Effects of low concentrations of O3, leaf age and water stress on leaf diffusive conductance and water use efficiency in soybean

Ozone, leaf age and water stress each affected leaf conductance in soybean [ Glycine max (L.) Merr. Hodgson], but there were no interactions among these factors. Exposure to increased concentrations of O₃ (0.01, 0.05, 0.09. and 0.13 μl l⁻¹) resulted in linear declines in abaxial and adaxial conductances in leaves of all ages. There were no differences in relative response to O₃ between the two leaf surfaces. For well-watered plants, water use efficiency also decreased with exposure to increased O₃ concentrations (water-stressed plants were not tested). Abaxial conductance increased as leaves aged from 4 to 10 days and then declined with further aging. Adaxial conductance decreased with all increases in leaf age beyond 4 days, and the ratio of abaxial/adaxial conductance increased continuously throughout the leaf lifespan. During water-stress cycles (water withheld for 2–3 days) leaves of water-stressed plants had lower conductances than those from well-watered plants, and there was no difference in relative response between abaxial and adaxial stomata.     

Leaf Water Diffusion Resistance in Clonal Tea (Camellia sinensis L.): Effects of Water Stress, Leaf Age and Clones

Leaf diffusion resistance was influenced by leaf age in well-wateredand water-stressed clonal tea plants. In well-watered plantsand in two of the three clones studied, young leaves showeda significantly lower diffusion resistance than old leaves.In water-stressed plants young leaves always exhibited a higherdiffusion resistance than old leaves. The highest diffusionresistance, irrespective of leaf age and water stress, occurredin clone DN which is known to be the most drought tolerant ofthe three clones studied, suggesting that drought tolerancein clone DN is caused, at least in part, by a stomatal regulationmechanism. Water release characteristic curves for the threeclones indicated differences in the water content-water potentialrelationship between young and old leaves as well as betweenclones. The drought tolerant clones had a higher relative watercontent for a given water potential compared with the drought-susceptibleclone. Camellia sinensis L., tea, diffussion resistance, water stress     

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.     

Leaf Wilting Movement Can Protect Water-Stressed Cotton (<Emphasis Type="Italic">Gossypium hirsutum</Emphasis> L.) Plants Against Photoinhibition of Photosynthesis and Maintain Carbon Assimilation in the Field

Under severe water stress, leaf wilting is quite general in higher plants. This passive movement can reduce the energy load on a leaf. This paper reports an experimental test of the hypothesis that leaf wilting movement has a protective function that mitigates against photoinhibition of photosynthesis in the field. The experiments exposed cotton (Gossypium hirsutum L.) to two water regimes: water-stressed and well-watered. Leaf wilting movement occurred in water-stressed plants as the water potential decreased to −4.1 MPa, reducing light interception but maintaining comparable quantum yields of photosystem II (PS II; Yield for short) and the proportion of total PS II centers that were open (qP). Predrawn F _v/F _m (potential quantum yield of PS II) as an indicator of overnight recovery of PS II from photoinhibition was higher than or similar to that in well-watered plants. Compared with water-stressed cotton leaves for which wilting movement was permitted, water-stressed cotton leaves restrained from such movement had significantly increased leaf temperature and instantaneous CO₂ assimilation rates in the short term, but reduced Yield, qP, and F _v/F _m. In the long term, predrawn F _v/F _m and CO₂ assimilation capacity were reduced in water-stressed leaves restrained from wilting movement. These results suggest that, under water stress, leaf wilting movement could reduce the incident light on leaves and their heat load, alleviate damage to the photosynthetic apparatus due to photoinhibition, and maintain considerable carbon assimilation capacity in the long term despite a partial loss of instantaneous carbon assimilation in the short term.     

