Reduction In Leaf Water Potential Research Articles

How do leaf hydraulics limit stomatal conductance at high water vapour pressure deficits?

A reduction in leaf stomatal conductance (g) with increasing leaf-to-air difference in water vapour pressure (D) is nearly ubiquitous. Ecological comparisons of sensitivity have led to the hypothesis that the reduction in g with increasing D serves to maintain leaf water potentials above those that would cause loss of hydraulic conductance. A reduction in leaf water potential is commonly hypothesized to cause stomatal closure at high D. The importance of these particular hydraulic factors was tested by exposing Abutilon theophrasti, Glycine max, Gossypium hirsutum and Xanthium strumarium to D high enough to reduce g and then decreasing ambient carbon dioxide concentration ([CO2]), and observing the resulting changes in g, transpiration rate and leaf water potential, and their reversibility. Reducing the [CO2] at high D increased g and transpiration rate and lowered leaf water potential. The abnormally high transpiration rates did not result in reductions in hydraulic conductance. Results indicate that low water potential effects on g at high D could be overcome by low [CO2], and that even lower leaf water potentials did not cause a reduction in hydraulic conductance in these well-watered plants. Reduced g at high D in these species resulted primarily from increased stomatal sensitivity to [CO2] at high D, and this increased sensitivity may mediate stomatal responses to leaf hydraulics at high D.

Influence of water stress on leaf photosynthetic characteristics in wheat cultivars differing in their susceptibility to drought

A gradual reduction in leaf water potential (Ψleaf), net photosynthetic rate (P N), stomatal conductance, and transpiration rate was observed in two drought tolerant (C 306 and K 8027) and two susceptible (RW 893 and 899) genotypes subjected to water stress. The extent of reduction was lower in K 8027 and C 306 and higher in RW 893 and RW 899. Rewatering the plants after 5 d of stress restored P N and other gas exchange traits in all four cultivars. Water stress had no significant effect on variable to maximum fluorescence ratio (Fv/Fm) indicating that water stress had no effect on primary photochemistry of photosystem 2 (PS2). However, water stress reduced the efficiency of excitation energy transfer (F'v/F'm) and the quantum yield of electron transport (ΦPS2). The reduction was more pronounced in susceptible cultivars. Water stress had no significant effect on photochemical quenching, however, the non-photochemical quenching increased by water stress.

Adaptation to Heat and Drought Stresses in Snap Bean (Phaseolus vulgaris) during the Reproductive Stage of Development

We screened snap bean (Phaseolus vulgaris L.) germplasm for heat and drought tolerance during the reproductive stage under extremely hot conditions as well as in the winter seasons in closed net houses to raise the temperatures under field conditions, a practice usually followed by vegetable growers. Substantial genotypic differences were found in morpho-physiological characteristics like phenology, plant water relations, photosynthetic parameters and shoot growth, which were related to the reproductive responses. The associations of days to podding and leaf water content with the number of pods per plant and seed yield were consistent across the different environments and experiments. The leaf water content was maintained by reductions in leaf water potential and shoot extension in response to heat and drought stress. Therefore, these traits can be used as a marker to screen germplasm for heat and drought tolerance. In this paper, we briefly reviewed the results of our studies carried out on heat and drought tolerance in snap bean at Okinawa Subtropical Station, Ishigaki.

Controlled Drought Affects Morphology and Anatomy of Salvia splendens

Polyethylene glycol 8000 (PEG-8000) was applied to a soilless growing medium at the concentrations of 0, 15, 20, 30, 42, or 50 g·L-1 to impose controlled drought. Salvia (Salvia splendens F. Sellow. ex Roem & Shult.) seeds were planted in the growing medium to determine if controlled drought affects morphology and anatomy of salvia. Polyethylene glycol decreased emergence percentage and delayed emergence up to 5 days. Stem elongation of salvia treated with the five lowest concentrations was reduced up to 35% (21 days after seeding), and salvia were a maximum of 53% shorter and the canopy was 20% more narrow compared to nontreated seedlings 70 days after seeding. These morphological changes were attributed to PEG-8000 mediated reduction in leaf water potential (Ψw). The growing medium Ψw ranged from -0.29 to -0.85 MPa in PEG-8000 treated plants, and plant height was positively correlated with Ψw 21 days after seeding. Stem diameter of PEG-treated seedlings was reduced up to 0.4 mm mainly due to reductions in vascular cross-sectional area. Xylem cross-sectional area decreased more than stem and phloem cross-sectional area. Polyethylene glycol 8000 reduced vessel element number, but not diameter.

