassica campestris L.). An artificial climate chamber was used to simulate the effects of low nocturnal temperature on seedling and stomatal morphology, chloroplast ultrastructure, photosynthetic parameters, and dry matter distribution and accumulation in two winter rapeseed cultivars, Longyou-7 (ultra coldresistant) and Tianyou-2 (weak cold resistance). Compared with those at diurnal/nocturnal temperatures of 20°/10°C (control), rapeseed seedlings at 20°/5°C had increased leaf chlorophyll content, deepened green leaf color, decreased stomatal conductance (G_s), intercellular CO₂ concentration (C_i), and photosynthetic rate (P_n), and improved root/shoot ratio; the majority of stomata remained open in Longyou-7 while those in Tianyou-2 were mostly closed or semi-closed. At diurnal/nocturnal temperatures of 20°/–5°C, rapeseed seedlings had decreased leaf chlorophyll content with increased C_i but decreased G_s and P_n; Tianyou-2 exhibited ruptured chloroplast membrane, dissolved grana, broken stroma lamella, and decreased root/shoot ratio, whereas Longyou-7 had chloroplasts retaining partial structure of grana with a small amount of starch granules in guard cells. Low nocturnal temperature damaged the photosynthetic membrane of chloroplasts and reduced P_n in the leaves of winter rapeseed influencing photosynthetic processes in this crop. The reduction of P_n was mainly related to stomatal limitation at diurnal/nocturnal temperatures of 20°/5°C and non-stomatal limitation at diurnal/nocturnal temperatures of 20°/–5°C.     

The effect of freezing nights on photosynthesis, stomatal conductance, and internal CO2 concentration in seedlings of Engelmann spruce (Picea engelmannii Parry)

Abstract The effect of freezing night temperatures on net photosynthesis, stomatal conductance, and internal CO₂ concentration was investigated in unhardened seedlings of Engelmann spruce. Exposure to – 2.5°C in the dark for 10 h caused a slight and reversible reduction in gas-exchange parameters on the following days. Substantial and irreversible inhibition of photosynthesis occurred after exposure to -4°C or –5°C. Despite a parallel decline in stomatal conductance and net photosynthesis, exposure to a hard freeze caused a decrease in the stomatal limitation to gas exchange. Hard-freeze conditions (less than – 4°C) also caused a decrease in carboxylation efficiency and apparent quantum yield, indicating a freeze-induced failure of the dark reactions and electron transport. There was no significant difference in the photosynthetic response to freezing temperatures in different elevational populations of spruce, although acclimatory adjustments were observed. Gas exchange in seedlings grown under cool conditions (14°C day/9°C night) was less affected and recovered more rapidly after exposure to a hard freeze than in seedlings grown under warm conditions (24°C day/19°C night).     

A Definition of Optimum Nutrient Requirements in Birch Seedlings

Birch seedlings (Betula verrucosa Enrh.) were grown in nutrient solutions with pH varied in the range 2.5 to 6.8 or temperature varied in the range 2.5 to 35°C. The criteria for maximum growth previously established for birch seedlings were used and maintained by means of automatic pH and conductivity titrations with stock solutions containing the optimum nutrient proportions. Both nitrogen sources, NH₄ and NO₃, were present in the solutions. Growth rate was maximum or close to maximum between pH 4.0 and 6.8, whether kept at a specific level or allowed to vary between the extremes. At pH 3.5 and lower, the calcium uptake was decreased and root damage was observed. The seedlings has also a high dry matter content and obviously an unsatisfactory water balance. pH 2.5 was rapidly lethal. Growth rate was linearly correlated with solution temperature up to 20°C. Temperatures above 30°C, especially in the range 32.5 to 35°C, resulted in rapid decrease in growth rate. The nutrient contents in the seedlings were strongly affected by solution temperature in the low as well as in the high range when expressed on a dry weight basis. However, this effect was almost entirely attributable to changes in dry matter content. When expressed on a fresh weight basis, nutrient uptake and nutrient status of the seedlings appeared to be optimum throughout, although a variation remained since the varying dry matter content is included in the fresh weight basis. The results indicate, in agreement with the literature, that disturbed water uptake and water balance is the way in which growth is affected by root medium temperature. Similarly, extremely low pH levels in the nutrient solution meant root damage, although birch seedlings appear comparatively insensitive to pH variations. Thus, the growth technique used supplied the seedlings with adequate nutrients, so that the criteria used in the definition of nutrient requirements in birch seedlings are valid within wide ranges of solution pH and temperature; and other factors than nutrition determine growth.     

