Heliotropic leaf movements in common beans controlled by air temperature

Heliotropic leaf movements were examined in common beans (Phaseolus vulgaris cv Blue Lake Bush) under outdoor and laboratory conditions. Heliotropic leaf movements in well-watered plants were partly controlled by temperature, and appeared to be independent of atmospheric humidity and CO₂ concentration. When environmental conditions were held constant in the laboratory, increased air temperature caused bean leaves to orient more obliquely to a light source. Ambient CO₂, intercellular CO₂, and net photosynthesis were not correlated with the temperature-induced changes in heliotropic movements, nor did they significantly affect these movements directly. The effect of air temperature on leaf movements need not be mediated through a change in leaf water potential, transpiration, or leaf conductance. Air temperature modified laminar orientation in light through its effect on tissue temperature in the pulvinal region, not that of the lamina or petiole. However, under darkness the temperature effects on leaf movements were not expressed. Active heliotropic movements in response to air temperature allowed lamina temperature to remain close to the thermal optimum of photosynthesis. This temperature effect underlies a commonly observed pattern of leaf movements under well-watered conditions: a tendency for leaves to face the sun more obliquely on hot days than cool days.     

Effect of drought and rewatering on photosynthetic physioecological characteristics of soybean

Field experiments were conducted on soybean Glycine max, yudou29, a major cultivated variety in the Henan Province of China to study the relationship between photosynthetic characteristics and other physioecological parameters of its leaves under soil drying and rewatering treatments. The study showed that the dawn water potential of soybean leaves under the drying treatment was very close to that of soybean leaves under well-watered treatments (CK) when soil water content was higher than 47% of field water capacity (FWC). But when soil water content dropped below 47% of FWC, the leaf water potential decreased rapidly, indicating a significant threshold reaction. The dawn water potential threshold of soybean leaves was about ?1.02 MPa. Below this, the leaf water potential and net photosynthesis ratio dropped rapidly. When the soil water content was 47%, the leaf water potential and net photosynthesis ratio were nearly as high as those in CK, but the transpiration ratio was 67% lower, indicating that transpiration was more sensitive to drought than photosynthesis. After rewatering, the water status of soybean leaves improved, the net photosynthesis ratio and transpiration ratio increased linearly, and leaf stomata conductance (Gs) also recovered quickly. These results showed that after stress removal, soybean had fast-growing characteristics.     

Change of shoot architecture during juvenile-to-adult phase transition in soybean

Juvenile-to-adult phase change is an indispensable event which guarantees a successful life cycle. Phase change has been studied in maize, Arabidopsis and rice, but is mostly unknown in other species. Soybean/Fabaceae plants undergo drastic changes of shoot architecture at the early vegetative stage including phyllotactic change and leaf type alteration from simple to compound. These characteristics make soybean/Fabaceae plants an interesting taxon for investigating vegetative phase change. Following the expansion of two cotyledons, two simple leaves simultaneously emerge in opposite phyllotaxy. The phyllotaxy of the third and fourth leaves is not fixed; both opposite and distichous phyllotaxis are observed within the same population. Leaves were compound from the third leaf. But the third leaf was rarely simple. Morphological and quantitative changes in early vegetative phase were recognized in leaf size, leaf shape, number of trichomes, stipule size and shape, and shoot meristem shape. Two microRNA genes, miR156 and miR172, are known to be associated with vegetative phase change. Examination of the expression level revealed that miR156 expression was high in the first two leaves and subsequently down-regulated, and that of miR172 showed the inverse expression pattern. These expression patterns coincided with the case of other species. Taken all data together, the first and second leaves represent juvenile phase, the fifth and upper leaves adult phase, and the third and fourth leaves intermediate stage. Further investigation of soybean phase change would give fruitful understandings on plant development.     

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.     

