Leaf Water Potential Response to Transpiration by Citrus

This paper reports on further studies of a model for interpreting leaf water potential data for Citrus. Experimental data confirmed the assumption that the ratio of vapor pressure deficit to leaf diffusion resistance adequately estimates transpiration when leaf-to-air temperature differences are small. Data collected diurnally indicated that the relationship between leaf water potential and transpiration followed a sequence of steady states without hysteresis. No difference in water transport characteristics was found for Valencia orange on three rootstocks in well-watered soil, but the two rootstocks Cleopatra mandarin and Rangpur gave slightly greater leaf water stress in Valencia orange leaves than‘Troyer’ citrange rootstock at high transpiration rates under mild soil water deficits. In laboratory studies, previously unstressed seedlings had higher leaf water potentials than field trees at equivalent transpiration rates. After several drying cycles, however, leaf water potentials were similar to those observed in the field.     

The Relationship between Transpiration, Root Water Uptake, and Leaf Water Potential

The effect of changing the transpiration rate on leaf waterpotential and water balance has been examined to show if permeabilityof the plant (predominantly the roots) is constant or varieswith the transpiration rate. Measurements of leaf effectivethickness, water potential, transpiration, and uptake of waterby roots were made on sunflower, barley, and maize plants grownin solution culture and subjected to a range of atmosphericconditions and root treatments: cooling, low osmotic potential,and removal of part of the root system. Leaf water potential changed little under a wide range of atmosphericconditions and rates of water flux in the three species, sothat the root permeability to water increases as the rate oftranspiration, and therefore flow across the root surface, increases.Equality between uptake and loss of water and thereby maintenanceof constant leaf water potential is assisted by stomatal changes,which appear to be in response to conditions at or in the rootrather than a direct response to changes in bulk leaf waterpotential.     

Anatomy and Transpiration of the Avocado Inflorescence

Structure and function of the inflorescence of cv. 'Hass' and'Fuerte' avocado (Persea americana Mill.) were examined by scanningelectron microcopy (SEM) and by porometry. Sepals and petalscould not be distinguished by their position in the flower,by visual gross morphology or by microscopic surface structureand were hence designated as tepals. These tepals were arrangedin two whorls of three, followed by two whorls of three outerand three inner stamens, each opposite a tepal. The most conspicuousfeature of tepals, developing leaves and peduncles was the densecover of hair which were most frequent on the adaxial tepalsurface (925-1200 trichomes mm^-2), followed by their abaxialsurface (625-1000 mm^-2) and peduncles (375-655 mm^-2). Stomatawere absent from the adaxial surfaces of both tepal and leaves,as well as peduncles. On the tepals, abaxial stomata appearedfunctional, small (8-9 x 11-13 µm) and scarce with 2·8-3·4stomata mm^-2, i.e. very low relative to avocado leaves (350-510stomata mm^-2) or young fruit (50-75 stomata mm^-2. However, flowersincluding tepals transpired 1·2-1·3 mmol underfield conditions in Southern California (1·6-2 kPa),i.e. in excess of leaves (0·7-1·1 mmol) and peduncles(0·6-0·8 mmolH₂O m^-2 s^-1). This situation wasattributed to the few small but functional abaxial stomata onthe tepal, in contrast to 80% closed stomata and dense epicuticularwax cover in form of rodlets on young and dendritic crystalson old leaves including the guard cells, and absence of stomatafrom the peduncle.Copyright 1993, 1999 Academic Press Persea americana Mill., avocado, bioenergetics, flower, fruit, leaf, peduncle, scanning electron microscopy, stomata, transpiration, petals, sepals, tepals     

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.     

Effects of Water Stress under Contrasting Environmental Conditions on Transpiration and Photosynthesis in Soybean

Continuous and simultaneous measurements of CO₂ exchange andtranspiration rates of whole soybean plants were made undercontrasting, controlled environmental conditions when waterstress was imposed by withholding water. Daytime temperaturesand vapour pressure deficits were 27.5 ° C/12 mbar; 27.5° C/5 mbar; 22.5 ° C/12 mbar, and 22.5 ° C/5 mbar.The experimental conditions were virtually the same as the conditionsunder which the plants had been grown. Under all four treatments photosynthesis and transpiration rateswere closely correlated as water stress increased, but in viewof the evidence for a significant mesophyll resistance to photosynthesisin- both stressed and unstressed plants it is not consideredthat this is due to total stomatal control. At — 0.4 bar soil water potential (_soll) the rates oftranspiration and photosynthesis became independent of the atmosphericconditions and were very similar under all treatments. Thisis attributed to slow movement of water into the root zone fromthe surrounding soil with associated stomatal closure limitingthe rates of water uptake and transpiration. With decreasing _soll, relative water content of the leaf (RWC)fell more rapidly and to lower levels under 27.5 ° C/12mbar conditions than under the other treatments. The least reductionin RWC was under the 22.5 ° C/5 mbar treatment. Increasingsoil water stress had the greatest relative effect on ratesof gaseous exchange under 27.5 ° C/12mbar conditions andleast under 22.5 ° C/5mbar conditions.     

