WATER RELATIONS OF WATER-STRESSED,SPLIT-ROOT C4 (SORGHUM BICOLOR; POACEAE) AND C3 (HELIANTHUS ANNUUS; ASTERACEAE) PLANTS

Sorghum [Sorghum bicolor (L.) Moench] and sunflower (Helianthus annuus L.) were grown in a greenhouse with roots divided between sand irrigated with nutrient solution (–0.097 MPa) or nutrient solution containing polyethylene glycol (PEG) (–0.570 MPa) to compare the effect of unequal root zone stress on plant water relations of a C₄ (sorghum) and a C₃ (sunflower) plant. Roots also were divided between two pots of sand irrigated only with nutrient solution (controls) or only with PEG in nutrient solution. In addition to plant water-status measurements, photosynthetic rate, growth (height, root, and shoot dry weights), and evolution of ethylene (a gaseous hormone indicative of stress) were measured. Under all three split-root treatments, sunflower had a lower leaf water potential and produced more ethylene than sorghum. Sunflower was able to survive the PEG stress if half of its root system was under nonstressed conditions. Sunflower with half its root system irrigated with PEG usually had values of leaf water potential, osmotic potential, stomatal resistance, transpiration rate, photosynthetic rate, ethylene evolution, height, and dry weights that were close to those of the control plants. Sunflower with all roots exposed to PEG was wilted severely. Sorghum was little affected by PEG stress applied either to half or all the root system. Growth of sorghum was the same under all treatments. Apparently because stomata of sorghum were more closed in the partial stress test than those of sunflower, sorghum conserved water and had a higher leaf water potential, which might have permitted growth with stress.     

The Nature and Location of Variable Hydraulic Resistance in Helianthus annuus L. (Sunflower)

Koide, R. 1985. The nature and location of variable hydraulicresistance in Helianthus annuus L. (sunflower).—J. exp.Bot. 36: 1430–1440. Hydraulic resistances for whole sunflower plants (Helianthusannuus L.) and sunflower leaves, stems, petioles and roots weremeasured. Whole plant hydraulic resistance was shown to declinewith an increase in transpiration. Leaf hydraulic resistancewas shown, with one technique employing transpiring leaves,to vary with transpiration and with another technique, employingpressure-induced flow in leaves, to be constant over a widerange of transpiration. Stem and petiole hydraulic resistanceswere constant over a wide range of exudation. Pressure-inducedflow through root systems was shown to be an inappropriate methodfor characterizing their hydraulic properties because flow mayoccur through unnatural paths. The technique employing measuredtranspiration rates and water potentials of non-growing leavesand soil is suggested to be better. The evidence presented inthis study suggests that the hydraulic resistance of the transpirationstream does vary and that the site of variability is the root Key words: Hydraulic resistance, sunflower, pressure-induced water flow     

Control of Leaf Expansion by Nitrogen Nutrition in Sunflower Plants : ROLE OF HYDRAULIC CONDUCTIVITY AND TURGOR

Nitrogen nutrition strongly affected the growth rate of young sunflower (Helianthus annuus L.) leaves. When plants were grown from seed on either of two levels of N availability, a 33% decrease in tissue N of expanding leaves was associated with a 75% overall inhibition of leaf growth. Almost all of the growth inhibition resulted from a depression of the daytime growth rate. Measurements of pressure-induced water flux through roots showed that N deficiency decreased root hydraulic conductivity by about half. Thus, N deficiency lowered the steady-state water potential of expanding leaves during the daytime when transpiration was occurring. As a result, N-deficient leaves were unable to maintain adequate turgor for growth in the daytime. N deficiency also decreased the hydraulic conductivity for water movement into expanding leaf cells in the absence of transpiration, but growth inhibition at night was much less than in the daytime. N nutrition had no detectable effects on plastic extensibility or the threshold turgor for growth.     

