Responses of apple leaf stomata: a model for single leaves and a whole tree

Abstract. An empirical model of stomatal response to environmental factors was developed from measurements of stomatal conductance ( g _s) made in a leaf chamber under controlled conditions. Results presented in a companion paper (Warrit, Landsberg & Thorpe, 1980) indicated that the model could be written in terms of only two factors, photon flux density ( Q _p) and leaf to air vapour pressure gradient ( D ). The response of Q _p was hyperbolic and that to D linear; combining these the equation of the model is where g _r is a reference conductance, α is the slope of the response to D and β indicates the sensitivity of g _s response to Q _p. Values of α were 0.20 and 0.30 kPa⁻¹ in June and August; the corresponding values of β were 59 and 79 μmol m⁻² s⁻¹.
The model was tested against mean values of g _s obtained with a porometer in the field, using environmental measurements as inputs. Correspondence between measured and calculated values was good. Transpiration rates were calculated from the Penman-Monteith equation, with stomatal resistance values calculated from the model, and compared with gravimetric measurements of tree water use. It was shown that transpiration could be calculated with acceptable accuracy. The effects of variations in stomatal resistance on transpiration rates under a range of conditions were explored using the model and the Penman- Monteith equation.     

Influence of NaCl, Cd(NO3)2 and air humidity on transpiration of Tamarix aphylla

Transpiration rates of young Tamarix aphylla (L.) Karst, plants grown in hydroponics were measured under NaCl- and Cd(NO₃)₂-stress. Transpiration rates were negatively correlated with the relative humidity of the ambient air at all NaCl concentrations investigated. Low and intermediate concentrations of Cd²⁺ (45 and 90 μ M , respectively) in the medium caused an increase in transpiration rates. This was particularly pronounced at low levels of relative humidity. At 180 μ M Cd²⁺, transpiration rates dropped, probably as a result of root damage due to Cd²⁺ toxicity. Since the transpiration rates differed by a factor of ca 3 between day and night, it is concluded that the stomata did not lose their ability to regulate transpiration under the influence of NaCl or of Cd(NO₃)₂. The transpiration behaviour of T. aphylla indicates that the effect of water vapour pressure (presented as relative humidity) on the degree of stomatal opening is small. Under conditions of ample water supply transpiration follows the evaporative demand of the ambient air and is influenced by the water uptake capacity of the root system as well as by other environmental factors, e.g. light.     

Stomatal responses to humidity in air and helox

Abstract. Stomatal responses to humidity were studied in several species using normal air and a helium: oxygen mixture (79:21 v/v, with CO₂ and water vapour added), which we termed 'helox'. Since water vapour diffuses 2.33 times faster in helox than in air, it was possible to vary the water-vapour concentration difference between the leaf and the air at the leaf surface independently of the transpiration rate and vice versa. The CO₂ concentration at the evaporating surfaces ( c_i ), leaf temperature and photon flux density were kept constant throughout the experiments. The results of these experiments were consistent with a mechanism for Stomatal responses to humidity that is based on the rate of water loss from the leaf. Stomata apparently did not directly sense and respond to either the water vapour concentration at the leaf surface or the difference in water vapour concentration between the leaf interior and the leaf surface. In addition, stomatal responses that caused reductions in transpiration rate at low humidities were accompanied by decreases in photosynthesis at constant c_i , suggesting heterogeneous (patchy) stomatal closure.     

Changes in leaf water status associated with salinity in mature, field grown Prunus salicina

Seasonal and diurnal measurements of leaf water potential (ψ₁), relative water content (RWC) and stomatal conductance (g_s) were made in the field on 19-year old Prunus salicina (L.) cv. Santa Rosa, a deciduous fruit tree species, irrigated with 3 different concentrations of saline water over a 3 year period (1985-1987). With the exception of stage III of fruit growth, little or no treatment difference in Φ₁, leaf turgor potential (Φ_p), or RWC was noted during the day. Seasonal averages of morning (0700-0900) and afternoon (1500-1700) Φ_p did not decline with increasing salinity, indicating long-term osmotic adjustment in this species. Maintenance of leaf water status under saline conditions was in part a consequence of increased stomatal closure, with a subsequent reduction in leaf transpiration rate. However, during stage III of fruit growth, an increase in mean afternoon (1200-1700) stomatal conductance of 26-117%, independent of salinity treatment, was observed in 1985 and again in 1987. Higher conductance values during this period may be associated with rapid fruit expansion and greater assimilate demand. The observed increase in conductance resulted in greater leaf water loss and larger measured differences in midday ψ₁ between salinity treatments. This research indicates that for Prunus salicina in the field, salinity stress resulted in leaf water deficits only during the final period of fruit expansion and ripening.     