Leaf movements and photoinhibition in relation to water stress in field-grown beans

Photoinhibition in plants depends on the extent of light energy being absorbed in excess of what can be used in photochemistry and is expected to increase as environmental constraints limit CO2 assimilation. Water stress induces the closure of stomata, limiting carbon availability at the carboxylation sites in the chloroplasts and, therefore, resulting in an excessive excitation of the photosynthetic apparatus, particularly photosystem II (PSII). Mechanisms have evolved in plants in order to protect against photoinhibition, such as non-photochemical energy dissipation, chlorophyll concentration changes, chloroplast movements, increases in the capacity for scavenging the active oxygen species, and leaf movement or paraheliotropism, avoiding direct exposure to sun. In beans (Phaseolus vulgaris L.), paraheliotropism seems to be an important feature of the plant to avoid photoinhibition. The extent of the leaf movement is increased as the water potential drops, reducing light interception and maintaining a high proportion of open PSII reaction centres. Photoinhibition in water-stressed beans, measured as the capacity to recover F(v)/F(m), is not higher than in well-watered plants and leaf temperature is maintained below the ambient, despite the closure of stomata. Bean leaves restrained from moving, increase leaf temperature and reduce qP, the content of D1 protein and the capacity to recover F(v)/F(m) after dark adaptation, the extent of such changes being higher in water-stressed plants. Data are presented suggesting that even though protective under water stress, paraheliotropism, by reducing light interception, affects the capacity to maintain high CO2 assimilation rates throughout the day in well-watered plants.     

Leaf anatomy of highbush blueberry grownin vitro and during acclimatization toex vitro conditions

Leaves of micropropagated highbush blueberry (Vaccinium corymbosum) cv. ‘Bluetta’ have been observed during the acclimatization phase. In vitro-developed leaf cells were circular and small, the spongy parenchyma was discontinuous and disorganized and formed by 1–2 layers of cells with large intercellular spaces and the palisade to spongy mesophyll thickness ratio was 1:1.5. After rooting ex vitro, the first leaves formed under natural conditions showed substantial changes in the anatomical characteristics. After 6 months, the plants produced leaves similar to those in field-grown plants. The palisade cells were rectangular, the spongy parenchyma was formed by 3–4 layers of cells and the intercellulars were around the stomata. Leaves from field-grown plants lost 24 % of water during 150 min after excision while leaves from in vitro shoots lost about 50 % of water in the same time. Leaves from in vitro shoots showed a higher number of smaller stomata (361 per mm²), with the guard cells forming a circular ring; the stomata frequency in field-grown leaves was 241 per mm² and the guard-cells were elliptical.     

Effects of Powdery Mildew and Water Stress on CO(2) Exchange in Uninfected Leaves of Barley

Net photosynthesis is stimulated in third seedling leaves of barley plants whose lower two leaves are heavily infected by Erysiphe graminis f.sp. hordei Marchal. Stimulation is greater in water-stressed than in well-watered plants. In stressed, but not in well-watered plants, stimulation is associated with the maintenance of high leaf water potential and high leaf conductance. A small part of the changes in net photosynthesis is attributable to changes in respiratory metabolism in the third leaf, and other possible causes are discussed.     

The influence of plant water stress on stomatal control of gas exchange at different levels of atmospheric humidity

Summary Leaves of well-watered and mildly water-stressed seedlings of Betula pendula Roth. and Gmelina aroborea L. were subjected to a range of vapour pressure deficits (VPD) between 10 and 24 kPa. The stomatal conductance of birch seedlings decreased as VPD was increased and at least in mildly-stressed seedlings this response seemed to be closely linked to the water status of the air rather than to the bulk water status of the plant. Mild water stressing enhanced the degree of the stomatal humidity-response and resulted in a significant increase in the efficiency of water use at high VPD. Stomata of Gmelina were apparently insensitive to variation in VPD, but were more sensitive to a decrease in bulk leaf water status than were stomata of birch. Water use efficiency of Gmelina seedlings was comparatively high, even when VPD was high and the stomata were fully open.     