Water Deficit and Associated Changes in Some Photosynthetic Parameters in Leaves of ;Valencia' Orange (Citrus sinensis [L.] Osbeck)

Photosynthetic CO₂ assimilation, transpiration, ribulose-1,5-bisphosphate carboxylase (RuBPCase), and soluble protein were reduced in leaves of water-deficit (stress) `Valencia' orange (Citrus sinensis [L.] Osbeck). Maximum photosynthetic CO₂ assimilation and transpiration, which occurred before midday for both control and stressed plants, was 58 and 40%, respectively, for the stress (−2.0 megapascals leaf water potential) as compared to the control (−0.6 megapascals leaf water potential). As water deficit became more severe in the afternoon, with water potential of −3.1 megapascals for the stressed leaves vs. −1.1 megapascals for control leaves, stressed-leaf transpiration declined and photosynthetic CO₂ assimilation rapidly dropped to zero. Water deficit decreased both activation and total activity of RuBPCase. Activation of the enzyme was about 62% (of fully activated enzyme in vitro) for the stress, compared to 80% for the control. Water deficit reduced RuBPCase initial activity by 40% and HCO₃⁻/Mg²⁺-saturated activity by 22%. However, RuBPCase for both stressed and control leaves were similar in K_cat (25 moles CO₂ per mole enzyme per second) and K_m for CO₂ (18.9 micromolar). Concentrations of RuBPCase and soluble protein of stressed leaves averaged 80 and 85%, respectively, of control leaves. Thus, reductions in activation and concentration of RuBPCase in Valencia orange leaves contributed to reductions in enzyme activities during water-deficit periods. Declines in leaf photosynthesis, soluble protein, and RuBPCase activation and concentration due to water deficit were, however, recoverable at 5 days after rewatering.     

Cyclic Variations in Plant Properties under Constant Environmental Conditions

Cyclic fluctuations in stomatal aperture, transpiration rate and leaf water potential under constant environmental conditions have been investigated in intact plants of cotton, pepper, and sunflower. Stomatal aperture and transpiration rate were least when leaf water potential was high and were greatest when leaf water potential was low. Lowest leaf water potential values lagged behind the occurrence of highest transpiration rates, and high overall resistance to water flow occurred in cycling plants. Both of these are considered essential for the occurrence of persistent cyclic behaviour. Hydropassive opening of stomates as the leaves wilted facilitated cycling in cotton and pepper, but not in sunflower, where hydropassive opening did not occur. The roots were identified as the site of the major resistance to water flow in the plant and further experiments directly showed the importance of this root resistance in initiating cycling by causing water stress in the leaves as the stomates opened. Root resistance varied diurnally, becoming increasingly important at night. Root resistance naturally rose to high levels in cotton. High levels were induced in pepper or sunflower by having the roots in deionized water for several days or by anoxia. Quantitative measurements of overall plant resistance were made from leaf water potential and transpiration rate data. The results are discussed and it suggested that plant resistance may indirectly be of importance in the movement of water from the plant to the air.     

Efficiency and regulation of water transport in some woody and herbaceous species

The efficiency with which plants transport water is related to the water potential differences required to drive water fluxes from the soil to the leaf. A comparative study of two woody and three herbaceous species (Citrus sinensis L. cv. Koethen, Pyrus kawakami L., Helianthus annuus L. cv. Mammoth Russian, Capsicum frutescens L. cv. Yolo Wonder, and Sesamum indicum L. cv. Glauca) indicated contrasts in water transport efficiency. Depression of leaf water potential in response to transpiration increases was found in the woody species; the herbaceous species, however, had more efficient water transport systems and presented no measurable response of leaf water potential to transpiration changes. Different maximum transpiration rates under the same climatic conditions were observed with different species and may be accounted for by stomatal response to humidity gradients between leaf and air. Leaf diffusion resistance in sesame increased markedly as the humidity gradient was increased, while leaf resistance of sunflower responded less to humidity. Stomata appeared to respond directly to the humidity gradient because changes in leaf water potential were not detected when leaf resistance increased or decreased.     