Genotypic Variability in Drought Performance and Recovery in Cowpea under Controlled Environment

AbstractMid‐season drought is a factor frequently limiting crop production in the moist to dry savannah zones of the tropical and subtropical regions of the world. Ten cowpea genotypes were subjected to a cycle of drought at flowering followed by re‐watering to study variation in drought performance and recovery. Drought caused a reduction in leaf assimilation rate, transpiration rate and stomatal conductance with genotypic variances of 75.4, 57.9, and 83.3 %, respectively. Only genotypic variance in stomatal conductance increased appreciably under drought. Reductions in leaf water potential as a consequence of drought positively correlated with a decline in assimilation rate, which was associated with stomatal closure. One week after re‐watering, the three gas exchange parameters of stressed plants recovered fully and attained values 10–30 % higher than the well‐watered plants with increased genotypic variability. Reductions in the total dry matter during the drought interval varied from 11 to 50 % among genotypes, but were of minor importance for the total dry matter at maturity. After stress, the gain in dry matter varied considerably among the stressed genotypes, with stressed plants showing higher gain than the unstressed plants during this interval. This was associated with increased availability of assimilates due to enhanced green leaf area duration after stress release. Variability in drought recovery among genotypes was found, and appears to be more important for final yield than responses during drought.

Effects of water stress and night temperature preconditioning on water relations and morphological and anatomical changes of Lotus creticus plants

The water relations and anatomical and morphological changes of Lotus creticus sub. creticus seedlings to two irrigation treatments (water stress and control) and temperature (unheated and night-heated greenhouse) was investigated in nursery conditions. Shoot and root growth, leaf stomatal density, leaf trichome density, xylem vessels in roots and stems and water relations were studied because of their relation with drought resistance. Water stress led to substantial losses in dry weight, leaf area, root dry weight and length. The limited osmotic adjustment reached by the plants may not maintain leaf turgor and growth. In general deficit irrigation produced Lotus plants with greater leaf trichome densities and number of xylem vessels of stems and roots. Night-time heating had little influence on these aspects, temperature reduced leaf water potential of water stressed plants but had no effect on well-watered plants. Deficit irrigation under nursery conditions induced a suite of morphological and physiological adaptations (reduction in leaf area, development of osmotic adjustment, increments of leaf hairs and number of xylem vessels of stem and roots) that might allow a considerable capacity to adapt to adverse conditions after transplanting.

Effect of water stress on partitioning of 14C-labelled photosynthates in Vitis vinifera.

The influence of fruits on export and distribution of photosynthates was studied in Vitis vinifera L. cv. Tempranillo using 14C labelling. Also, the influence of water stress was analysed in fruiting and fruitless plants of Tempranillo and fruiting plants of cv. Alfonso Lavallée. In fruitless plants of Tempranillo, reserve organs (trunk, roots and lower shoot) represented 80% of total plant dry matter (DM), and imported up to 90% of the total 14C exported from the fed leaf. Therefore, the distribution pattern of photosynthates in these plants reflected mainly the sink size. However, the presence of fruits in Tempranillo strongly stimulated 14C export and changed the distribution pattern of assimilates. Fruits imported up to 70-80% of the total 14C exported, while representing only 25% of the total plant DM. Therefore, the strength of fruits as carbon sinks was independent of sink size, and it is discussed on the basis of a water potential gradient theory.

