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.     

Responses of Soybean Leaf Angle, Photosynthesis and Stomatal Conductance to Leaf and Soil Water Potential

The hypothesis that soil water potential (_s) is better correlatedto heliotropic leaf orientation, photosaturated photosyntheticCO₂ assimilation and stomatal conductance during periods oflimited water availability than is bulk leaf water potential(₁) was examined in greenhouse-grown soybean (Glycine max) plants,submitted to a progressive drought. Paired plants were exposedto either 1000 or 100 µmol m^–2 s^–1 photonflux densities (PFD) for 45–60 mins. The higher irradianceinduced short-term decreases in ₁, due to increased transpiration,while _l in the plant exposed to low PFD did not decrease. Thesechanges in ₁ occurred independently of changes in soil waterstatus. Concurrent to the light treatments, a single attachedleaf from each of the two plants was isolated from the restof the plant by shading, and the pulvinus of its terminal leafletwas exposed to a perpendicular PFD of 500 µmol m–²S^–1. Leaf movement of this leaflet was recorded in responseto this light, until a stable leaflet angle was achieved. Valuesof _s and _l (before and after light treatment), and photosaturatedrates of photosynthesis and stomatal conductance, were thenmeasured on these leaves. Leaflet angle and gas exchange werebetter correlated with _s (r² = 0.50, 0.50 and 0.57 for angle,photosynthesis and conductance, respectively) than with _l especiallywhen _l was the result of short-term, high-light induced changesin leaf water status (r² = 0.36, 0.32 and 0.49, for the sameparameters). Leaflet angle was also correlated with stomatalconductance (r² = 0.61) and photosynthetic rate (r² = 0.60),suggesting a close association between leaf orientation, leafmetabolism and soil water availability. Glycine max (L.) Merr. cv. Essex, soybean, heliotropism, water potential, photosynthesis, stomatal conductance, solar tracking     

Photosynthesis and Carbohydrate Partitioning for the C3 Desert Shrub Encelia farinosa under Current and Doubled CO2 Concentrations

Changes in photosynthesis (A) and carbohydrate partitioning were studied for Encelia farinosa, a common C3 desert shrub in the southwestern United States, after a 3-month exposure to the current or a doubled CO2 concentration (750 [mu]L L-1). A remained unchanged under the current CO2 concentration but decreased during the day under the doubled CO2 concentration, resulting in a 46% enhancement in the early morning, 26% at midday, and 15% in the late afternoon by the elevated CO2. The decrease during the day under the doubled CO2 concentration may represent end-product inhibition, because the sucrose and the starch contents increased during the day proportionally more than under the current CO2 concentration. The 14CO2 activity in sink leaves was maximal 3 h after labeling under the doubled and at 5 h under the current CO2 concentration, indicating faster movement of photosynthate out of source leaves and into sink tissues under the doubled CO2 concentration, which may have been responsible for the sustained enhancement in A under the doubled CO2 concentration.     

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.     

Pubescence and leaf spectral characteristics in a desert shrub,Encelia farinosa

Summary The effects of leaf hairs (pubescence) on leaf spectral characteristics were measured for the drought-deciduous desert shrub Encelia farinosa. Leaf absorptance to solar radiation is diminished by the presence of pubescence. The pubescence appears to be reflective only after the hairs have dried out. There are seasonal changes in leaf absorptance; leaves produced at the beginning of a growing season have high absorptances, whereas leaves produced during the growing season are more pubescent and have lower absorptances. The decrease in leaf absorptance is the result of an increase in pubescence density and thickness. Between 400 and 700 nm (visible wavelengths), pubescence serves as a blanket reflector. However, over the entire solar spectrum (400–3000 nm), the pubescence preferentially reflects near infrared radiation (700–3000 nm) over photosynthetically useful solar radiation (400–700 nm). Leaf absorptance to solar radiation (400–3000 nm) varies between 46 and 16%, depending on pubescence; whereas leaf absorptance to photosynthetically useful radiation (400–700 nm) may vary from 81 to 29%.C.I.W.-D.P.B. Publication No. 612     

Leaf Response to Water Deficits in Soybeans

Soybeans [Glycine max (L.) Merrill cv. Wayne] plants were subjected to an extended drying cycle in the field to investigate the leaf sensitivity to water deficits. Soybeans in irrigated plots were superior to those in non-irrigated plots in the average size and number of leaflets per plant. Apparent differences in the leaf area distributions in the canopy seemed to be mediated by moisture stress effects associated with leaf senescence and light penetration in the lower depths of the canopy. A major decrease in leaf enlargement occurred near a leaf-water potential of -8 bars, and at - 12 bars, the growth was completely halted. Similar decreases were observed at a stomatal conductance of 0.4 cm/s and at 0.2 cm/s no enlargement was observed.     