Osmoregulation in Cotton in Response to Water Stress : II. LEAF CARBOHYDRATE STATUS IN RELATION TO OSMOTIC ADJUSTMENT

Diurnal changes in tissue water potential components, photosynthesis, and specific leaf carbohydrates were examined in water stress-adapted and nonadapted cotton plants. Adapted plants exhibited lower daily minimum leaf water potentials and maintained turgor to lower leaf water potentials than nonadapted plants. Because of this turgor maintenance, photosynthesis continued in adapted plants at leaf water potentials that inhibited photosynthesis in nonadapted plants. Adapted plants exhibited lower rates of photosynthesis than did nonadapted plants when leaves were fully turgid. The inhibition was not due to stomatal restriction of CO₂ diffusion because leaf conductances of nonadapted and adapted leaves were similar at high leaf water potentials.     

The growth response of C4 plants to rising atmospheric CO2 partial pressure: a reassessment

Despite mounting evidence showing that C₄ plants can accumulate more biomass at elevated CO₂ partial pressure (p(CO₂)), the underlying mechanisms of this response are still largely unclear. In this paper, we review the current state of knowledge regarding the response of C₄ plants to elevated p(CO₂) and discuss the likely mechanisms. We identify two main routes through which elevated p(CO₂) can stimulate the growth of both well-watered and water-stressed C₄ plants. First, through enhanced leaf CO₂ assimilation rates due to increased intercellular p(CO₂). Second, through reduced stomatal conductance and subsequently leaf transpiration rates. Reduced transpiration rates can stimulate leaf CO₂ assimilation and growth rates by conserving soil water, improving shoot water relations and increasing leaf temperature. We argue that bundle sheath leakiness, direct CO₂ fixation in the bundle sheath or the presence of C₃-like photosynthesis in young C₄ leaves are unlikely explanations for the high CO₂-responsiveness of C₄ photosynthesis. The interactions between elevated p(CO₂), leaf temperature and shoot water relations on the growth and photosynthesis of C₄ plants are identified as key areas needing urgent research.     

Leaf Senescence Induced by Mild Water Deficit Follows the Same Sequence of Macroscopic, Biochemical, and Molecular Events as Monocarpic Senescence in Pea

We have compared the time course of leaf senescence in pea (Pisum sativum L. cv Messire) plants subjected to a mild water deficit to that of monocarpic senescence in leaves of three different ages in well-watered plants and to that of plants in which leaf senescence was delayed by flower excision. The mild water deficit (with photosynthesis rate maintained at appreciable levels) sped up senescence by 15 d (200 degrees Cd), whereas flower excision delayed it by 17 d (270 degrees Cd) compared with leaves of the same age in well-watered plants. The range of life spans in leaves of different ages in control plants was 25 d (340 degrees Cd). In all cases, the first detected event was an increase in the mRNA encoding a cysteine-proteinase homologous to Arabidopsis SAG2. This happened while the photosynthesis rate and the chlorophyll and protein contents were still high. The 2-fold variability in life span of the studied leaves was closely linked to the duration from leaf unfolding to the beginning of accumulation of this mRNA. In contrast, the duration of the subsequent phases was essentially conserved in all studied cases, except in plants with excised flowers, where the degradation processes were slower. These results suggest that senescence in water-deficient plants was triggered by an early signal occurring while leaf photosynthesis was still active, followed by a program similar to that of monocarpic senescence. They also suggest that reproductive development plays a crucial role in the triggering of senescence.     

Changes of epicuticular wax induced by enhanced UV-B radiation impact on gas exchange in Brassica napus