REGULATION OF PLANT GROWTH IN CONTAINER-GROWN ORNAMENTALS THROUGH THE USE OF CONTROLLED IRRIGATION

The aim of this research was to determine whether shoot growth could be regulated and plant quality improved through two controlled irrigation techniques: Regulated Deficit Irrigation (RDI) or Partial Root Drying (PRD). An additional benefit of such techniques is that they would also improve the efficiency of irrigation application and reduce the volume of water used on commercial nurseries. Results from two ornamental woody plant species (Cotinus and Forsythia) demonstrated that plant quality could be significantly improved when RDI was applied at ≤ 60% of potential evapo-transpiration (ETp). Stomatal closure and reduced leaf and internode growth rates were associated with both the RDI and PRD techniques, but reduced leaf water potential was only recorded in the RDI system. Changes in xylem sap pH and ABA concentrations were correlated with changes in shoot physiology, and thought to be generated by those roots exposed to drying soil. By adopting such controlled irrigation systems on commercial holdings it is estimated that water consumption could be reduced by 50 to 90%.

Photosynthetic Responses of a Temperate Liana to Xylella fastidiosa Infection and Water Stress

Abstract Xylella fastidiosa is a xylem‐limited bacterial plant pathogen that causes bacterial leaf scorch in its hosts. Our previous work showed that water stress enhances leaf scorch symptom severity and progression along the stem of a liana, Parthenocissus quinquefolia, infected by X. fastidiosa. This paper explores the photosynthetic gas exchange responses of P. quinquefolia, with the aim to elucidate mechanisms behind disease expression and its interaction with water stress. We used a 2 × 2‐complete factorial design, repeated over two growing seasons, with high and low soil moisture levels and infected and non‐infected plants. In both years, low soil moisture levels reduced leaf water potentials, net photosynthesis and stomatal conductance at all leaf positions, while X. fastidiosa‐infection reduced these parameters at basally located leaves only. Intercellular CO2 concentrations were reduced in apical leaves, but increased at the most basal leaf location, implicating a non‐stomatal reduction of photosynthesis in leaves showing the greatest disease development. This result was supported by measured reductions in photosynthetic rates of basal leaves at high CO2 concentrations, where stomatal limitation was eliminated. Repeated measurements over the summer of 2000 showed that the effects of water stress and infection were progressive over time, reaching their greatest extent in September. By reducing stomatal conductances at moderate levels of water stress, P. quinquefolia maintained relatively high leaf water potentials and delayed the onset of photosynthetic damage due to pathogen and drought‐induced water stress. In addition, chlorophyll fluorescence measurements showed that P. quinquefolia has an efficient means of dissipating excess light energy that protects the photosynthetic machinery of leaves from irreversible photoinhibitory damage that may occur during stress‐induced stomatal limitation of photosynthesis. However, severe stress induced by disease and drought eventually led to non‐stomatal decreases in photosynthesis associated with leaf senescence.

Changes of leaf water potential and endogenous cytokinins in young apple trees treated with or without paclobutrazol under drought conditions

Leaf water potential and endogenous cytokinins in xylem sap of young apple trees treated with or without paclobutrazol (PBZ) under drought stress conditions were investigated. Three drought and rewatering cycles were used in the experiments and leaf water potential was measured every 2–4 days during the experimental period (39 days). Zeatin (Z) and zeatin riboside (ZR) from the xylem sap in the trunks were measured during the drought periods. The results showed that leaf water potential of the plants decreased significantly when drought was applied, while for the PBZ treatment the reduction of leaf water potential was significantly less in all the three drought cycles, particularly in the first and second cycles. The concentrations of Z and ZR in xylem sap of the drought treated plants decreased significantly in the first drought cycle. In the second cycle, Z continued to decrease but ZR did not change significantly by day 4 but increased at day 8 for both treatments compared with the control. In the third cycle, there was no difference in Z concentration between the drought treatment alone and the control, but Z increased significantly for the PBZ treatment. ZR increased significantly for the drought and drought plus PBZ treatments compared with the control in the third cycle.