Stomatal crypts may facilitate diffusion of CO2 to adaxial mesophyll cells in thick sclerophylls

In some plants, stomata are exclusively located in epidermal depressions called crypts. It has been argued that crypts function to reduce transpiration; however, the occurrence of crypts in species from both arid and wet environments suggests that crypts may play another role. The genus Banksia was chosen to examine quantitative relationships between crypt morphology and leaf structural and physiological traits to gain insight into the functional significance of crypts. Crypt resistance to water vapour and CO₂ diffusion was calculated by treating crypts as an additional boundary layer partially covering one leaf surface. Gas exchange measurements of polypropylene meshes confirmed the validity of this approach. Stomatal resistance was calculated as leaf resistance minus calculated crypt resistance. Stomata contributed significantly more than crypts to leaf resistance. Crypt depth increased and accounted for an increasing proportion of leaf resistance in species with greater leaf thickness and leaf dry mass per area. All Banksia species examined with leaves thicker than 0.6 mm had their stomata in deep crypts. We propose that crypts function to facilitate CO₂ diffusion from the abaxial surface to adaxial palisade cells in thick leaves. This and other possible functions of stomatal crypts, including a role in water use, are discussed.     

Ecophysiological and morphological parameters related to survival in grass species exposed to an extreme climatic event

An experiment was performed to elucidate interspecific differences in survival time of grass species subjected to an extreme climatic event. We exposed eight grass species to a simulated heat wave in the field ('free air' temperature increase at 11°C above ambient) combined with drought. We determined whether interspecific differences in survival time were related to the responses of the species to the imposed stress or could be explained by their ecophysiological or morphological characteristics in unstressed conditions. Surprisingly, there was no effect of specific leaf area, but species with a higher total leaf area survived longer. This may arise from a greater water reserve in the plant as a whole, which could delay the desiccation of the meristem, or from reduced evaporation due to a higher leaf area index. Species in which the decrease in light-saturated stomatal conductance ( g _s ) and photosynthetic CO₂ uptake rate ( A _max ) was strongly related to the decrease in soil water availability (measured as soil relative water content and stress duration) survived longer than species in which g _s and A _max likewise declined but responded more to daily fluctuations in irradiance, temperature, and vapor pressure deficit during the heat wave. We, therefore, hypothesize that interspecific differences in stress survival time might be related to the extent to which stomata react to changes in soil water conditions relatively to changes in other environmental and physiological factors. The results suggest that resistance to extremes is governed by other mechanisms than resistance to moderate drought.     

Changes in leaf nitrogen and carbohydrates underlie temperature and CO2 acclimation of dark respiration in five boreal tree species

We tested the hypothesis that acclimation of foliar dark respiration to CO₂ concentration and temperature is associated with adjustments in leaf structure and chemistry. Populus tremuloides Michx. , Betula papyrifera Marsh. , Larix laricina (Du Roi) K. Koch , Pinus banksiana Lamb., and Picea mariana (Mill.) B.S.P. were grown from seed in combined CO₂ (370 or 580 μ mol mol^–1) and temperature treatments (18/12, 24/18, or 30/24 °C). Temperature and CO₂ effects were predominately independent. Specific respiration rates partially acclimated to warmer thermal environments through downward adjustment in the intercept, but not Q ₁₀ of the temperature–response functions. Temperature acclimation of respiration was larger for conifers than broad-leaved species and was associated with pronounced reductions in leaf nitrogen concentrations in conifers at higher growth temperatures. Short-term increases in CO₂ concentration did not inhibit respiration. Growth in the elevated CO₂ concentration reduced leaf nitrogen and increased non-structural carbohydrate concentrations. However, for a given nitrogen concentration, respiration was higher in leaves grown in the elevated CO₂ concentration, as rates increased with increasing carbohydrates. Across species and treatments, respiration rates were a function of both leaf nitrogen and carbohydrate concentrations ( R ² = 0·71, P < 0·0001). Long-term acclimation of foliar dark respiration to temperature and CO₂ concentration is largely associated with changes in nitrogen and carbohydrate concentrations.     