The Effects of Leaf Position and Water Vapour Density Deficit on the Transpiration Rate of Detached Leaves

The drying of detached leaves may be measured by the loss ofweight taking place over a period of time, and expressed byleaf conductance kl as a function of relative water contentRWC. A parameter is defined which is proportional to the tendencyof the leaf to dry, and expresses high conductance, low finalRWC, or both combined. Observations have been made on Zea maysand on three species of Solanum with the intention of exploringthe possibility of using this technique for comparative physiology.The position of the leaves on the parent shoot has been foundto affect the drying characteristics of the leaves, and theearly values of leaf conductance are reduced when evaporationis brought about by higher values of water vapour density deficit.Both leaf sampling and the evaporative conditions must be standardizedif the method is to yield useful results for comparative purposes.     

Effect of Excision on Leaf Water Potential

Improvements of thermocouple hygrometric techniques for in situleaf water potential measurement in the field now allows forcontinuous monitoring of water potential in response to an externalperturbation, such as leaf excision. Using Citrus jambhiri plants,measured leaf water potentials of completely excised leaf portionsimmediately increased when the petiole was excised or incisionswere made either transverse or parallel to the midrib. Incisionsparallel to the midrib were on the side nearest the hygrometeror opposite it if preceded by a petiole excision. Midrib incisionswere 100–150 mm long with the nearest cut edge being 20–50mm from the hygrometer cavity. All excisions were such thatleaf tissue was removed from the leaf with water potential onone of the leaf portions being measured continuously prior toand after excising. The peak increase in measured water potentialof the excised leaf portions ranged between 20 kPa and 80 kPabut averaged 50 kPa. In uncovered leaves, particularly underfield conditions with the associated high evaporative demand,measured leaf water potential declined rapidly after the initialincrease. The increase in measured water potential immediatelyfollowing various types of excision was confirmed for dark andlight conditions (laboratory and field respectively) using bothpsychrometric and dewpoint modes and occurred for secondaryexcisions, but to a lesser extent. Discovery of this phenomenonimplies that water potential measured on detached leaves maynot always represent accurately in situ leaf water potential. Key words: Leaf water potential, Thermocouple hygrometers, Leaf excision effects     

Leaf Elongation in Relation to Leaf Water Potential in Soybean

Leaf water potential, turgor pressure, and leaf elongation ratewere measured in soybeans growing in controlled environmentchambers, greenhouses, and outdoors. Plants in chambers hadthe highest water potentials and turgor pressures, and plantsoutdoors the lowest. In all three environments there was a linearrelationship between elongation rate and turgor pressure. Leavesof plants in drier environments required less turgor for elongation,and showed a greater increase in elongation rate per unit increasein turgor. Elongation rates over a 72 h period were equal inthe three environments. Leaves reached the largest final sizein the greenhouse (intermediate in water potential). Epidermalcells were larger in chamber- and greenhouse-grown leaves thanin leaves of plants grown outdoors. The number of epidermalcells per leaf was greater in the greenhouse and outdoors thanin the chamber. Leaf elongation characteristics of greenhouseplants were duplicated by mildly stressing chamber plants, andleaf elongation characteristics of field plants were duplicatedby more severely stressing chamber plants. Leaves of mildlystressed chamber plants also reached a larger final size thanleaves of more severely stressed chamber plants, or leaves ofcontrol plants in the chamber. Water stress in the chamber increasedthe number of epidermal cells per leaf. More severe water stressin the chamber reduced epidermal cell size. Based on the waterstress experiments it is concluded that the differences in plantwater status in the chamber, greenhouse, and field caused differencesin elongation characteristics, and were responsible for thedifferences in leaf size.     

环境和遗传对烟草叶片结构的影响

叶片性状很大程度上反映了植物的功能及其对环境的适应。叶片性状的变异受环境及遗传特性的影响,但是很多研究未能很好区分出它们的相对影响。本研究通过将同一烟草品种种植于不同地点来认识环境对烟草叶片性状的影响;将不同的烟草品种种植于同一环境来了解遗传差异对烟草叶片性状的影响。研究发现,叶脉密度和叶片厚度受环境的影响较大,气孔密度和长度受环境的影响较小。气孔密度和叶片厚度受遗传的影响较大,叶脉密度和气孔长度受遗传的影响较小。在生长温度高的地方,烟草叶脉密度越大。研究结果对于认识植物对环境的适应性具有重要意义。     

Effects of Water Stress and Abscisic Acid on Transpiration Regulation in Wheat

The transpiration response to recurrent light periods was studied'n water-stressed wheat seedlings. Seedlings were stressed by three methods: addition of mannitol to the root medium, root cooling and drving of the roots in air. All three methods induced almost equal effects on transpiration regulation during alternating dark and light intervals. Exogenous abscisic acid supplied to the shoots of excised plants had qualitatively the same effect as water stress. Water stress and ABA increased the time lapse between light-on and the onset of transpiration increase and lowered the amplitude of transpiration increase in light. Weak light introduced before strong light shortened the delay times.     