Influence of water deficit and low air humidity in the nursery on survival of Rhamnus alaternus seedlings following planting

SummaryThe effect of irrigation and air humidity on the water relations and root and shoot growth of Rhamnus alaternus L. during the nursery phase was considered to evaluate the resulting degree of hardening obtained by these treatments. R. alaternus seedlings were pot-grown in two greenhouses of equal characteristics. In one of these greenhouses air humidity was controlled using a dehumidifying system, while in the other one the environmental conditions were not artificially modified. In each greenhouse, two irrigation treatments were used. Thus, four different treatments were applied during the nursery phase (January-May): 1) control air humidity + control irrigation; 2) control air humidity + deficit irrigation; 3) low air humidity + control irrigation; 4) low air humidity + deficit irrigation. In May, plants of all treatments were transplanted and grown in good environmental and irrigation conditions for one month (17 May–20 June), after which they received no irrigation until the end of the experiment (14 July). Low air humidity and water deficit reduced all shoot growth parameters during the nursery phase, however the root growth was not significantly affected by air humidity and even increased under the water deficit. The reduction in leaf water potential under water stress was induced by tissue dehydration since leaf turgor potential also decreased and non-osmotic adjustment was observed. The drought effects on water relations were similar in both low and high air humidity. The leaf stomatal conductance was also reduced by both types of stress, leading to a decrease in the rate of photosynthesis at the end of the nursery phase. Both water deficit and low air humidity showed their value as nursery acclimation processes, improving the survival of seedlings following transplanting and non-irrigation conditions (establishment phase). The stomatal regulation and a shift in the allocation of assimilates from shoot to root were the acclimation mechanisms showed by R. alaternus under both types of stress. The accumulated effects in low air humidity and water deficit plants could explain the highest percentage of survival at the end of the establishment period (97%) for the combined treatment.

Water deficit effects on osmotic adjustment and solute accumulation in leaves of annual clovers

Annual clovers are very popular forage crops in the Mediterranean areas. However, their herbage and seed yields are often reduced by high temperatures and water stress occurring during spring and summer. Field experiments using the four most widely grown forage legumes, berseem ( Trifolium alexandrinum L.), crimson clover ( T. incarnatum L.), Persian clover ( T. resupinatum L.) and squarrosum clover ( T. squarrosum L.), were conducted in 1992 and 1993 at Foggia (Italy) to evaluate the osmotic adjustment capability and the relevant contribution of inorganic and organic solutes in response to water deficits. Soil water depletion reduced leaf water potential ( Ψ w) and leaf osmotic potential at full turgor ( Ψ π 100) in both years. In particular, berseem and squarrosum clovers showed lower values (−1.46 and −1.51 MPa on average, for Ψ w and Ψ π 100, respectively). Furthermore, water-stressed plants showed an increase in potassium, reducing sugars and proline concentrations and a decrease in non-reducing sugar contents. Generally, about 80% of the measured cellular osmotic potential was attributable to assayed osmotically active solutes; the inorganic ions represented the major contributors (about 59%). However, only proline levels appeared to change clearly in terms of relative contribution to Ψ π 100, showing an increase of about 2.2%, under water-stressed conditions. Statistically significant differences ( P≤0.05) in the osmotic adjustment trait were not found for either species or year. Because clover species showed the same value for solute accumulation capability (0.34 MPa), degree of tolerance to dehydration and turgor maintenance but differed in leaf water parameters, other physiological and/or morphological traits will have to be investigated to better explain the performance of each species under water-stress conditions. However, the tendency of crimson and Persian clovers to maintain high leaf water potentials could be an example of a stress-avoidance mechanism.