Photosynthesis and ultrastructure of photosynthetic apparatus in tomato leaves under elevated temperature

The microstructure of leaves and ultrastructure of chloroplasts were examined in tomato (Lycopersicon esculentum L.) plants treated with elevated temperature. Plants were exposed to 35°C for 30 d after florescence. The plants grown continuously under 25°C served as controls. Compared with the controls, the net photosynthetic rate (P _N) in stressed plants decreased significantly. Stomatal conductance, intercellular CO₂ concentrations, the rate of transpiration, and the limitation of stomatal conductance showed that the decrease in P _N was caused mainly by nonstomatal restrictions. Meanwhile, stomata density increased significantly in the stressed plants. The stomata status of opening and closing became disorganized with a prolonged 35°C exposure. The damage of chloroplast membrane occurred earlier and was more serious in the plants under elevated temperature. At the same time, the thylakoids were loosely distributed with lesser grana, but the number of lipid droplets increased in chloroplasts. The number of starch grains in chloroplasts increased first and then decreased. In addition, the length of the main nerve in leaves increased and the main vein showed distortion in the plants stressed by 35°C. An increase was observed in the number of cells on the abaxial side of the main vein and these cells were overly congregated. The thickness of a vertical section became thinner in the stressed leaves. The cells of the upper epidermis thinned, and the ratio of palisade tissue to spongy tissue decreased. Generally, the photosynthetic apparatus of tomato changed significantly and the changed chloroplast ultrastructure might be one of the important reasons that caused the decrease of P _N under 35°C.     

The Role of Endogenous Abscisic Acid in the Response of Plants to Stress

When a continuous stream of warm air (38°C) was directedon to the leaves of dwarf bean seedlings they wilted and thengradually regained turgor. This process of adaptation was accompaniedby an increasing abscisic acid (ABA) level in the leaves andan increase in leaf resistance (R_L). It is suggested that theleaf-water deficit induced by the warm-air treatment causedthe increase in ABA level and that the latter was responsiblefor stimulating stomatal closure, enabling the plants to regainfull turgor. A similar type of adaptation, brought about byan increased level of ABA in the leaves, is believed to occurin tomato, dwarf bean, and wheat plants when they are flooded.Predictably, in rice, a species adapted to a flooded environment,seedlings showed no increase in ABA level as a result of flooding. It is proposed that adaptation may involve the formation ofan equilibrium between ABA and its conjugate form (i. e. theglucose ester). The ABA-conjugate was observed to disperse slowlyfrom leaves recovering from a water deficit and therefore itmay act as a metabolic ‘back-stop’, enabling the‘free’ ABA level to remain high for a period evenwhen the leaves have regained turgor. Abscisic acid appears to be responsible for alleviating theeffects of water stress in plants, making it possible for plantsto pass through periods of stress with little harm.     

Effect of the nitrogen-containing salt on the content of cytokinins in detached wheat leaves

The dynamics of the cytokinin content in detached leaves of wheat (Triticum durum, cv. Bezenchukskaya 139) seedlings moistened with ammonium nitrate or water (control) was studied by immunoenzyme analysis. Leaf treatment with water was accompanied by a transient accumulation of cytokinins, maybe due to their release from their O-glucosylated forms. An increase in the contents of zeatin and its riboside after their initial decrease in detached leaves treated with ammonium nitrate could not occur due to their release from stored forms (nucleotides or O-glucosides) because the contents of zeatin and its riboside increased simultaneously with the content of stored cytokinins. The accumulation of isopentenyladenosine and zeatin nucleotide, which occurred simultaneously with an increase in the content of zeatin and zeatin riboside, permits a supposition that cytokinins can be synthesized in detached wheat leaves treated with ammonium nitrate.     

Effect of high temperature on active oxygen species,senescence and photosynthetic properties in cucumber leaves

To gain a physiological understanding of the effects of high temperatures on cucumber (Cucumis sativus L.), we subjected seedlings to heat treatment at daytime temperatures of 28 °C, 32 °C, and 36 °C for 7 h a day for 30 days. The amount of active oxygen species, indicators of senescence, and photosynthetic properties in the second and third leaves were determined at the start of temperature treatment and on the 15th and 30th days of treatment. The amount of active oxygen species superoxide in leaves was greatest in the high temperature zones on the 15th day of treatment, and the amount of hydrogen peroxide was greatest in the high temperature zones on both the 15th and 30th days of treatment. The reduction in the amount of protein and the increase in the amount of malondialdehyde, both indicators of senescence, were greatest in the high temperature zones on both the 15th and 30th days of treatment, and the amount of chlorophyll was lowest in the 36 °C zone on the 15th day, and lower in the high temperature zones on the 30th day. It is clear from these results that a large amount of active oxygen species is generated and accumulated in the leaves at high temperatures, and senescence is significantly accelerated. The photosynthetic properties of stomatal conductance, sub-stomatal CO₂ concentration, and transpiration rate were at the same level on both the 15th and 30th days of treatment in all three temperature treatment zones. No significant difference was seen in the net photosynthesis rate between the 28 °C and 32 °C zones, was lower in the 36 °C zone than the 32 °C zone on the 15th day, and lowest in the 36 °C zone on the 30th day. CO₂ intake and water absorption are only mildly affected by high temperatures, and the reduction in net photosynthesis rate due to the 36 °C high temperature stress suggests that the large amount of active oxygen species induces inhibition of photosynthesis and damage to the mechanism of photosynthesis.