The epicuticular wax covering on plant surface plays important roles in protecting plants against UV radiation. However, the role of epicuticular wax in affecting leaf gas exchange under enhanced ultraviolet-B (UV-B) radiation remains obscure. In the present study, different aged leaves of Brassica napus were used to analyze the responses of crystal structure and chemical constituents of epicuticular wax to UV-B radiation and the effects of such responses on gas exchange indices. Enhanced UV-B radiation significantly decreased the amount of esters in all leaves except the first leaf, amount of secondary alcohols in the second, third and fourth leaves, and amount of primary alcohols in the second and third leaves, while increased the amounts of ketones and aldehydes in the first leaf. Enhanced UV-B level had no significant effect on the amounts of alkanes and total wax in all leaves. Exposure to UV-B radiation resulted in wax fusion on adaxial leaf and stomata opening on abaxial leaf. Fusions of plates and rods on adaxial leaf surface covered most of the stomata, thereby influencing the photosynthesis in the upper mesophyll of leaves. Enhanced UV-B level significantly reduced the net photosynthesis rate (P _N) but increased the stomata conductance (g _s), concentrations of intercellular CO₂ (C _i ), and transpiration rate (E) in all leaves. Both UV-B radiation and the wax fusion induced by enhanced UV-B radiation resulted in different stomata status on abaxial and adaxial leaf surface, causing decrease of P _N, and increase of g _s, C _i and E in leaves.     

Analysis of Stomatal and Nonstomatal Components in the Environmental Control of CO(2) Exchange in Leaves of Welwitschia mirabilis

In well-watered plants of Welwitschia mirabilis, grown in the glass-house under high irradiance conditions, net CO₂ assimilation was almost exclusively observed during the daytime. The plants exhibited a very low potential for Crassulacean acid metabolism, which usually resulted in reduced rates of net CO₂ loss for several hours during the night. In leaves exposed to the diurnal changes in temperature and humidity typical of the natural habitats, CO₂ assimilation rates in the light were markedly depressed under conditions resembling those occurring during midday, when leaf temperatures and the leaf-air vapor pressure differences were high (36°C and 50 millibars bar⁻¹, respectively). Studies on the relationship between CO₂ assimilation rate and intercellular CO₂ partial pressure at various temperatures and humidities showed that this decrease in CO₂ assimilation was largely due to stomatal closure. The increase in the limitation of photosynthesis by CO₂ diffusion, which is associated with the strong decline in stomatal conductance in Welwitschia exposed to midday conditions, may significantly contribute to the higher ¹³C content of Welwitschia compared to the majority of C₃ species.     

Abscisic Acid is not the only stomatal inhibitor in the transpiration stream of wheat plants

Xylem sap was collected from the transpiration stream of wheat (Triticum aestivum L.) plants and assayed for the presence of an inhibitor of transpiration using leaves detached from well-watered plants. Transpiration of detached leaves was reduced by nearly 60% by sap collected from plants in drying soil, and to a lesser extent (about 25%) by sap from plants in well-watered soil. As the soil dried the abscisic acid (ABA) concentration in the sap increased by about 50 times to 5 × 10⁻⁸ molar. However, the ABA in the sap did not cause its inhibitory activity. Synthetic ABA of one hundred times this concentration was needed to reduce transpiration rates of detached leaves to the same extent. Furthermore, inhibitory activity of the sap was retained after its passage through an immunoaffinity column to remove ABA. Xylem sap was also collected by applying pressure to the roots of plants whose leaf water status was kept high as the soil dried. Sap collected from these plants reduced transpiration to a lesser extent than sap from nonpressurised plants. This suggests that the inhibitory activity was triggered partly by leaf water deficit and partly by root water deficit.     

Photosynthesis and water relations of well-watered orange plants as affected by winter and summer conditions

The aim of this study was to evaluate how the summer and winter conditions affect the photosynthesis and water relations of well-watered orange trees, considering the diurnal changes in leaf gas exchange, chlorophyll (Chl) fluorescence, and leaf water potential (Ψ) of potted-plants growing in a subtropical climate. The diurnal pattern of photosynthesis in young citrus trees was not significantly affected by the environmental changes when compared the summer and winter seasons. However, citrus plants showed higher photosynthetic performance in summer, when plants fixed 2.9 times more CO₂ during the diurnal period than in the winter season. Curiously, the winter conditions were more favorable to photosynthesis of citrus plants, when considering the air temperature (< 29 °C), leaf-to-air vapor pressure difference (< 2.4 kPa) and photon flux density (maximum values near light saturation) during the diurnal period. Therefore, low night temperature was the main environmental element changing the photosynthetic performance and water relations of well-watered plants during winter. Lower whole-plant hydraulic conductance, lower shoot hydration and lower stomatal conductance were noticed during winter when compared to the summer season. In winter, higher ratio between the apparent electron transport rate and leaf CO₂ assimilation was verified in afternoon, indicating reduction in electron use efficiency by photosynthesis. The high radiation loading in the summer season did not impair the citrus photochemistry, being photoprotective mechanisms active. Such mechanisms were related to increases in the heat dissipation of excessive light energy at the PSII level and to other metabolic processes consuming electrons, which impede the citrus photoinhibition under high light conditions.     