Alleviation of drought stress of Chile ancho pepper ( Capsicum annuum L. cv. San Luis) with arbuscular mycorrhiza indigenous to Mexico

Selecting indigenous mycorrhizal fungi that enhance plant water status is important in Mexico for sustainable production systems of Chile ancho pepper ( Capsicum annuum L. cv. San Luis). To determine mycorrhizal enhancement of drought resistance, plants were either non-inoculated (NonAMF), or inoculated with arbuscular mycorrhizal fungi (AMF): Glomus fasciculatum and a mixed Glomus spp. from Mexico (ZAC-19). Plants were then exposed to a 20-day drought cycle. To equalize growth and minimize tissue-P differences, NonAMF plants received higher P than AMF plants. Drought reduced leaf water potential ( Ψ leaf), tissue relative water content (RWC), stomatal conductance ( g s), whole plant transpiration (mg H 2O m −2 s −1), leaf transpirational surface area and plant biomass. Only plants colonized with ZAC-19 had enhanced drought resistance, as indicated by higher Ψ leaf and fewest plants with visible wilting during peak drought stress. A higher root/shoot ratio occurred with ZAC-19 plants (despite equal total plant biomass among droughted plants), which may have also contributed to drought resistance. Drought enhanced arbuscule formation and hyphae development of ZAC-19, while reducing colonization of G. fasciculatum. Tissue P was not a contributing factor to drought resistance. AMF did not enhance water-use efficiency (WUE) as-determined gravimetrically on a whole plant basis (g carbon/kg H 2O) or by carbon isotope discrimination ( Δ).

Abscisic acid in leaves and roots of willow: significance for stomatal conductance.

Excised leaves and roots of willow (Salix dasyclados Wimm.) accumulated abscisic acid (ABA) in response to desiccation. The accumulation of ABA was greater in young leaves and roots than in old leaves and roots. In mature leaves, ABA accumulation was related to the severity and duration of the desiccation treatment. Water loss equal to 12% of initial fresh weight caused the ABA content of mature leaves to increase measurably within 30 min and to double in 2.5 h. The drying treatment caused significant (P = 0.05) reductions in leaf water potential and stomatal conductance. Recovery of leaf water potential to the control value occurred within 10 min of rewatering the dehydrated leaves, but recovery of stomatal conductance took an hour or longer, depending on the interval between dehydration and rewatering. The addition of ABA to the transpiration stream of well-watered excised leaves was sufficient to cause partial stomatal closure within 1 h and, depending on ABA concentration, more or less complete stomatal closure within 3 h. When the ABA solution was replaced with water, stomatal conductance increased at a rate inversely related to the concentration of the ABA solution with which the leaves had been supplied.

Water use of young ‘Fuji’ apple trees at three soil moisture regimes in drainage lysimeters

Studies were conducted during 4 months of each growing season in 1994 and 1995 to measure water use of young apple trees ( Malus domestica Borkh. cv ‘Fuji’) growing under different soil moisture regimes in temperate climate conditions and to evaluate monthly crop coefficients of such conditions. To do so, double pot lysimeters under a transparent rain shield were designed and installed. The three soil moisture regimes in three replicates each were: (A) drip-irrigation at −50 kPa of soil matric potential (IR50); (B) drip-irrigation at −80 kPa of soil matric potential (IR80); and (C) constant shallow water table at 0.45 m below the soil surface (WT45). In each treatment, soil surface was maintained with or without turf grasses. Monthly water use was not different in drip-irrigated treatments (IR50 and IR80), but greatest in the WT45 treatment. Monthly crop coefficients increased linearly in time for drip-irrigated apple trees ( r 2 values of 0.76 ∗∗∗ for IR50 and of 0.77 ∗∗∗ for IR80), while those obtained in the WT45 treatment fluctuated. Leaf water potential (LWP) of drip-irrigated trees was similar until 63 days after treatment (DAT), but the values for IR80 trees began to decline thereafter. The LWP of WT45 trees decreased from 48 DAT. Temporal variations in leaf water content (LWC) was similar to that of LWP, except for two abrupt decreases in IR80 trees. The LWC of WT45 trees began to decrease from 59 DAT, and this occurred 2 weeks after the reduction in LWP. Average shoot length of IR50 trees was greater than that of IR80 and WT45 trees. The results of this study provided water use and crop coefficients for apple trees in relation to soil moisture regimes under temperate climate.

Effect of liana cutting on water potential and growth of adult Senna multijuga (Caesalpinioideae) trees in a Bolivian tropical forest.