Diurnal Expansion and Contraction of Leaves and Fruits of English Morello Cherry

Changes in leaf thickness and fruit diameter were measured ingrowth chambers under varying conditions of light, humidity,temperature, and soil moisture for potted trees of English Morellocherry (Prunus cerasus L. grafted on Prunus mahaleb root stock).Decrease in leaf thickness of well-watered plants was inducedby illumination following a period of darkness. In the light,leaf thickness decreased when vapour-pressure deficit (VPD)was rising or high. The magnitude of diurnal change in leafthickness was related to amount of change in VPD of the air.Leaf thickness increased in the dark when stomata were closed.When changes in VPD were minimized, leaves under 8-h days and16-h nights still decreased in thickness during the day andexpanded at night, but the degree of change was not as greatas when VPD also fluctuated diurnally. When severe internal water deficits developed in trees duringprolonged droughts the correlations of changes in leaf thicknesswith VPD and light intensity were low. Marked decline in leafthickness resulted with little re-expansion in the dark at lowVPD. Soil irrigation resulted in rapid hydration and expansionof leaves. Changes in fruit diameter in contrast were positivelycorrelated with VPD fluctuations and were not influenced byphotoperiod.     

1-Aminocyclopropane-1-Carboxylic Acid Transported from Roots to Shoots Promotes Leaf Abscission in Cleopatra Mandarin (Citrus reshni Hort. ex Tan.) Seedlings Rehydrated after Water Stress

The effect of water stress and subsequent rehydration on 1-aminocyclopropane-1-carboxylic acid (ACC) content, ACC synthase activity, ethylene production, and leaf abscission was studied in Cleopatra mandarin (Citrus reshni Hort. ex Tan.) seedlings. Leaf abscission occurred when drought-stressed plants were allowed to rehydrate, whereas no abscission was observed in plants under water stress conditions. In roots of water-stressed plants, a high ACC accumulation and an increase in ACC synthase activity were observed. Neither increase in ACC content nor significant ethylene production were detected in leaves of water-stressed plants. After rehydration, a sharp rise in ACC content and ethylene production was observed in leaves of water-stressed plants. Content of ACC in xylem fluid was 10-fold higher in plants rehydrated for 2 h after water stress than in nonstressed plants. Leaf abscission induced by rehydration after drought stress was inhibited when roots or shoots were treated before water stress with aminooxyacetic acid (AOA, inhibitor of ACC synthase) or cobalt ion (inhibitor of ethylene-forming enzyme), respectively. However, AOA treatments to shoots did not suppress leaf abscission. The data indicate that water stress promotes ACC synthesis in roots of Cleopatra mandarin seedlings. Rehydration of plants results in ACC transport to the shoots, where it is oxidized to ethylene. Subsequently, this ethylene induces leaf abscission.     

Farnesol-like antitranspirant activity and stomatal behaviour in maize and Sorghum lines of differing drought tolerance

Abstract Potted seedlings of four lines of maize and Sorghum of differing drought tolerance were subjected to a single soil drying cycle and were only rewatered when the plants showed the first signs of wilting. Other plants remained well-watered throughout the experimental period. As plant water potentials decreased in the unwatered plants of three of the lines investigated (Sorghum Piper and M35-1, V-4146 and maize Farz 27), endogenous levels of farnesol-like antitranspirants increased. Closure of stomata correlated well with the increase in endogenous antitranspirant. In the fourth line (Sorghum M35-1, V-4184), stomata did not close as the level of plant water stress increased, although leaf diffusion resistance of even the well-watered plants of this line was quite high. In this line, there was no consistent relationship between plant water stress and antitranspirant level or between stomatal behaviour and antitranspirant level. The involvement of farnesol-like antitranspirants in the control of stomatal behaviour in water-stressed plants is discussed.     

