THE INFLUENCE OF LOW PLANT WATER POTENTIAL ON THE GROWTH AND NITROGEN METABOLISM OF THE NATIVE CALIFORNIA SHRUB LOTUS SCOPARIUS (NUTT. IN T & G) OTTLEY

Reduced plant water potential, induced by polyethylene glycol in hydroponics, inhibited growth and decreased the number of leaves per branch in the southern California drought-deciduous species Lotus scoparius (Nutt. in T & G) Ottley. Decreasing plant water potential diminished the proportion of large leaves per branch and therefore reduced the leaf area. Nitrate uptake rate decreased with decreasing water potential, although the nitrate ion concentration increased in the roots and the leaves. Ammonium ion concentration increased significantly in the roots at −5 bars and lower osmotic potentials in the root medium. Kjeldahl nitrogen remained the same in all treatments and tissues over the experimental period. It is suggested that the increase in ammonium ion in the roots was due to a decreased rate of ammonium transport caused by low plant water potential. The slight increase in nitrate ion in the roots may correspond to a decrease in nitrate reductase activity. This study indicates that some of the changes in nitrogen metabolism associated with low water potentials in agricultural plants occur also in a plant which experiences frequent droughts in its native habitat.     

Nitrate Reductase Activity and Polyribosomal Content of Corn (Zea mays L.) Having Low Leaf Water Potentials

Desiccation of 8- to 13-day-old seedlings, achieved by withholding nutrient solution from the vermiculite root medium, caused a reduction in nitrate reductase activity of the leaf tissue. Activity declined when leaf water potentials decreased below −2 bars and was 25% of the control at a leaf water potential of −13 bars. Experiments were conducted to determine whether the decrease in nitrate reductase activity was due to reduced levels of nitrate in the tissue, direct inactivation of the enzyme by low leaf water potentials, or to changes in rates of synthesis or decay of the enzyme.     

Recovery of photosynthesis in sunflower after a period of low leaf water potential

Photosynthesis was studied in sunflower plants subjected to 1 to 2 days of desiccation and then permitted to recover. The leaf water potential to which leaves returned after rewatering was dependent on the severity of desiccation and the evaporative conditions. Under moderately evaporative conditions, leaf water potential returned to predesiccation levels after 3 to 5 hours when desiccation was slight. Leaf water potentials remained below predesiccation levels for several days after rewatering when leaf water potentials decreased to −13 to −19 bars during desiccation. Leaf water potential showed no sign of recovery when leaf water potentials decreased to −20 bars or below during desiccation. The lack of full recovery of leaf water potential was attributable to increased resistance to water transport in the roots and stem. The resistance ultimately became large enough to result in death of the leaves because net water loss continued even after the soil had been rewatered.     

Interactive Effects of Thiourea and Phosphorus on Clusterbean Under Water Stress

Effects of phosphorus and thiourea application (either alone or in combination) were studied on clusterbean (Cyamopsis tetragonoloba Taub.) plants subjected to water stress by withholding irrigation at pre- and post-flowering stages in pot culture trial. Water stress significantly decreased shoot water potential, relative water content of leaves, net photosynthetic rate, contents of total chlorophyll, starch and soluble proteins as well as nitrate reductase activity at both the growth stages. Application of phosphorus and thiourea or combined application increased most of these parameters. Results revealed synergistic effects of P and thiourea in enhancing net photosynthesis, leaf area, chlorophyll content and nitrogen metabolism leading to significant improvement in plant growth and seed yield under water stress condition.     

Nitrate reductase activity and chlorophyll content in sun leaves of subtropical Australian closed-forest (rainforest) and open-forest communities