Lianas, or woody climbing plants, are a major constituent of seasonally dry tropical forests, and are thought to impact negatively their host trees. In this study we evaluated whether liana presence was associated with reduced leaf water potentials and growth in adult Senna multijuga trees during the dry season in a lowland Bolivian forest. We used leaf water potentials in trees as a first approach to assess trees' water status, under the assumption that leaf water potentials become more negative when water losses (via transpiration) exceed gains (by uptake). We measured relative growth in girth at 1.5 m height (gbh) to quantify tree growth. At the beginning of the 1996 dry season (early June), we selected 20 S. multijuga trees 10-20 cm dbh, and measured their gbh. We also recorded pre-dawn and mid-day leaf water potentials in these trees. In ten experimental trees all lianas were then cut, while the remaining trees were used as controls. Pre-dawn and mid-day water potentials were re-measured 1 day after liana-cutting, and then every week in all trees for 1 month and then at 3 and 5 months, until the beginning of the next rainy season (November); gbh was measured again in July 1997 to estimate relative growth rate. Liana removal was associated with less negative pre-dawn (-0.3 vs -0.4 MPa) and mid-day (-0.5 vs -0.7 MPa) water potentials in trees during the dry season. This difference appeared as early as 1 day after cutting, and disappeared once the rainy season began. Liana-cut trees grew more (0.4 mm/mm year) than liana-uncut trees (0.2 mm/mm year). These findings suggest that lianas may interfere with water availability to these trees during the dry season, and may also hinder tree growth.

Characterization of the photosynthetic induction response in a Populus species with stomata barely responding to light changes

The photosynthetic induction response is constrained by stomatal and biochemical limitations. However, leaves in some plants like Populus koreana x trichocarpa cv. Peace (a hybrid clone) may have little stomatal limitation because their stomata barely respond to changes in photon flux density (PFD). We examined the induction responses of leaves of well-watered and dehydrated P. koreana x trichocarpa plants grown in a high-light or a low-light regime. With an increase in PFD from 50 to 500 micromol m(-2) s(-1), steady-state stomatal conductance (g(s)) increased by only 0.25-8.2%, regardless of the initial g(s), but steady-state assimilation rate (A) increased by 550-1810%. Photosynthetic induction times required to reach 50% (IT50) and 90% (IT90) of A at high PFD were 60-90 s and 210-360 s, respectively. Examination of the dynamic relationships between A and g(s), and between A and intercellular CO2 concentration, indicated that the induction limitation was imposed completely by the biochemical components within 30-40 s after the PFD increase. Values of IT50 and IT90 were significantly higher in low-light leaves than in high-light leaves, whereas the induction state at 60 s and the induction efficiency at 60 and 120 s after the increase in PFD were lower in low-light leaves than in high-light leaves. Dehydration reduced leaf water potential (psi) significantly, resulting in a significantly decreased initial g(s). Leaf water potential had no significant effects on induction time in high-light leaves, but a low psi significantly reduced the induction time in low-light leaves. We conclude that the photosynthetic induction response was limited almost completely by biochemical components because the stomata barely responded to light changes. The biochemical limitation appeared to be higher in low-light leaves than in high-light leaves. Mild water stress may have reduced steady-state A and g(s), but it had little effect on the photosynthetic induction response in high-light leaves.

Photosynthesis and water‐relation traits of the summer annual C4 grasses, Eleusine indica and Digitaria adscendens, with contrasting trampling tolerance

Two summer annual C4 grasses with different trampling susceptibilities were grown as potted plants, and diurnal leaf gas exchange and leaf water potential in each grass were compared. The maximum net photosynthetic rate, leaf conductance and transpiration rate were higher in the trampling‐tolerant Eleusine indica (L.) Gaertn. than in trampling sensitive Digitaria adscendens (H. B. K.) Henr. Leaf water potential was much lower in E. indica than in D. adscendens. There were no differences in soil‐to‐leaf hydraulic conductance and leaf osmotic potential at full turgor as obtained by pressure–volume analysis. However, the bulk modulus of elasticity in cell walls was higher in E. indica leaves than in D. adscendens leaves. This shows that the leaves of E. indica are less elastic. Therefore, the rigid cell walls of E. indica leaves reduced leaf water potential rapidly by decreasing the leaf water content, supporting a high transpiration rate with high leaf conductance. In trampled habitats, such lowering of leaf water potential in E. indica might play a role in water absorption from the compacted soil. In contrast, the ability of D. adscendens to colonize dry habitats such as coastal sand dunes appears to be due to its lower transpiration rate and its higher leaf water potential which is not strongly affected by decreasing leaf water content.