Photosynthesis in Encelia farinosa Gray in Response to Decreasing Leaf Water Potential

Photosynthetic responses of intact leaves of the desert shrub Encelia farinosa were measured during a long term drought cycle in order to understand the responses of stomatal and nonstomatal components to water stress. Photosynthetic rate at high irradiance and leaf conductance to water vapor both decreased linearly with declining leaf water potential. The intercellular CO₂ concentration (c_i) remained fairly constant as a function of leaf water potential in plants subjected to a slow drought cycle of 25 days, but decreased in plants exposed to a 12-day drought cycle. With increasing water stress, the slope of the dependence of photosynthesis on c_i (carboxylation efficiency) decreased, the maximum photosynthetic rates at high c_i became saturated at lower values, and water use efficiency increased. Both the carboxylation efficiency and photosynthetic rates were positively correlated with leaf nitrogen content. Associated with lower leaf conductances, the calculated stomatal limitation to photosynthesis increased with water stress. However, because of simultaneous changes in the dependence of photosynthesis on c_i with water stress, increased leaf conductance alone in water-stressed leaves would not result in an increase in photosynthetic rates to prestressed levels. Both active osmotic adjustment and changes in specific leaf mass occurred during the drought cycle. In response to increased water stress, leaf specific mass increased. However, the increases in specific leaf mass were associated with the production of a reflective pubescence and there were no changes in specific mass of the photosynthetic tissues. The significance of these responses for carbon gain and water loss under arid conditions are discussed.     

The photosynthesis of ryegrass leaves grown in a simulated sward

Plants were taken from simulated swards of perennial ryegrass (Lolium perenne) grown in a controlled environment and the rates of photosynthesis of the youngest fully expanded leaves, and the second and third youngest leaves on the same tillers were measured. The youngest leaves had the highest rates and the third the lowest, with the second leaves intermediate. The rate of photosynthesis in bright light of successive youngest expanded leaves decreased as the swards increased in leaf area, but did not when plants were grown so that the main stem was not shaded. When plants were grown at different densities and the photosynthetic rates of leaves of a particular ontogenetic rank were measured, it was found that leaves on plants from higher densities had lower rates of photosynthesis. Also leaves on plants grown in bright light had higher photosynthetic rates than those on plants grown in dim light. It is concluded that the decline in the photosynthetic capacity of successive leaves in a rapidly growing simulated sward is due to the intense shading to which they are subjected during their expansion.     

Osmotic adjustment, symplast volume, and nonstomatally mediated water stress inhibition of photosynthesis in wheat