Summary A study of the sun leaves of two closed-forest (rainforest) and eight open-forest communities in subtropical southeast Queensland, Australia, showed that a large number of pioneer woody species in closed-forests had high levels of nitrate reductase (EC 1.6.6.1), whereas only a few herbaceous species in the open-forests showed high levels. There was a continuously declining gradient in nitrate reductase activity from pioneer to mature-forest species in all communities, associated with a decrease in Leaf Specific Area. The level of nitrate reductase activity was lower in certain plant families (including sclerophyllous monocotyledons, small-leaved composites and legumes), but still showed the same general relationship with Leaf Specific Area. The decrease in Leaf Specific Area is associated with an increase in both the dry weight: fresh weight ratio and the chlorophyll a: chlorophyll b ratio of the leaves. Three groups of plants can be recognised by nitrate reductase activity plotted against water content (% fresh weight) of their leaves-(1) pioneer, (2) mature-forest and (3) semi-sclerophyllous species. As the proportion of cytoplasm to structural tissue (indicated by water content) in leaves increases, there is a continuous increase in (a) nitrate reductase activity (b) total chlorophyll (per unit dry weight) (c) the proportion of chlorophyll b to chlorophyll a and (b) chloroplastic isoform of glutamine synthetase. These attributes are associated with high nitrogen content in the leaves and high photosynthetic potentials, resulting in rapid growth rates of pioneer species.     

The effect of soil moisture tension and nitrogen supply on nitrate reduction and accumulation in wheat seedlings

Summary The effect of soil moisture tension on nitrate reductase and on nitrate accumulation in wheat plants was studied. Nitrate reductase activity was inhibited when soil moisture tension was increased to about 3.0 bars associated with a drop in leaf relative water content to about 90 per cent. The decrease in nitrate reductase activity did not result in nitrate accumulation in short-term experiments (10 days) when plants were exposed to only 1–2 cycles of elevated soil moisture tensions. However, when the period of different moisture regimes was extended up to the flag-leaf stage, nitrate accumulated in stressed plants.Significant increase in plant nitrate concentration as a result of increased moisture tensions was only found at the high levels of added nitrogen. On the other hand, moisture tensions had no effect on the content of total nitrogen in wheat shoots, implying that nitrate reduction was rather limiting under stress conditions.An effect of soil moisture tension and nitrogen nutrition on dry matter production by wheat seedlings was also found in the long-term experiment. At the highest dose of soil nitrogen an increase in maximal soil moisture tension from 0.1 to 0.33 bars reduced plant growth; at intermediate nitrogen doses only tension higher than 2 bars reduced growth. Under complete nitrogen deficiency, plant dry matter production was very low and was not affected by soil moisture tensions.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. 1972 Series, No. 2185-E.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. 1972 Series, No. 2185-E.     

Effects of Nitrate Application on Amaranthus powellii Wats. : I. Changes in Photosynthesis, Growth Rates, and Leaf Area

Physiological effects of different nitrate applications were studied using the C₄ plant, Amaranthus powellii Wats. Plants were grown in a controlled environment chamber and watered daily with nutrient solutions containing 45, 10, 5, or 1 millimolar nitrate. Chloride and sulfate were used to keep the cation and phosphate concentrations constant. Total leaf nitrogen concentration, chlorophyll concentration, specific leaf mass, leaf area, relative growth rate, relative leaf growth rate, unit leaf rate (increase of dry mass per unit leaf area per day), net photosynthetic rate, and incident quantum yield decreased with decreasing nitrate concentration. The per cent decrease of unit leaf rate was similar to the decrease of light-saturated net photosynthetic rate; however, the decrease in relative growth rate was less than that of unit leaf rate because leaf area ratio (leaf area per unit dry mass) increased with decreasing nitrate concentration. Essential mineral concentrations per unit leaf area were about equal among all treatments. Leaf expansion, determined by stomatal density, decreased except for the 1 millimolar treatment which showed relatively more cell expansion but less cell division. Decreased nitrate application was correlated with higher osmotic potentials and lower pressure potentials (determined by pressure-volume curves), whereas leaf water potentials were equal among treatments. Even though total leaf area and shoot mass decreased with decreasing applied nitrate, the increase of the leaf area ratio may be related to selection for the highest possible growth rate.     