High vapour pressure deficit results in a rapid decline of leaf water potential and photosynthesis of carrots grown on free-draining, sandy soils

Two carrot (Daucus carota L.) genotypes (Nantes and Imperator) were grown in the field on a coarse-textured, sandy soil. Experiments were conducted over 2 consecutive seasons, one providing cool growing conditions and the other much warmer growing conditions during which the vapour pressure deficit was up to 2-fold higher than in the first season. Changes in growth, soil water content, and environmental conditions were monitored for both seasons, and diurnal measurements of leaf water potential and leaf photosynthesis were taken near maturity. Frequent irrigation maintained bulk soil water content above, or near, field capacity, with the sum of rainfall and irrigation exceeding potential evaporation by 1.4- and 1.3-fold during the cool and warm seasons, respectively. Even under such well-watered conditions, a large diurnal variation in leaf water potential (1200 and 1800 kPa for the cool- and warm-season crops, respectively) was recorded. During the cool season, withholding irrigation for up to 60 h resulted in further reductions in midday leaf water potential. However, there was no effect of withholding irrigation on leaf water potential during the warm season. During both seasons, leaf photosynthetic rate of well-watered plants peaked at around 20 µmol/m2.s in the early morning (0900 hours) and then decreased throughout the day, with the magnitude of the decline associated with the prevailing vapour pressure deficit. Under well-watered conditions, leaf water potential and photosynthesis were both negatively correlated with vapour pressure deficit, for both genotypes. Leaf water potential and photosynthesis were positively correlated with each other and we conclude that a high hydraulic resistance in the plant or soil results in a vapour pressure deficit-induced reduction in leaf water potential, which in turn reduces the rate of leaf photosynthesis.

Interactive effects of nitrate and long-term waterlogging on growth, water relations, and gaseous exchange properties of maize (Zea mays L.)

The interactive effect of additional amount of NO3-N and long-term waterlogging on maize was studied in glasshouse conditions. Forty-two-day-old plants were subjected to continuous flooding for 21 days at three different NO3-N regimes (196, 294 and 392 mg N kg−1 soil). Shoot fresh mass and leaf area of waterlogged plants reduced significantly at two higher NO3 regimes. Leaf water potential was generally decreased, whereas osmotic potential increased in all waterlogged plants but much reduction in leaf water potential was found at the highest external NO3 regime. Leaf turgor potential decreased due to waterlogging but this decrease progressed with increase in external NO3 concentration. Chlorophylls ‘a’ and ‘b’ increased in non-waterlogged plants with increase in NO3 concentration of the growth medium, but these two pigments decreased significantly due to waterlogging particularly at the two higher NO3 regimes. Chlorophyll a/b ratio increased linearly in non-waterlogged plants with increase in external NO3 regimes but the ratio remained almost unchanged due to waterlogging. Waterlogging caused a reduction in net photosynthesis and stomatal conductance but no effect of additional amount of NO3 was observed on these two variables. Transpiration was also decreased as a result of waterlogging but a marked reduction in this variable was observed at the highest external NO3 concentration. Water-use efficiency increased with increase in external NO3 concentration in both waterlogged and non-waterlogged plants. Although waterlogging caused a reduction in substomatal CO2 concentration, it generally increased in both waterlogged and non-waterlogged plants due to supplementary NO3, particularly at its highest concentration. From these results, it is clear that supplementary NO3 in the growth medium of maize plants experiencing long-term waterlogging had injurious effect on growth.