At low water potential (ψ_w), dehydration reduces the symplast volume of leaf tissue. The effect of this reduction on photosynthetic capacity was investigated. The influence of osmotic adjustment on this relationship was also examined. To examine these relationships, comparative studies were undertaken on two wheat cultivars, one that osmotically adjusts in response to water deficits (`Condor'), and one that lacks this capacity (`Capelle Desprez'). During a 9-day stress cycle, when water was withheld from plants grown in a growth chamber, the relative water content of leaves declined by 30% in both cultivars. Leaf osmotic potential (ψ_s) declined to a greater degree in Condor plants. Measuring ψ_s at full turgor indicated that osmotic adjustment occurred in stressed Condor, but not in Capelle plants. Two methods were used to examine the degree of symplast (i.e. protoplast) volume reduction in tissue rapidly equilibrated to increasingly low ψ_w. Both techniques gave similar results. With well-watered plants, symplast volume reduction from the maximum (found at high ψ_w for each cultivar) was the same for Condor and Capelle. After a stress cycle, volume was maintained to a greater degree at low ψ_w in Condor leaf tissue than in Capelle. Nonstomatally controlled photosynthesis was inhibited to the same degree at low ψ_w in leaf tissue prepared from well-watered Condor and Capelle plants. However, photosynthetic capacity was maintained to a greater degree at low ψ_w in tissue prepared from stressed Condor plants than in tissue from stressed Capelle plants. Net CO₂ uptake in attached leaves was monitored using an infrared gas analyzer. These studies indicated that in water stressed plants, photosynthesis was 106.5% higher in Condor than Capelle at ambient [CO₂] and 21.8% higher at elevated external [CO₂]. The results presented in this report were interpreted as consistent with the hypothesis that there is a causal association between protoplast (and presumably chloroplast) volume reduction at low ψ_w and low ψ_w inhibition of photosynthesis. Also, the data indicate that osmotic adjustment allows for maintenance of relatively greater volume at low ψ_w, thus reducing low ψ_w inhibition of chloroplast photosynthetic potential.     

Dirunal leaf movement, chlorophyll fluorescence and carbon assimilation in soybean grown under different nitrogen and water availabilities

Diurnal heliotropic leaf movements, photosynthetic gas exchange, and the ratio of variable fluorescence to maximum fluorescence (Fv/Fm) of unrestrained and horizontally restrained leaves from soybean (Glycine max cv. Cumberland) plants grown in two different water and two different nitrogen treatments were measured. Leaves of plants grown in low water or low nitrogen availability treatments displayed more pronounced diaheliotropism (solar tracking) in the afternoon and a longer period of paraheliotropism (light avoiding) at midday relative to those of well-watered, high-nitrogen-grown plants. Photosaturated photosynthetic rates and the photon flux required to saturate photosynthesis were reduced by water stress and nitrogen deficiency. Compared to horizontal leaves, irradiance on orienting leaves was nearer to the breakpoint of the photosynthetic light response curve, where photosynthesis is co-limited by ribulose biphosphate regeneration and carboxylation. This would increase the carbon return on investments of nitrogen into photosynthesis. A positive linear relationship between Fv/Fm and quantum yield of photosynthesis was measured. Leaves of low-nitrogen-grown plants had earlier and more prolonged reductions in Fv/Fm at midday compared to leaves of high nitrogen grown plants of the same water treatment. Within the same water and nitrogen treatment, horizontally restrained leaves had lower midday Fv/Fm in relation to orienting leaves. Nitrogen deficiency and water stress enhanced this difference such that horizontally restrained leaves of low water and low nitrogen grown plants had earlier and longer midday depressions in Fv/Fm.     

Influence of leaf age on photosynthesis, enzyme activity, and metabolite levels in wheat

The rate of photosynthesis under high light (1000 micromole quanta per square meter per second) and at 25°C was measured during development of the third leaf on wheat plants and compared with the activity of several photosynthetic enzymes and the level of metabolites. The rate of photosynthesis reached a maximum the 7th day after leaf emergence and declined thereafter. There was a high and significant correlation between the rate of photosynthesis per leaf area and the activities of the enzymes ribulose 5-phosphate kinase (r = 0.91), ribulose 1,5-bisphosphate (RuBP) carboxylase (r = 0.94), 3-phosphoglycerate (PGA) kinase (r = 0.82), and fructose 1,6-bisphosphatase (r = 0.80) per leaf area. There was not a significant correlation of photosynthesis rate with chlorophyll content. The rate of photosynthesis was strongly correlated with the level of PGA (r = 0.85) and inversely correlated with the level of triose phosphate (dihydroxyacetone phosphate and glyceraldehyde 3-phosphate) (r = 0.92). RuBP levels did not change much during leaf development; therefore photosynthesis rate was not correlated with the level of RuBP. The rate of photosynthesis was at a maximum when the ratio of PGA/triose phosphate was high, and when the ratio of RuBP/PGA was low. Although several enzymes change in parallel with leaf development, the metabolite changes suggest the greatest degree of control may be through RuBP carboxylase. The sucrose content of the leaf was highest under high rates of photosynthesis. There was no evidence that later in leaf development, photosynthesis (measured under high light and at 25°C) was limited by utilization of photosynthate.     