A DEVELOPMENTAL STUDY OF THE LEAVES OF NICOTIANA GLUTINOSA AS RELATED TO THEIR SMOG–SENSITIVITY

Glater , Ruth Bobrov , Richard A. Solberg , and Flora M. Scott . (U. California, Los Angeles, and Los Angeles County Air Pollution Control District.) A developmental study of the leaves of Nicotiana glutinosa as related to their smog-sensitivity. Amer. Jour. Bot. 49(9): 954–970. Illus. 1962.—Plants growing in the fields of Los Angeles County as well as those experimentally fumigated in the laboratory show gross markings in response to smog which vary from species to species, from a glistening appearance of the leaf undersurface due to a temporary accumulation of water in the affected cells through complete necrosis. In dicotyledonous leaves, “silvering,” “bronzing,” brown-black mottling or an increase in anthocyanin may be seen. In monocotyledons, transverse banding, tan in color, or longitudinal streaking of leaves are the usual responses. This damage appears in a characteristic pattern on the leaves, different from that produced by other phytotoxicants. Nicotiana glutinosa plants were grown in the air-filtered greenhouses at UCLA. The normal anatomical development of the foliage was studied and correlated with its susceptibility to smog injury. On a given plant, leaves of different ages show damage in different positions. Very young leaves at the apex of the plant and old leaves at the base of the plant are not sensitive. Expanding leaves between young and old in age are sensitive; in this group a distinct pattern of damage is discernible. Damage markings in the youngest leaves appear only at the tip; in leaves somewhat older, close to midblade; in fully mature leaves, only at the base. This localization of damage is shown to be correlated with the gradient of cellular differentiation from tip toward base as the leaf matures. Those cells which have just attained maximum size (young mature) are sensitive; damage, therefore, is a function of cellular development and maturity. The following anatomical details were analyzed: (1) differentiation and distribution of stomata and their opening and closing on both upper and lower epidermal surfaces and (2) development of intercellular air spaces in palisade and spongy parenchyma tissue. These studies indicate that damage occurs in the region of the leaf where stomata have just become functional and ambient polluted air can make direct contact with interior leaf tissues by virtue of large substomatal chambers and intercellular air spaces.     

Uniconazole-induced tolerance of soybean to water deficit stress in relation to changes in photosynthesis, hormones and antioxidant system

This study investigated whether uniconazole confers drought tolerance to soybean and if such tolerance is correlated with changes in photosynthesis, hormones and antioxidant system of leaves. Soybean plants were foliar treated with uniconazole at 50 mg L-1 at the beginning of bloom and then exposed to water deficit stress at pod initiation for 7 d. Uniconazole promoted biomass accumulation and seed yield under both water conditions. Plants treated with uniconazole showed higher leaf water potential only in water-stressed condition. Water stress decreased the chlorophyll content and photosynthetic rate, but those of uniconazole-treated plants were higher than the stressed control. Uniconazole increased the maximum quantum yield of photosystemand ribulose-1,5-bisphosphate carboxylase/oxygenase activity of water-stressed plants. Water stress decreased partitioning of assimilated 14C from labeled leaf to the other parts of the plant. In contrast, uniconazole enhanced translocation of assimilated 14C from labeled leaves to the other parts, except stems, regardless of water treatment. Uniconazole-treated plants contained less GA3, GA4 and ABA under well-watered condition than untreated plants, while the IAA and zeatin levels were increased substantially under both water conditions, and ABA concentration was also increased under water stressed condition. Under water-stressed conditions, uniconazole increased the content of proline and soluble sugars, and the activities of superoxide dismutase and peroxidase in soybean leaves but not the malondialdehyde content or electrical conductivity. These results suggest that uniconazole-induced tolerance to water deficit stress in soybean was related to the changes of photosynthesis, hormones and antioxidant system of leaves.     