Leaf stomatal responses to vapour pressure deficit under current and CO2-enriched atmosphere explained by the economics of gas exchange

Using the economics of gas exchange, early studies derived an expression of stomatal conductance ( g ) assuming that water cost per unit carbon is constant as the daily loss of water in transpiration ( f _e) is minimized for a given gain in photosynthesis ( f _c). Other studies reached identical results, yet assumed different forms for the underlying functions and defined the daily cost parameter as carbon cost per unit water. We demonstrated that the solution can be recovered when optimization is formulated at time scales commensurate with the response time of g to environmental stimuli. The optimization theory produced three emergent gas exchange responses that are consistent with observed behaviour: (1) the sensitivity of g to vapour pressure deficit ( D ) is similar to that obtained from a previous synthesis of more than 40 species showing g to scale as 1 −  m  log( D ), where m   ∈  [0.5,0.6], (2) the theory is consistent with the onset of an apparent 'feed-forward' mechanism in g , and (3) the emergent non-linear relationship between the ratio of intercellular to atmospheric [CO₂] ( c _i/ c _a) and D agrees with the results available on this response. We extended the theory to diagnosing experimental results on the sensitivity of g to D under varying c _a.     

Physiological effects of long-term exposure to low and moderate concentrations of atmospheric NH3 on poplar leaves

Abstract. Poplar shoots ( Populus euramericana L.) obtained from cuttings were exposed for 6 or 8 weeks to NH₃ concentrations of 50 and 100 μgm⁻³ or filtered air in fumigation chambers. After this exposure the rates of NH₃ uptake, transpiration, CO₂ assimilation and respiration of leaves were measured using a leaf chamber. During the long-term exposure also modulated chlorophyll fluorescence measurements were carried out to obtain information about the photosynthetic performance of individual leaves. Both fluorescence and leaf chamber measurements showed a higher photosynthetic activity of leaves exposed to 100 μg NH₃ m⁻³. These leaves showed also a larger leaf conductance and a larger uptake rate of NH₃ than leaves exposed to 50 μg m⁻³ NH₃ or filtered air. The long-term NH₃ exposure did not induce an internal resistance against NH₃ transport in the leaf, nor did it affect the leaf cuticle. So, not only at a short time exposure, but also at a long-term exposure NH₃ uptake into leaves can be calculated from data on the boundary layer and stomatal resistance for H₂O and ambient NH₃-concentration. Furthermore, the NH₃ exposure had no effect on the relation between CO₂-assimilation and stomatal conductance, indicating that NH₃ in concentrations up to 100 μg m⁻³ has no direct effect on stomatal behaviour; for example, by affecting the guard or contiguous cells of the stomata.     

Theory and practice of a portable photosynthesis instrument

Abstract. Theory and practice of non-steady-state portable photosynthesis instruments (LI-6000 and 6200, LI-COR Inc., Nebraska, U.S.A.) are presented. Mass balance equations for the time dependence of H₂O and CO₂ mol fractions within the leaf chamber were used to describe instrument function. Measurements for each run were fitted to an exponential function to estimate average rates of CO₂ assimilation and transpiration during the measurement period. Stomatal conductances and intercellular CO₂ mol fractions were also computed. Linear data analysis used in the LI-6200 produced similar results for assimilation rates, stomatal conductances and intercellular CO₂ concentrations compared to a more rigorous nonlinear analysis, provided humidity within the chamber was kept constant during the measurement period. Instrument performance for CO₂ fluxes was confirmed by injecting pure CO₂ at steady rates from a microsyringe into the chamber. Miniature evaporimeters were designed to check H₂O flux measurements. Significant discrepancies were observed between LI-6200 estimates of H₂O fluxes and direct measurement and errors were attributed to adsorption desorption of water vapour on chamber walls or to leaks. The leaf chamber should be stored at humidities and temperatures similar to those during measurement conditions for maximum reliability of results.     

δ 13C of CO2 respired in the dark in relation to δ13C of leaf carbohydrates in Phaseolus vulgaris L. under progressive drought

The variations in δ ¹³C in both leaf carbohydrates (starch and sucrose) and CO₂ respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in ¹³C) with decreasing stomatal conductance and decreasing p _i/ p _a during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in ¹³C). This may be explained by increased p _i/ p _a but other possible explanations are also discussed. Interestingly, the variations in δ ¹³C of CO₂ respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ ¹³C of CO₂ respired in the dark and sucrose, respired CO₂ always being enriched in ¹³C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in ¹³C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing ¹³C-enriched CO₂; and that (ii) this leads to ¹³C depletion in the remaining leaf material.     