Water Potential and Stomatal Resistance of Sunflower and Soybean Subjected to Water Stress during Various Growth Stages

Plants of two varieties of soybean (Glycine max (L.) Merr.) and two varieties of sunflower (Helianthus annuus L.) were grown in controlled environments and subjected to water stress at various stages of growth. Leaf resistances and leaf water potentials were measured as stress developed. In soybeans the upper leaf surface had a higher resistance than the lower surface at all leaf water potentials and growth stages. Resistance of the upper surface began to increase at a higher water potential and increased more than the resistance of the lower surface. Resistances returned to prestress values 4 days after rewatering. In sunflowers upper and lower leaf surfaces had similar resistances at all water potentials and growth stages. Leaf resistances were higher in sunflower plants stressed before flowering than in those stressed later. Sunflower plants stressed to −16 bars recovered their prestress leaf resistance and water potential a few days after rewatering, but leaves of sunflower plants stressed to −23 bars died. Leaves of soybean and sunflower plants stressed before flowering suffered less injury than those of older plants and sunflowers stressed after flowering suffered more injury than soybeans.     

Nitrate reductase activity and protein concentration of two populas clones

Nitrate reductase activity and protein percentage of various tree parts of two Populus clones were determined in relation to nitrate ion activity. Nitrogen was supplied as NH(4)NO(3) in a nutriculture system. Wisconsin-5 had significantly greater nitrate reductase activity than Tristis No. 1. Protein percentages of leaf plastochron index 10 leaves (tenth leaf below first leaf lamina exceeding 20 mm in length), bottom leaves, and roots in relation to nitrate ion activity were not appreciably different between clones. The nitrate reductase activity and protein percentage of Tristis No. 1 apex started to level off at the same nitrate ion activity, about 0.09 mm. In Wisconsin-5 apex protein percentage continued to increase at nitrate ion activities where nitrate reductase activity decreases sharply, suggesting that protein nitrogen was being supplied by ammonium ion. The difference in nitrate reductase activity between clones was probably due to genetically determined ability to synthesize nitrate reductase in response to nitrate ion. The expression of nitrate reductase activity was not an index of nitrogen assimilation ability but may be a useful index of growth potential when nitrate ion does not limit nitrate reductase synthesis.     

Influence of Water Stress on Water Relations, Photosynthetic Parameters and Nitrogen Metabolism of Moth Bean Genotypes

Effects of water stress at pre-flowering stage were studied in three genotypes (RMO-40, Maru moth and CZM-32 E) of moth bean [Vigna aconitifolia (Jacq.) Marechal]. Increasing water stress progressively decreased plant water potential, leaf area, net photosynthetic rate, starch and soluble protein contents and nitrate reductase activity while contents of reducing sugars, total soluble sugar, free amino acids and free proline progressively increased. Significant genotypic differences were observed and genotype CZM-32-E displayed a better drought tolerance than other genotypes.     

Drought-induced changes in xylem pH, ionic composition, and ABA concentration act as early signals in field-grown maize (Zea mays L.).

Early signals potentially regulating leaf growth and stomatal aperture in field-grown maize (Zea mays L.) subjected to drought were investigated. Plants grown in a field lysimeter on two soil types were subjected to progressive drought during vegetative growth. Leaf ABA content, water status, extension rate, conductance, photosynthesis, nitrogen content, and xylem sap composition were measured daily. Maize responded similarly to progressive drought on both soil types. Effects on loam were less pronounced than on sand. Relative to fully-watered controls, xylem pH increased by about 0.2 units one day after withholding irrigation (DAWI) and conductivity decreased by about 0.25 mS cm(-1) 1-3 DAWI. Xylem nitrate, ammonium, and phosphate concentrations decreased by about 50% at 1-5 DAWI and potassium concentration decreased by about 50% at 7-8 DAWI. Xylem ABA concentration consistently increased by 45-70 pmol ml(-1) at 7 DAWI. Leaf extension rate decreased 5 DAWI, after the changes in xylem chemical composition had occurred. Leaf nitrogen significantly decreased 8-16 DAWI in droughted plants. Midday leaf water potential and photosynthesis were significantly decreased in droughted plants late in the drying period. Xylem nitrate concentration was the only ionic xylem sap component significantly correlated to increasing soil moisture deficit and decreasing leaf nitrogen concentration. Predawn leaf ABA content in droughted plants increased by 100-200 ng g(-1) dry weight at 7 DAWI coinciding with a decrease in stomatal conductance before any significant decrease in midday leaf water potential was observed. Based on the observed sequence, a chain of signal events is suggested eventually leading to stomatal closure and leaf surface reduction through interactive effects of reduced nitrogen supply and plant growth regulators under drought.     