Mitigation of Tomato Spotted Wilt Virus Infection and Symptom Expression by Water Stress

The effects of water stress on the infection and systemic movement of tomato spotted wilt virus (TSWV) in vegetative tomato plants are examined. Two groups of plants: water stressed (water potential ?1.31 MPa) and well-watered ones (water potential ?0.65 MPa), were mechanically inoculated on the third expanded leaf from the top. Each group was then divided into a well-watered and a water stressed one. TSWV inoculation was effective both on water-stressed and well-watered plants. In all groups, infection was first detected in roots, then in shoot apices and subsequently in the rest of the leaves. Systemic virus infection was somewhat slower in the water stressed plants. Post inoculation stress treatments had a significant effect in reducing infection as well as in attenuating TSWV symptoms. Results from the present work suggest that water stress may be effective in controlling virus concentration and systemic infection symptoms in tomato plants and could thus be used as a tool to study the regulation of development of TSWV infection in this system.     

Scanning Electron Microscopy of Pseudomonas syringae pv, morsprunorum on Sweet Cherry Leaves

Scanning electron microscopy indicated that sub-stomatal cavities on sweet cherry leaves are “protected sites” which shelter resident populations of Pseudomonas syringae pv. morsprunorum. Bacteria entered the stomata, multiplied in the cavities and emerged in a mass 6 days after inoculation. There were no visible symptoms, suggesting that the pathogen colonized the host in “sub-clinical” numbers to generate populations which were then released onto the leaf surface under suitable conditions.     

Characteristics of Crassulacean Acid Metabolism in the Succulent C(4) Dicot, Portulaca oleracea L

Crassulacean acid metabolism (CAM) was investigated in leaves and stems of the succulent C₄ dicot Portulaca oleracea L. Diurnal acid fluctuations, CO₂ gas exchange, and leaf resistance were monitored under various photoperiod and watering regimes. No CAM activity was seen in well watered plants grown under 16-hour days. Under 8-hour days, however, well watered plants showed a CAM-like pattern of acid fluctuation with amplitudes of 102 and 90 microequivalents per gram fresh weight for leaves and stems, respectively. Similar patterns were also observed in detached leaves and defoliated stems. Leaf resistance values indicated that stomata were open during part of the dark period, but night acidification most likely resulted from refixation of respiratory CO₂. In water-stressed plants maximum acid accumulations were reduced under both long and short photoperiods. At night, these plants showed short periods of net CO₂ uptake and stomatal opening which continued all night long during preliminary studies under natural environmental conditions. Greatest acid fluctuations, in P. oleracea, with amplitudes of 128 microequivalents per gram fresh weight, were observed in water-stressed plants which had been rewatered, especially when grown under short days. No net CO₂ uptake took place, but stomata remained open throughout the night under these conditions. These results indicate that under certain conditions, such as water stress or short photoperiods, P. oleracea is capable of developing an acid metabolism with many similarities to CAM.     

Environmental scanning electron microscopy of the surface of normal and vitrified leaves of Gypsophila paniculata (Babies Breath) cultured in vitro

Leaf surfaces of non-tissue-cultured, vitrified and non-vitrified plantlets of Gypsophila paniculata (Babies Breath) were examined using an environmental scanning electron microscope. Non-tissue-cultured plants had a complete epidermal surface, recessed stomata and wax present on the leaf surface. The surface of tissue-cultured plantlets appeared similar to non-tissue-cultured plants excepting stomata were slightly protruding and less wax appeared to be present. In both non-tissue-cultured and tissue-cultured plants stomata were found both opened and closed and were observed closing. In contrast vitrified plantlets had abnormal, malformed stomata which appeared non-functional. The ventral surfaces of leaves seemed more normal than the dorsal, this may be due to the former receiving more light. Additionally, discontinuities were found in the epidermis. Often epidermal holes were found in association with stomatal apertures. It is suggested that the main cause of desiccation of vitrified G. paniculata plantlets ex vitro is due to loss of water from the discontinuity in epidermis and not because of non-functional stomata. Liquid water could be seen through the epidermal holes indicating that at least some of the extra water in vitrified plantlets is contained in the intercellular spaces.Abbreviations ESEM Environmental scanning election microscope - IAA Indole acetic acid - NTC Non-tissue-cultured - TC Tissue-cultured - V vitrified     

Mycorrhizal colonisation improves fruit yield and water use efficiency in watermelon (Citrullus lanatus Thunb.) grown under well-watered and water-stressed conditions