Intraspecific competitive effects on water relations,growth and reproduction in Encelia farinosa

Summary An experiment was conducted to assess the importance of intraspecific competition on water relations, growth and reproductive output in Encelia farinosa, a common deciduous-leaved shrub of the Sonoran Desert. Nearest neighbor analyses in monospecific stands indicated that plants exhibited a clumped distribution. Plant size and nearest neighbor distance were positively correlated, inferring intraspecific competition. Removal experiments monitored for two years indicated that plants now without neighbors had higher leaf water potentials, higher leaf conductances, and a greater leaf area than control plants. As a consequence, growth rates and reproductive output were significantly higher in plants without neighbors. These data strongly support the notion that warm desert plants with a contagious spatial distribution compete for water.     

The Response of Leaf Water Potential and Crassulacean Acid Metabolism to Prolonged Drought in Sedum rubrotinctum

Plants of Sedum rubrotinctum R. T. Clausen were studied in a green-house over a 2-year period without watering. Only the apical leaves survived and were turgid at the end of the experiment. The midday leaf water potential of these apical leaves was −1.20 megapascals, while the leaf water potential of comparable leaves on well-watered control plants was −0.20 megapascals. The unwatered plants appear to have maintained turgor by means of an osmotic adjustment. After 2 years without water the plants no longer exhibited a nocturnal accumulation of titratable acidity. However, the daytime levels of titratable acidity of the unwatered plants were more than 2-fold greater than the levels in well-watered control plants. Well-watered plants of S. rubrotinctum exhibited seasonal shifts in biomass stble carbon isotope ratios, indicating a greater proportion of day versus night CO₂ uptake in the winter than in the summer. The imposition of water stress prevented the expression of this seasonal rhythm and restricted the plants to dark CO₂ uptake.     

Diurnal and Seasonal Changes in Leaf Water Potential Components and Elastic Properties in Response to Water Stress in Apple Trees

Apple trees are very drought tolerant, having the capability to grow and carry on photosynthesis even at low water potentials. Much of the tolerance is due to the ability of apple leaves to maintain turgor potentials at levels conducive to growth and stomatal opening. Diurnally, leaf turgor is maintained through decreases in osmotic potentials (due to active solute accumulation), osmotic adjustment, or to concentration of solutes via tissue water loss. These two processes combined may decrease osmotic potentials by as much as 1.65 MPn during the day. Seasonally, osmotic potentials remain fairly constant, but leaf elasticity increases, allowing growth to continue and stomata to remain open us water and turgor potentials become progressively lower. Release of stored water from plant tissues to the transpiration stream is another means of preventing water potentials from reaching critical values for stomatal closure. A combination of a number of these physiological adaptations may account for much of the drought tolerance in apple trees.     

Influence of Rate of Development of Leaf Water Deficits upon Photosynthesis, Leaf Conductance, Water Use Efficiency, and Osmotic Potential in Sorghum

This study reports the effect of rate of development of leaf water deficits in soil-grown sorghum (Sorghum bicolor) on the relationship of net photosynthesis, leaf conductance, and water use efficiency to leaf water potential, and on the degree of solute accumulation (osmotic adjustment). Recovery of these processes on rewatering, and responses during a second stress cycle were also studied. The most rapid rate of stress (1.2 MPa day^?1) resulted in no solute accumulation and the lowest rate of net photosynthesis and leaf conductance for any given leaf water potential during stress. Stress at 0.7 and 0.15 MPa day^?1 led to equal solute accumulations of approximately 0.6 MPa, but net photosynthesis, leaf conductance, and water use efficiency at a given leaf water potential were lower with the faster rate of stress (0.7 MPa day^?1). Additionally, leaf conductance at a given leaf turgor potential was lowest at the 1.2 MPa day^?1 stress rate, slightly higher at the intermediate rate of stress, and clearly highest at the slowest rate of stress. Recovery of both net photosynthesis and leaf conductance upon rewatering was rapid, taking less than 3 days, but full recovery of osmotic potential took between 6 and 11 days. One slow stress cycle had no influence on relationships during a second cycle. The concept of a threshold leaf water potential for stomatal closure is discussed and the conclusion reached that stomatal closure occurs slowly over a wide range of leaf water potential (> 1.0 MPa), the range being greater for slower rates of stress.     