Water status of soil and vegetation in a shortgrass steppe

Summary In an attempt to describe some major relationships between soil and plant compartments in a shortgrass steppe, the process of water loss from the system and plant water relations throughout a drying cycle were studied. The water supply was manipulated and some soil and plant variables monitored throughout a drying cycle. Leaf conductance and leaf water potential of blue grama (Bouteloua gracilis) were measured periodically at predawn and noon. Soil water content and water potential of different layers were also monitored.Three different periods were distinguished in the water loss process throughout a drying cycle. These distinctions were made taking into account the relative contribution of different soil layers. Leaf conductance and water potential at noon slowly declined throughout the first 50 days of plant growth. After that, they rapidly decreased, reaching values of 0.29 mm s^-1 and-5.0 MPa, respectively. The predawn leaf water potential remained unchanged around-0.5 MPa during the first 45 days, then rapidly decreased. This occurred when soil water of the wettest soil layer was near depletion.Predawn leaf water potentials were highly correlated with water potentials of the wettest layer. Leaf conductance and water potential at noon were correlated with effective soil water potential (soil water potential weighted by the root distribution in the profile). We concluded that root surface area limited the water flow through an important part of the day in this semiarid ecosystem. Axial root resistance did not appear important in determining the equilibrium status between leaves and the wettest soil layer.     

WATER RELATIONS AND GROWTH CHARACTERISTICS OF PROSOPIS GLANDULOSA VAR. TORREYANA IN A SIMULATED PHREATOPHYTIC ENVIRONMENT

Recent studies of Prosopis glandulosa have demonstrated a unique system of a deeply rooted species with significant water stress tolerance. Several growth and developmental characteristics have been correlated with water stress and nitrogen availability during field studies. Here we present a lab experiment in which a phreatophytic regime is simulated and the availability of nitrogen and water are varied. Increased ground water salinity caused lower plant water potentials and greater osmotic adjustment without significant increases in leaf Na⁺ concentrations. Leaf conductance was higher in the higher salinity treatments. Low water potential was also associated with reduced leaf size, reduced leaf area per plant and increased root to shoot ratio. Specific leaf weight and the transpiration ratio were unaffected by the low water potentials induced by increased salinity. Increasing nitrogen availability caused increased growth rates but did not influence water use efficiency. Net assimilation rates increased with increasing nitrogen availability but relative growth rates were more dependent on overall plant size than treatment conditions. The responses of P. glandulosa to the simulated phreatophytic environment were similar to those predicted by field measurements.     

An Improved Method of Soil Moisture Control with Observations on Tomato Growth and Water Uptake

The control of soil moisture depended on the arrangement ofnumerous auto-irrigation cells radially around the plant, allowingrelatively unrestricted spread of the root system, the cellsbeing so placed that no point in the soil was more than about1 in from a water-supplying surface. Water for the cells wassupplied from barostat reservoirs fixed at the levels requiredto give the target hydrostatic pressures of –27, –84,–149, –266 mb at the centre of the cells. The degree of soil moisture control achieved was assessed bysoil moisture tensiometers. Three months after sowing mean matricpotentials were reduced below target levels to minimum valuesof only –33, –104, –178, and –329 mb.At this time some of the tomato plants had leaf areas in excessof 2000 cm² per plant and water uptakes of 200 ml per day perplant in the –27 mb and of 150 ml in the –266 mbtreatment. Leaf area per plant decreased with decrease in matric potential;leaf dry weight decreased with matric potentials less than –84mb; root growth, observed in glass panels, was depressed atboth the largest and smallest matric potentials. The reductionin water uptake with decreasing matric potential was more thancould be accounted for by a smaller leaf area alone.     