The effect of arbuscular mycorrhizal (AM) colonisation by Glomus clarum on fruit yield and water use efficiency (WUE) was evaluated in watermelon (Citrullus lanatus) cv. Crimson Sweet F1 under field conditions. Treatments were: (1) well-watered plants without mycorrhizae (WW-M), (2) well-watered plants with mycorrhizae (WW+M), (3) water- stressed plants without mycorrhizae (WS-M) and (4) water-stressed plants with mycorrhizae (WS+M). When soil water tension readings reached –20 and –50 kPa for well-watered (WW) and water-stressed (WS) treatments, respectively, irrigation was initiated to restore the top soil to near field capacity. Water stress reduced watermelon shoot and root dry matter, fruit yield, water use efficiency but not total soluble solids (TSS) in the fruit, compared with the non-stressed treatments. Mycorrhizal plants had significantly higher biomass and fruit yield compared to nonmycorrhizal plants, whether plants were water stressed or not. AM colonisation increased WUE in both WW and WS plants. Macro- (N, P, K, Ca and Mg) and micro- (Zn, Fe and Mn) nutrient concentrations in the leaves were significantly reduced by water stress. Mycorrhizal colonisation of WS plants restored leaf nutrient concentrations to levels in WW plants in most cases. This is the first report of the mitigation of the adverse effect of water stress on yield and quality of a fruit crop.     

Paraheliotropism can protect water-stressed bean (Phaseolus vulgaris L.) plants against photoinhibition

In order to estimate the importance of leaf movements on photosynthesis in well-watered and water-stressed field grown bean cultivars (Arroz Tuscola (AT), Orfeo INIA (OI), Bayos Titan (BT), and Hallados Dorado (HD)), CO2 assimilation, leaf temperature, and capacity for the maximum quantum yield recovery, measured as Fv/Fm, were assessed. Leaf water potential was lower in water-stressed compared to control plants throughout the day. Water status determined a decrease in the CO2 assimilation and stomatal conductance as light intensity and temperature increased up to maximal intensities at midday. Both parameters were lower in stressed compared to control plants. Even though high light intensity and water-stress induced stomatal closure is regarded as a photoinhibitory condition, the recovery of variable to maximal fluorescence (Fv/Fm) after 30min of darkness was nearly constant in both water regimes. In fact, higher values were observed in OI and AT when under stress. Photochemical and non-photochemical fluorescence quenching resulted in minor changes during the day and were similar between watered and stressed plants. It is concluded that paraheliotropism, present in the four bean cultivars, efficiently protects stressed plants from photoinhibition in the field and helps maintain leaf temperatures far below the ambient temperatures, however, it may also be responsible for low CO2 assimilation rates in watered plants.     

Contribution of osmotic adjustment to the dehydration tolerance of water-stressed pigeon pea (Cajanus cajan (L.) millsp.) leaves

Abstract Water-stressed pigeonpea leaves have high levels of osmotic adjustment at low leaf water potentials. The possible contribution of this adjustment of dehydration tolerance of leaves was examined in plants grown in a controlled environment. Osmotic adjustment was varied by withholding water from plants growing in differing amounts of soil, which resulted in different rates of decline of leaf water potential. The level of osmotic adjustment was inversely related to leaf water potential in all treatments. In addition, at any particular water potential, plants that had experienced a rapid development of stress exhibited less osmotic adjustment than plants that experienced a slower development of stress. Leaves with different levels of osmotic adjustment died at water potentials between –3.4 and –6.3 MPa, but all leaves died at a similar relative water content (32%). Consequently, leaves died when relative water content reached a lethal value, rather than when a lethal leaf water potential was reached. Osmotic adjustment delayed the time and lowered the leaf water potential when the lethal relative water content occurred, because it helped maintain higher relative water contents at low leaf water potentials. The consequences of osmotic adjustment for leaf survival in water-stressed pigeonpea are discussed.