Leaf expansion,net CO2 uptake,Rubisco activity,and efficiency of long-term biomass gain for the common desert subshrub Encelia farinosa

Encelia farinosa is one of the most abundant and highly studied species of the Sonoran Desert, yet characteristics of its leaf development and long-term photosynthetic capacity are relatively unknown. The net CO₂ uptake rate and the Rubisco activity per unit leaf area for E. farinosa in a glasshouse increased in parallel for about 18 days after leaf emergence (leaf area was then 5 cm²), after which both were constant, suggesting that Rubisco levels controlled net CO₂ uptake. Instantaneous net CO₂ uptake rates at noon for well-watered E. farinosa in the glasshouse at different temperatures and light levels correctly predicted differences in daily net CO₂ uptake at four seasonally diverse times for transplanted plants under irrigated conditions in the field but overpredicted the daily means by 13%. After this correction, seasonally adjusted net CO₂ uptake per unit leaf area multiplied by the estimated monthly leaf area predicted that 42% of the net carbon gain was incorporated into plant dry weight over a 17-month period. The ecological success of E. farinosa apparently reflects an inherently high daily net CO₂ uptake and retention of a substantial fraction of its leaf carbon gain.     

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 Mulberry Leaf Metabolism in Response to Water Stress

A series of experiments were conducted to characterize the water stress-induced changes in the activities of RuBP carboxylase (RuBPCO) and sucrose phosphate synthase (SPS), photosystem 2 activity, and contents of chlorophylls, carotenoids, starch, sucrose, amino acids, free proline, proteins and nucleic acids in mulberry (Morus alba L. cv. K-2) leaves. Water stress progressively reduced the activities of RuBPCO and SPS in the leaf extracts, the chlorophyll content, and PS2 activity in isolated chloroplasts. Plants exposed to drought showed lower content of starch and sucrose but higher total sugar content than control plants. While the soluble protein content decreased under water stress, the amino acid content increased. Proline accumulation (2.5-fold) was noticed in stressed leaves. A reduction in the contents of DNA and RNA was observed. Reduced nitrogen content was associated with the reduction in nitrate reductase activity. SDS-PAGE protein profile showed few additional proteins (78 and 92 kDa) in the water stressed plants compared to control plants.     

周期性土壤干旱和叶片水势对气孔响应木质部ABA灵敏度的影响

研究了周期性土壤干旱期间气孔对木质部ＡＢＡ响应的灵敏度的变化以及叶片水势对灵敏度的影响。实验结果证明了木质部ＡＢＡ浓度是反映根系周围土壤水分状况的一个指标的结论。土壤周期性干旱不影响木质部ＡＢＡ浓度对土壤水分状况的依赖关系,但显著地提高了气孔对木质部ＡＢＡ 响应的灵敏度。根据对实测数据的数学模拟结果显示,引起气孔导度下降５０％ 所需的木质部ＡＢＡ浓度从第一轮土壤干旱的７５０ ｎｍｏｌ／Ｌ降至第二轮土壤干旱的５５０ ｎｍｏｌ／Ｌ。分根实验的结果表明,叶片水分亏缺显著提高了气孔对木质部ＡＢＡ 的响应的灵敏程度,全根干旱中引起气孔导度下降５０ ％ 所需的木质部ＡＢＡ 浓度比半根干旱的小２ ～４ 倍。这表明,气孔对木质部ＡＢＡ响应的灵敏度不是一个固定的特性,可随植物生长环境及许多其他因素的变化而表现出很大的差异     

The Effect of Epidermal Cell Water Potential on Stomatal Response to Illumination of Leaf Discs of Vicia faba

Illuminated leaf discs of Vicia faba were brought into equilibrium with a series of mannitol solutions. The width of stomatal aperture and the osmotic potential of guard cells and epidermal cells were determined. It was found that the maximal aperture was obtained when epidermal cells were at about incipient plasmolysis and that any increase in their turgor pressure brought about a decrease in stomatal aperture. These findings emphasize the importance of epidermal cells in determining the width of the stomatal pore.     

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     

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.     

Architecture of coastal and desert Encelia farinosa (Asteraceae): consequences of plastic and heritable variation in leaf characters

The shrub Encelia farinosa (Asteraceae) exhibits geographic variation in aboveground architecture and leaf traits in parallel with environmental variation in temperature and moisture. Measurements of plants occurring across a natural gradient demonstrated that plants in desert populations produce smaller, more pubescent leaves and are more compact and branched than plants in more mesic coastal environments. This phenotypic variation is interpreted in part as adaptive genetic differentiation; small size and pubescence reduce leaf temperature and thus increase water-use efficiency but at the cost of lower photosynthetic rate, which results in slower growth and more compact growth form. We explored the basis of phenotypic variation by planting seed offspring from coastal and desert populations in common gardens in both environments. Phenotypic differences among populations persisted in both common gardens, suggesting a genetic basis for trait variation. Desert offspring outperformed coastal offspring in the desert garden, suggesting superior adaptation to hot, dry conditions. Herbivore damage was greater for all offspring in the coastal garden. Phenotypic characters also showed plastic responses; all offspring had smaller, more pubescent leaves and more compact growth form in the desert garden. Our results confirm that leaf size and pubescence are heritable characters associated with pronounced variation in plant architecture.