Water stress-induced modifications of leaf hydraulic architecture in sunflower: co-ordination with gas exchange

The hydraulic architecture, water relationships, and gas exchange of leaves of sunflower plants, grown under different levels of water stress, were measured. Plants were either irrigated with tap water (controls) or with PEG600 solutions with osmotic potential of -0.4 and -0.8 MPa (PEG04 and PEG08 plants, respectively). Mature leaves were measured for hydraulic resistance (R(leaf)) before and after making several cuts across minor veins, thus getting the hydraulic resistance of the venation system (R(venation)). R(leaf) was nearly the same in controls and PEG04 plants but it was reduced by about 30% in PEG08 plants. On the contrary, R(venation) was lowest in controls and increased in PEG04 and PEG08 plants as a likely result of reduction in the diameter of the veins' conduits. As a consequence, the contribution of R(venation) to the overall R(leaf) markedly increased from controls to PEG08 plants. Leaf conductance to water vapour (g(L)) was highest in controls and significantly lower in PEG04 and PEG08 plants. Moreover, g(L) was correlated to R(venation) and to leaf water potential (psi(leaf)) with highly significant linear relationships. It is concluded that water stress has an important effect on the hydraulic construction of leaves. This, in turn, might prove to be a crucial factor in plant-water relationships and gas exchange under water stress conditions.     

Ethephon-Induced Gummosis in Sour Cherry (Prunus cerasus L.) : II. Flow Characteristics of Gum Solutions

Flow of sour cherry (Prunus cerasus L. cv. Montmorency) gum solutions through a glass capillary was Newtonian for pressure gradients from 0 to 1.8 megapascals per meter, and hydraulic conductance was inversely proportional to solution viscosity in this range. However, flow became plastic at pressure gradients above 1.8 megapascals per meter, resulting in a decrease in solution viscosity. The magnitude of this effect diminished as gum concentration increased. Flow of water, a solution of the component sugar monomers of sour cherry gum, and sucrose solutions remained Newtonian over the entire pressure gradient range examined (0-4 megapascals per meter). Plastic flow of gum solutions in the vessels of intact sour cherry shoots is possible under pressure gradients induced by transpiration when high resistance to flow occurs over short distances.     

Hydraulic resistance of two sorghums varying in drought resistance

Genotypes of sorghum [Sorghum bicolor (L.) Moench] vary in drought resistance. Yet it is not known if their hydraulic resistances vary. The objective of this study was to determine if the hydraulic resistance of a drought-resistant sorghum was the same as that of a drought-sensitive sorghum. Leaf water and osmotic potentials were measured daily, during a 14-d period, in leaves of a drought-resistant (‘KS9’) and a drought-sensitive (‘IA25’) sorghum, which had the roots in pots with a commercial potting soil that was either well watered or allowed to dry. Soil water potential, adaxial stomatal resistance, and transpiration rate were determined daily. Hydraulic resistance of the plants was calculated from the slope of the line relating soil water potential minus leaf water potential versus transpiration rate. When the soil was not watered, the drought-sensitive sorghum had a water potential that averaged −0.50 MPa lower and an osmotic potential that averaged −0.57 MPa lower, but a similar adaxial stomatal resistance (1.19 s mm⁻¹), compared with the drought-resistant sorghum. Seven days after the beginning of the experiment, the water potential of the soil with the drought-sensitive sorghum was −0.25 MPa lower than that of the soil with the drought-resistant sorghum. With the water-limited conditions, the drought-sensitive sorghum depleted the soil-water reserve more quickly and died 2 d before the drought-resistant sorghum. Under well watered conditions, the two sorghums had similar water potentials (−1.64 MPa), osmotic potentials (−2.83 MPa), and adaxial stomatal resistances (0.78 s mm⁻¹). The calculated hydraulic resistance of the two sorghums did not differ and averaged 3.4 × 10⁷ MPa s m⁻¹. The results suggested that the variation in susceptibility to drought between the two genotypes was due to differences in rate of soil-water extraction. Contribution No. 86-249-J from the Kansas Agricultural Experiment Station. The paper is dedicated to the memory of Dr Dan M Rodgers.     