An experimental test of the eddy correlation technique over a Mediterranean macchia canopy

Abstract. Flux densities of water vapour and carbon dioxide were measured for a Mediterranean macchia canopy. Results show good agreement between the measured available energy and the sum of latent sensible and heat flux densities determined with the eddy correlation technique. Joint evaluation of the Bowen ratio, aerodynamic resistance, canopy resistance and the 'omega factor' suggests that the macchia canopy is intermediate in aerodynamic roughness between coniferous and deciduous canopies. Maximum daytime carbon flux densities ranged from -14 to -22(μnol m⁻² s⁻¹ on a ground area basis. The ratio of transpiration to assimilation (E/A) was a function of incident photo-synthetic photon flux density below about 400 μmol m⁻²s⁻¹ and above it was fairly constant at 272 mol mol⁻¹ (H₂O/CO₂). The relationship between carbon influx and canopy conductance was linear. Results show promising applications of the eddy correlation technique for evaluating physiological features of canopies, treated as unitary functional systems.     

Three parameters comprehensively describe the temperature response of respiratory oxygen reduction

Using an exponential model that relies on Arrhenius kinetics, we explored Type I, Type II and dynamic (e.g. declining Q ₁₀ with increasing temperature) responses of respiration to temperature. Our Arrhenius model provides three parameters: R _REF (the base of the exponential model, nmol g⁻¹ s⁻¹), E ₀ (the overall activation energy of oxygen reduction that dominates its temperature sensitivity, kJ mol⁻¹) and δ (that describes dynamic responses of E ₀ to measurement temperature, 10³ K²). Two parameters, E ₀ and δ , are tightly linked. Increases in overall activation energy at a reference temperature were inversely related to changes in δ . At an E ₀ of ca. 45 kJ mol⁻¹, δ approached zero, and respiratory temperature response was strictly Arrhenius-like. Physiologically, these observations suggest that as contributions of AOX to combined oxygen reduction increase, E ₀( REF ) decreases because of different temperature sensitivities for V _max, and δ increases because of different temperature sensitivities for K _1/2 of AOX and COX. The balance between COX and AOX activity helps regulate plant metabolism by adjusting the demand for ATP to that for reducing power and carbon skeleton intermediates. Our approach enables determination of respiratory capacity in vivo and opens a path to development of process-based models of plant respiration.     

Nitrogen allocation in response to partial shading of a plant: Possible mechanisms

The significance of photosynthetic and transpiration rates for the perception by plants of light gradients in leaf canopies has been investigated with regard to nitrogen allocation and re-allocation. A gradient of photon flux density (PFD) over a plant's foliage was simulated by shading one leaf of a pair of primary leaves of bean ( Phaseolus vulgaris L. cv. Rentegever). Photosynthetic rate was manipulated independently of PFD and, to some extent, also of transpiration, by subjecting the leaf to different CO₂ concentrations. Transpiration rate was changed independently of PFD and photosynthetic rate by subjecting the leaf to different vapour pressure differences (VPD). A reduced partial pressure of CO₂ reduced specific leaf mass (SLM) as did a decreased PFD, but did not change leaf N per unit area (N_LA) and light saturated rate of photosynthesis (A_max). A reduced VPD caused several effects consistent with the effect of PFD. It decreased N_LA and A_max and increased the chlorophyll to N ratio in old and young leaves. Furthermore, it decreased the chlorophyll a to b ratio and inhibited leaf growth in young leaves. The transpiration stream is partitioned among the leaves of a plant according to their transpiration rates. The results suggest that relative rates of import of xylem sap into leaves of a plant play an important role in the perception of partial shading of a plant, a situation normally found in dense vegetations. The possible role of cytokinin influx into leaves as controlled by transpiration rate, is discussed.     