Tolerance and physiological responses of Phragmites australis to water deficit

The water stress tolerance of Phragmites australis (Cav.) Trin ex. Steud. grown in the laboratory were investigated by examining effects of different levels of imposed water deficits on growth, photosynthesis and various physiological traits related to water stress. Individual plants were grown under conditions of unrestricted water supply and compared with groups of plants receiving 60, 30, 15 or 5% of previous daily water requirements, respectively.Water deficit was found to reduce the leaf area and the leaf biomass per plant due to decreased production of new leaves, increased leaf shedding and reduced average leaf size. Leaf production and leaf expansion growth were very sensitive to water availability and were reduced when plants were subjected to fairly mild water deficit. Osmolality in sap expressed from leaves and the concentration of proline in leaves were only significantly increased in severely stressed plants, indicating that osmotic adjustment was of minor importance until a critical stress level was reached. Photosynthetic parameters were rather unaffected until the water availability was very low and led to the assertion that reduced CO₂ assimilation was mainly due to stomatal closure and not biochemical changes. Water stress had no effect on the activity of Rubisco. The CO₂ assimilation rate and stomatal conductance decreased in such a way that the intrinsic water use efficiency (A/g_s) increased, indicating efficient CO₂ utilization in water stressed plants. The apparent quantum yield (φ_i) was reduced in leaves of the most stressed plants, probably due to a decrease in the CO₂ molar fraction in the chloroplasts following stomatal closure.The initial response of P. australis to water deficit is a reduction in leaf area, the remaining leaves staying physiological rather well functioning until they are severely stressed. A high intrinsic water use efficiency and the ability to maintain some capacity for photosynthesis under severe water stress can undoubtedly contribute to the survival of P. australis under dry conditions. Taken together with its well-developed adaptations to flooding, P. australis seems very well adapted to grow in wetland areas with a widely fluctuating hydroperiod. P. australis grows very well in rather deep water, but can also tolerate extensive periods of drought with reduced availability of water.     

Effects of water stress on gas exchange and water relations of a succulent epiphyte,Kalanchoë uniflora

Summary Measurements of gas exchange, xylem tension and nocturnal malate synthesis were conducted with well-watered and droughted plants of Kalanchoë uniflora. Corresponding results were obtained with plants grown in 9 h and 12 h photoperiods. In well-watered plants, 50 to 90% of total CO₂-uptake occurred during the light period. Nocturnal CO₂-uptake and malate synthesis were higher and respiration rate was lower in old leaves (leaf pairs 6 to 10) compared to young leaves (leaf pairs 1 to 5). Within four days of drought distinct physiological changes occurred. Gas exchange during the light period decreased and CO₂-uptake during the dark period increased. Nocturnal malate synthesis significantly increased in young leaves.Respiration rate decreased during periods of drought, this decrease being more pronounced in young leaves compared to old leaves. Restriction of gas exchange during the light period resulted in a decrease of transpiration ratio from more than 100 to about 20. The difference between osmotic pressure and xylem tension decreased in young leaves, indicating a reduction in bulk leaf turgor-pressure.We conclude that both the CAM-enhancement in young leaves and the decrease of respiration rate are responsible for the increase of nocturnal CO₂-uptake during water stress. During short drought periods, which frequently occur in humid habitats, the observed physiological changes result in a marked reduction of water loss while net CO₂-uptake is maintained. This might be relevant for plant growth in the natural habitat.Abbreviations LP light period - DP dark period - CAM crassulacean acid metabolism