Gas exchange of root hemi-parasite Striga hermonthica and its host Sorghum bicolor under short-term soil water stress

The gas exchange of the upper fully expanded leaf of the root parasite Striga hermonthica and of its host Sorghum bicolor was measured under wet and dry conditions to identify the mechanisms of the devastating effects of the parasite on its hosts under drought. The short-term water stress severely reduced photosynthetic rate in infected sorghum, but less in S. hermonthica. Soil water stress did not affect leaf respiration rate in either S. hermonthica or infected sorghum. This suggests that under dry conditions both infected sorghum and S. hermonthica decreased autotrophic carbon gain. The transpiration rate of S. hermonthica, a major driving force for assimilate uptake from the host, was higher and less affected by water stress than that of infected sorghum. Stomatal density on the abaxial surfaces of the leaves was higher in S. hermonthica than in sorghum. Both S. hermonthica infection and water stress decreased stomatal conductance of the sorghum leaves. S. hermonthica, irrespective of soil water status, had greater stomatal aperture on the adaxial and abaxial surfaces of its leaves than infected sorghum. These results indicate that the higher transpiration rate of S. hermonthica even under water stress, achieved through higher stomatal density on the abaxial surfaces of the leaves and greater stomatal aperture on both surfaces of the leaves, may induce the maintenance of water and solute transfers from the host to the parasite leading to severe damage to the host under drought.     

Plant morphological and biochemical responses to field water deficits: I. Responses of glutathione reductase activity and paraquat sensitivity

The effects of water deficits on plant morphology and biochemistry were analyzed in two photoperiodic strains of field-grown cotton (Gossypium hirsutum L.). Plants grown under dryland conditions exhibited a 40 to 85% decrease in leaf number, leaf area index, leaf size, plant height, and total weight per plant. Gross photosynthesis decreased from 0.81 to 0.47 milligram CO₂ fixed per meter per second and the average midday water, osmotic, and turgor potentials decreased to −2.1, −2.4, and 0.3 megapascals, respectively.

There was a progressive increase in glutathione reductase activity and in the cellular antioxidant system in the leaves of stressed plants compared to the irrigated controls. The stress-induced increases in enzyme activity occurred at all canopy positions analyzed.

Irrigation of the dryland plots following severe water stress resulted in a 50% increase in leaf area per gram fresh weight in newly expanded leaves of both strains over the leaves which had expanded under the dryland conditions. Paraquat resistance (a relative measure of the cellular antioxidant system) decreased in the strain T25 following irrigation. Glutathione reductase activities remained elevated in the T25 and T185 leaves which were expanded fully prior to irrigation and in the leaves which expanded following the irrigation treatment.

     

The response of sorghum and sunflower to short-term waterlogging

Summary The effect of waterlogging on water use and nutrient uptake in sunflower and sorghum was investigated in relation to stage of development of the crops and the timing and duration of waterlogging. Waterlogging at the vegetative and floral initiation stages of plant growth induced a reduction in water use of sunflower, with corresponding declines in leaf expansion and leaf water potential; in sorghum, the transpiration rates were much lower than for sunflower and relatively unaffected by waterlogging. Waterlogging at anthesis, however, caused an immediate reduction in water use in sunflower with a similar but delayed effect in sorghum. The differences in response of these two species are discussed in relation to the relative importance of water stress and nutrient uptake. Plant analysis at maturity indicated that waterlogging at any growth stage reduced both total and seed phosphorus in sunflower; similar effects were recorded with sorghum, with the exception of anthesis waterlogging which did not reduce nutrient uptake. Waterlogging effects on plant potassium levels were variable.     