Carbon and Nitrogen Assimilation by Vicia faba L. at Low Temperature: the Importance of Concentration and Form of Applied-N

Low temperature (6 C) growth was examined in two cultivarsof Vicia faba L. supplied with 4 and 20 mol m^–3 N as nitrateor urea. Both cultivars showed similar growth responses to increasedapplied-N concentration regardless of N-form. Total leaf areaincreased, as did root, stem and leaf dry weight, total carboncontent and total nitrogen content. In contrast to findingsat higher growth temperatures, 20 mol m^–3 urea-N gavesubstantially greater growth (all parameters measured) than20 mol m^–3 nitrate-N. The increased carbon content per plant associated with increasedapplied nitrate or urea concentration, or with urea in comparisonto nitrate, was due to a greater leaf area per plant for CO₂uptake and not an increased CO₂, uptake per unit area, carbon,chlorophyll or dry weight, all of which either remained constantor decreased. Nitrate reductase activity was substantial inplants given nitrate but negligible in plants given urea. Neitherfree nitrate nor free urea contributed greatly to nitrogen levelsin plant tissues. It is concluded that there is no evidence for a restrictionin nitrate reduction at 6 C, and it is likely that urea givesgreater growth than nitrate because of greater rates of uptake. Vicia faba, broad bean, low temperature growth, carbon assimilation, nitrogen assimilation     

Effect of two contrasted N fertilisations on rapeseed growth and nitrate metabolism

Winter oilseed rape was grown under two nitrogen fertilisation conditions. The N1-plants and N5-plants were respectively supplied with 4.5 g N per plant (N-limiting condition) and 22.5 g N per plant (non-N-limiting condition). Growth parameters and nitrate reducing capacity were monitored at five sampling stages interspersed with ammonium nitrate applications. N5-plants showed a higher growth rate producing more leaves and stems, early flower and silique formation and delayed leaf senescence. They also contained more nitrate and a higher nitrate reductase activity (NRA) especially in leaves which represented the main site of nitrate reduction before flowering. However, stems and siliques contributed to NRA especially in nitrogen-limited plants that lost their leaves early. This present study outlines the importance of siliques as individual sinks reducing nitrate essentially in the pod walls. The soluble protein content decreased in senescing leaves which was indicative of the reallocation of proteinic nitrogen towards stems and siliques. In non-limiting conditions, other nitrogen compounds of leaves may account for such a reallocation. Hence, the timing of leaf fall could contribute to the low nitrogen recovery in rapeseed.     

Relationships between Nitrate Reductase Activity, Plant Weight and Nitrogen Content in Seedlings of Triticum, Aegilops and Triticale

Seedlings of 12 genotypes were grown in pots and watered withnutrient solutions providing 0, 1, 6 and 20 mg equivalents ofnitrate per I. Increasing the external nitrate supply broughtabout increases in plant weight, nitrate, reduced nitrogen concentrationsand in vivo nitrate reductase activity. When given solutioncontaining 6 mg equivalents of nitrate per litre, the plantscontained approximately 0.1 per cent nitrate, a concentrationsimilarto that found in field-grown plantsat thesamestage of growth.At the 6 mg equivalent level nitrate supply, nitrate reductaseactivity was strongly positively correlated with the concentrationsof nitrate and reduced nitrogen and negatively correlated withplant weight. Similar, though weaker, correlations were foundat the lower and higher levels of nitrate supply. The two Triticalegenotypes however, had higher than average plant weights andnitrate reductase activities, while plants of the two Aegilopsspecies weighed much less, especially at the higher levels ofnitrate supply, than the average of all 12 genotypes and generallyhad correspondingly greater nitrate and reduced nitrogen concentrationsand nitrate reductase activities. For individual genotypes,plant weight at a given level of nitrate supply was stronglycorrelated with weight at all other levels. In a second experiment seedlings of 150 genotypes were grownin compost watered with 10 mM Ca(NO₃)₂ Nitrate and reduced nitrogenconcentrations were negatively correlated with plant weightbut there was no significant correlation between nitrate reductaseactivityand either plant weight, nitrate or reduced nitrogen concentration. The results are taken to indicate that genetic factors, otherthan those determining the supply of reduced nitrogen, werelimiting growth and that as a consequence small plants accumulatednitrate and reduced nitrogen compounds in greater concentrationsthan large ones. The greater nitrate concentrations in smallplants may have induced the increased nitrate reductase activityfound in these, as compared with larger plants. Because plantweight varied more than did reduced nitrogen concentration,variation in reduced nitrogen per plant was more highly correlatedwith plant weight than with per cent reduced nitrogen.     