Matric Water Potential of Leaf Tissue--Measurement and Significance

A thermocouple psychrometer method, previously described foruse in determining tissue and sap water potentials, has beenadapted for determining matric potentials. Matric values ranging from approximately zero to –10 x10⁵ Pa were observed with wilting wheat leaves. Mean valuesof total tissue water potential and of its solute (osmotic),matric, and cell wall pressure components were –16.6,–18.3, –30 and +4.7 Pa x 10⁵, respectively. Matric potentials are often ignored in investigations of plantwater relations. This practice is shown to result in underestimationof wall pressures (in this case by a mean of 64 per cent) andsometimes in spuriously negative values.     

Water Potential and Leaf Elongation in Radiata Pine and Wheat

The rate of leaf elongation in radiata pine (Pinus radiata) and wheat seedlings was closely related to the osmotic potential of the rooting solution. Sudden stress application and removal treatments caused immediate changes in the leaf elongation rate and a new steady-state rate independent of the old was quickly established. The osmotic potential in the active elongation zones of a leaf differed from that of the remainder of the leaf and was considered more appropriate for deriving turgor pressure to be related to leaf elongation. Shifts in elongation rate were associated with corresponding shifts in total leaf water potential in both species and with corresponding shifts in turgor pressure in wheat. There was a linear correlation between the pitch of the helical thickenings in the protoxylem of radiata pine needles and their elongation rate. A record of elongation rate is, therefore, preserved in the needle. Developmental aspects of the deposition of the helix are discussed.     

Diurnal Changes in Citrus Leaf Thickness, Leaf Water Potential and Leaf to Air Temperature Difference

Diurnal changes in leaf water potential and leaf thickness ofwell-watered citrus trees were found to be highly correlated.Midday decreases in leaf thickness of about 30–35 µm reflected midday decreases in leaf water potential of about1.1–1.3 MPa from predawn values. Leaf water potentialwas also correlated with changes in leaf-to-air temperaturedifference and ambient vapour pressure deficit. Leaf thicknessas well as leaf to air temperature difference could possiblybe used to monitor leaf water status continuously as an indicatorof citrus tree water stress.     

Changes in Transpiration, Net Carbon Dioxide Assimilation and Leaf Water Potential Resulting from Application of Hydrostatic Pressure to Roots of Intact Pepper Plants

Hydrostatic pressures varying from 0 to 6.0 bar were applied to roots of intact Capsicum annuum L. cv. California Wonder plants growing in nutrient solution and the rates of transpiration, and net CO₂ assimilation, apparent compensation point and leaf water potential measured. Increasing the pressure on the roots of plants with roots in solution with either -0.5 or -5.0 bar osmotic potential with 1 bar increments resulted in a decrease in transpiration. With the application of 1 or 2 bar pressure the rate of transpiration returned to near or above the original rate. An application of 3 or 4 bar pressure reduced the rate of transpiration of all plants. The transpiration of plants with roots in solution with -0.5 bar osmotic potential remained at the reduced rate for as long as these pressures were maintained. The transpiration of plants with roots in solution with -5.0 bar was only temporarily suppressed at these pressures. Changing the applied pressure from 3 or 4 bar to 0 resulted in a rapid increase in transpiration which lasted approximately 15 minutes. This was followed by a decrease in transpiration to a rate lower than before the pressure was applied. The pattern of response was similar for plants at low or high light intensity or at normal or low CO₂ concentrations. When leaf diffusive resistance was 6.0 s cm^?1 or greater, changes in net CO₂ assimilation were similar to those of transpiration. The apparent CO₂ compensation point increased as pressure was applied and decreased with a release in pressure. Leaf water potential increased with an increase in pressure and decreased with a decrease in pressure. The changes in leaf water potential were frequently but not always proportional to changes in pressure. It is postulated that the respouses noted were due to changes in resistance to flow of water from xylem terminals through the mesophyll cells and stomatal cavities to the atmosphere.     

水稻叶片水势的QTL定位与候选基因分析

为探究叶片水势(LWP)相关基因在水稻(Oryza sativa)抗旱中的作用及其遗传机制,以热研2号(Nekken2)和华占(HZ)为亲本以及构建的120个重组自交系(RILs)群体为实验材料,对水稻分蘖期叶片水势进行检测,并利用前期基于高通量测序构建的分子遗传连锁图谱进行数量性状基因座(QTL)分析。结果表明,共检...