The Comparative Drought Resistance of Landraces of Sorghum and Millet From Dry and Humid Regions

It may be that land-races of sorghum (Sorghum sp.) and millet[Pennisetum americanum (L.) Leeke] which evolved along geographicalgradients of rainfall in Africa and India, differ in their droughtresistance. Any physiological attributes found to be correlatedwith low rainfall might be important and effective characteristicsfor crop production in dry regions. Twenty land-races were chosen which evolved along geographicalgradients of rainfall, seven millets from India, six sorghumsfrom Mali, and seven sorghums from the Sudan. Races were evaluatedfor their growth potential and plant water relations under hydroponicsconditions in a growth chamber. A water stress treatment wasimposed by adding polyethylene glycol-8000 to the nutrient solution,giving a solute water potential of -0.5 MPa, compared with acontrol solution at 003 MPa. Drought resistance, in terms of relatively less growth inhibitionunder stress, was higher in races from dry regions than in racesfrom humid regions. Of all the physiological variables measured[carbon exchange rate, (CER), transpiration, transpiration ratio(CER/transpiration), leaf diffusive resistance, leaf water potentialand osmotic adjustment], only osmotic adjustment under stresswas generally correlated with average rainfall at each race'sorigin, indicating greater osmotic adjustment in land-racesfrom drier regions. Races with a greater capacity for osmoticadjustment were characterized by smaller plants with high ratesof transpiration and low rates of leaf senescence under stress. The carbon exchange rate per unit leaf area increased as liveleaf area decreased under stress due to leaf senescence. Thus,drought resistant races under stress tended to have lower CERper unit live leaf area (but not per plant) than susceptibleraces. Transpiration ratios under stress were lower in resistantthan in susceptible races, mainly because resistant races hadhigher transpiration. The results for the measured variables showed a general trendfor greater drought resistance in sorghum than in millet, indicatingthat the commonly observed adapation of the millets to dry environmentsmay be due to other factors, such as drought escape or heattolerance. Sorghum sp. Pennisetum americanum L. (Leeke), water stress, osmotic adjustment, photosynthesis, transpiration, evolution, drought resistance     

Homeostasis in leaf water potentials on leeward and windward sides of desert shrub crowns: water loss control vs. high hydraulic efficiency

Phenotypic plasticity in morphophysiological leaf traits in response to wind was studied in two dominant shrub species of the Patagonian steppe, used as model systems for understanding effects of high wind speed on leaf water relations and hydraulic properties of small woody plants. Morpho-anatomical traits, hydraulic conductance and conductivity and water relations in leaves of wind-exposed and protected crown sides were examined during the summer with nearly continuous high winds. Although exposed sides of the crowns were subjected to higher wind speeds and air saturation deficits than the protected sides, leaves throughout the crown had similar minimum leaf water potential (Ψ_L). The two species were able to maintain homeostasis in minimum Ψ_L using different physiological mechanisms. Berberis microphylla avoided a decrease in the minimum Ψ_L in the exposed side of the crown by reducing water loss by stomatal control, loss of cell turgor and low epidermal conductance. Colliguaja integerrima increased leaf water transport efficiency to maintain transpiration rates without increasing the driving force for water loss in the wind-exposed crown side. Leaf physiological changes within the crown help to prevent the decrease of minimum Ψ_L and thus contribute to the maintenance of homeostasis, assuring the hydraulic integrity of the plant under unfavorable conditions. The responses of leaf traits that contribute to mechanical resistance (leaf mass per area and thickness) differed from those of large physiological traits by exhibiting low phenotypic plasticity. The results of this study help us to understand the unique properties of shrubs which have different hydraulic architecture compared to trees.     

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.     

Evidence that root pressure flow is required for calcium transport to head leaves of cabbage

Young cabbage plants (Brassica oleracea L. var. capitata) that were exposed to an atmosphere at 50% relative humidity transpired freely and accumulated significant quantities of ⁴⁵Ca in the leaves. Plants that were enclosed by plastic bags to stop transpiration from all leaves exhibited guttation with the development of root pressure and also accumulated significant quantities of ⁴⁵Ca in the leaves. ⁴⁵Ca accumulation increased in the leaves and tended to decrease in roots and stems with increasing quantities of water transpired or guttated by the plant. When plants were only partially enclosed so that some leaves were covered and the remainder exposed, only the exposed leaves that were transpiring accumulated significant quantities of ⁴⁵Ca. The covered leaves of partially enclosed plants exhibited no guttation and accumulated little ⁴⁵Ca with no measurable ⁴⁵Ca at the margins of the leaves. The results demonstrate that root pressure flow is required to transport adequate amounts of Ca to those tissues in plants that are not undergoing transpirational water loss.     