Water relations of cotton plants under nitrogen deficiency

Nitrogen deficiency in cotton plants (Gossypium hirsutum L.) increased the threshold water potentials for both stomatal closure and leaf senescence (defined as loss of chlorophyll and protein) during drought. These studies attempted to answer two questions: (1) What is the basis for the N/water interaction on senescence? (2) Is there a direct relationship between stomatal closure and senescence? Young and old leaves from N-deficient and N-sufficient plants maintained their relative sensitivities to water stress when excised leaf discs were floated on solutions of polyethylene glycol in dim light. In this leaf disc system, both leaf aging and N deficiency increased the threshold water potential for senescence. Leaf aging and N deficiency also decreased the concentration of exogenous abscisic acid necessary to initiate senescence in discs. A role for cytokinins in controlling senescence could not be clearly shown. In young leaves of both N-deficient and N-sufficient plants, stomata closed at water potentials much higher than those causing senescence. During leaf aging, the water potentials causing senescence increased more than those causing stomatal closure. The two processes thus occurred at about the same potentials in the oldest leaves. These data argue against a general cause-and-effect relationship between stomatal closure and senescence. Rather, each process apparently responded independently to absicsic acid accumulated during drought.     

Nitrogen fertilization affects interactions between the components of an insect–fungus–plant tripartite system

1. The chrysomelid beetle, Gastrophysa viridula and the rust fungus, Uromyces rumicis both occur on leaves of Rumex obtusifolius growing in a wide range of soil nutrient conditions. We investigated the effect of fertilizing plants with eight nitrate and four ammonium concentrations on the components of this tripartite interaction in a controlled environment.
2. Leaf weight, area and total plant weight increased as both nitrate and ammonium concentrations increased up to 15mmol l^–1. Between 15 and 50mmol l^–1 added nitrate, leaf and total plant weight decreased. Total plant weight was unaffected by increasing ammonium fertilization from 15 to 25mmol l^–1.
3. The density and percentage of U. rumicis pustules sporulating 8 days after infection decreased with increasing nitrate but were unaffected by increasing the concentration of ammonium fertilization.
4. Leaf area consumed and number of eggs laid by G. viridula decreased as the concentration of nitrate fertilization increased. Increasing the concentration of ammonium decreased leaf area eaten.
5. First instar mortality of G. viridula was increased and gregariousness and adult fecundity was decreased additively by the combination of U. rumicis infection and decreasing the nitrate concentration fed to plants from 10 to 1mmol l^–1.     

Effects of Extended Periods of Osmotic Stress on Water Relationships of Pepper

Pepper plants grown to uniform size in a controlled environment were subjected to an osmotic stress for periods of 1 to 10 days. Polyethylene glycol 400 was used as the osmotic agent. Leaf area of the plants, grown under uniform conditions, was proportional to the weight of the plants. This relationship was not altered by reduction in rate of growth due to a decrease in osmotic potential of the nutrient solution. The rate of transpiration of the pepper plants decreased as the osmotic potential of the nutrient solution was decreased. The reduction in rate of transpiration was most rapid when the osmotic potential was reduced from ?0.5 to ?7.5 bars. There was continued reduction in the rate of transpiration with change in potential to ?12.5 bar but this change was less than that at the higher potentials. The rate of transpiration remained at a reduced rate for as long as the plants were growing in the solution with low osmotic potential. Alternating the osmotic potential of the nutrient solution between ?0.5 and ?5.0 bar did not change the response to the ?5.0 tension. The reduction in rate of transpiration resulting from the lowering of the osmotic potential by addition of NaCl was similar to that produced by addition of polyethylene glycol. Water potential, osmotic potential, relative water content and stomatal movement were all in dynamic equilibrium with the water content of the leaves. The water content of the leaves was regulated by the supply and demand. In these investigations the demand remained constant. The supply was altered by decreasing the difference in water potential between leaf and substrate and by an increase in resistance to flow of water in the roots as a result of the decrease in osmotic potential of the nutrient solution.