A Quantitative Study of the Resistances to Transpirational Water Movement in Sunflower (Helianthus annuus L.)

Hydroponic sunflower plants were used in a quantitative studyof the relationship between total plant and leaf resistancesto transpirational water movement and transpiration rate. Theresults demonstrate that both resistances are flux-dependentand decline 5–6-fold during a comparable increase in transpiration.The resistance of excised leaves including the petiole was approximatelyhalf the total plant resistance. Subsequent analyses of the water potential gradients and transpirationalfluxes in whole plants permitted calculation of the magnitudeof the partial resistances imposed by roots, stem, petiole,and leaf. The root and leaf resistances were approximately 50%and 30% of the total resistance respectively. Stem and petiolarresistances were relatively small and both influenced watermovement to the upper leaves similarly. The values obtainedare compared with previous published results obtained usingdiverse experimental techniques.     

Biophysical limitation of cell elongation in cereal leaves

Grass leaves grow from the base. Unlike those of dicotyledonous plants, cells of grass leaves expand enclosed by sheaths of older leaves, where there is little or no transpiration, and go through developmental stages in a strictly linear arrangement. The environmental or developmental factor that limits leaf cell expansion must do so through biophysical means at the cellular level: wall-yielding, water uptake and solute supply are all candidates. This Botanical Briefing looks at the possibility that tissue hydraulic conductance limits cell expansion and leaf growth. A model is presented that relates pathways of water movement in the elongation zone of grass leaves to driving forces for water movement and to anatomical features. The bundle sheath is considered as a crucial control point. The relative importance of these pathways for the regulation of leaf growth and for the partitioning of water between expansion and transpiration is discussed.     

The Effect of Leaf Water Potential, Leaf Temperature and Light Intensity on Leaf Diffusion Resistance and the Transpiration of Leaves of Malus sylvestris

The relationship between leaf resistance to water vapour diffusion and each of the factors leaf water potential, light intensity and leaf temperature was determined for leaves on seedling apple trees (Malus sylvestris Mill. cv. Granny Smith) in the laboratory. Leaf cuticular resistance was also determined and transpiration was measured on attached leaves for a range of conditions. Leaf resistance was shown to be independent of water potential until potential fell below — 19 bars after which leaf resistance increased rapidly. Exposure of leaves to CO₂-free air extended the range for which resistance was independent of water potential to — 30 bars. The light requirement for minimum leaf resistance was 10 to 20 W m^?2 and at light intensities exceeding these, leaf resistance was unaffected by light intensity. Optimum leaf temperature for minimum diffusion resistance was 23 ± 2°C. The rate of change measured in leaf resistance in leaves given a sudden change in leaf temperature increased as the magnitude of the temperature change increased. For a sudden change of 1°C in leaf temperature, diffusion resistance changed at a rate of 0.01 s cm^?1 min^?1 whilst for a 9°C leaf temperature change, diffusion resistance changed at a rate of 0.1 s cm^?1 min^?1. Cuticular resistance of these leaves was 125 s cm^?1 which is very high compared with resistances for open stomata of 1.5 to 4 s cm^?1 and 30 to 35 s cm^?1 for stomata closed in the dark. Transpiration was measured in attached apple leaves enclosed in a leaf chamber and exposed to a range of conditions of leaf temperature and ambient water vapour density. Peak transpiration of approximately 5 × 10^?6 g cm^?2 s^?1 occurred at a vapour density gradient from the leaf to the air of 12 to 14 g m^?3 after which transpiration declined due presumably to increased stomatal resistance. Leaves in CO₂-free air attained a peak transpiration of 11 × 10^?6 g cm^?2 s^?1 due to lower values of leaf resistance in CO₂ free air. Transpiration then declined in these leaves due to development of an internal leaf resistance (of up to 2 s cm^?1). The internal resistance was masked in leaves at normal CO₂ concentrations by the increase in stomatal resistance.