Stomatal responses of pearl millet (Pennisetum americanum [L.] Leeke) to leaf water status and environmental factors in the field

Abstract. Factors affecting stomatal conductance (g₁) of pearl millet ( Pennisetum americanum [L.] Leeke), cultivar BJ 104, were examined in the field in India during the dry season.
Diurnal changes in g₁ were evaluated for upper expanded leaves at flowering on two occasions using plants subjected to varying degrees of water stress. Except for the most severely stressed treatment, diurnal changes in g₁ closely matched changes in irradiance ( I ), the promotive effect of which largely overcame opposing influences on g₁ of increasing atmospheric vapour pressure deficit, and decreasing leaf water and turgor potentials (Ψ, Ψ_p).
Two main effects of water stress on g₁ were evident: (i) a decrease in the amplitude of the mid-day peak in g₁, and (ii) a decrease in the time over which high g₁ was maintained, resulting in early (mid-day) closure and hysteresis in the relationship between g₁ and I .
Leaf conductance was greatest for upper leaves and decreased down the canopy. At equivalent depths in the canopy g₁ was higher in flowering than in photoperiodically-retarded plants of the same age. The magnitude of water stress-induced stomatal closure increased down the plant, and was more marked in retarded than in flowering plants.
Within individual stress treatments Ψ of upper leaves decreased linearly as transpiration flux increased. It is concluded that stomatal behaviour of upper leaves of pearl millet at flowering largely operates to maximize assimilation rather than to minimize water loss.     

Low carbon dioxide concentrations can reverse stomatal closure during water stress

Leaf water potentials below threshold values result in reduced stomatal conductance (g_s). Stomatal closure at low leaf water potentials may serve to protect against cavitation of xylem. Possible control of g_s by leaf water potential or hydraulic conductance was tested by drying the rooting medium in four herbaceous annual species until g_s was reduced and then lowering the [CO₂] to determine whether g_s and transpiration rate could be increased and leaf water potential decreased and whether hydraulic conductance was reduced at the resulting lower leaf water potential. In all species, low [CO₂] could reverse the stomatal closure because of drying despite further reductions in leaf water potential, and the resulting lower leaf water potentials did not result in reductions in hydraulic conductance. The relative sensitivity of g_s to internal [CO₂] in the leaves of dry plants of each species averaged three to four times higher than in leaves of wet plants. Two species in which g_s was reputed to be insensitive to [CO₂] were examined to determine whether high leaf to air water vapor pressure differences (D) resulted in increased stomatal sensitivity to [CO₂]. In both species, stomatal sensitivity to [CO₂] was indeed negligible at low D, but increased with D, and low [CO₂] partly or fully reversed closure caused by high D. In no case did low leaf water potential or low hydraulic conductance during drying of the air or the rooting medium prevent low [CO₂] from increasing g_s and transpiration rate.     

Measurements of electrical leaf surface conductance reveal re-condensation of transpired water vapour on leaf surfaces

Electrical conductance ( λ ) was measured continuously and in vivo on leaf surfaces of Vicia faba and Aegopodium podagraria . λ increased with rise and decreased with fall in humidity, exhibiting a hysteresis during an applied humidity cycle [90–20–-90% relative humidity (r.h.)]. After treatment with NaNO₃ aerosols, a sudden increase in λ was observed at 73% r.h., which is close to the deliquescence point of the salt. Transpiration and electrical conductance of untreated leaves were measured simultaneously under conditions of constant r.h., while the photosynthetic photon flux density and CO₂ concentration of the air were varied to induce changes of stomatal aperture. At 35% r.h., changes of light and CO₂ level revealed a strong correlation between stomatal conductance ( g _S) and λ for Vicia faba leaves. This was also found at 90, 75, 60, 45 and 25% r.h. on the lower but not on the astomatous, upper surface of Aegopodium podagraria . The correlation between g _S and λ for stomata-bearing leaf surfaces indicates that an equilibrium exists between the ambient water vapour phase and the liquid water phase on and within the cuticle. This is modified by transpired water vapour influencing the air humidity inside the boundary layer. Our results imply re-condensation of transpired water vapour to salts on the leaf surface and its sorption to the cuticle.     

Nitrogen allocation in response to partial shading of a plant: Possible mechanisms

The significance of photosynthetic and transpiration rates for the perception by plants of light gradients in leaf canopies has been investigated with regard to nitrogen allocation and re-allocation. A gradient of photon flux density (PFD) over a plant's foliage was simulated by shading one leaf of a pair of primary leaves of bean ( Phaseolus vulgaris L. cv. Rentegever). Photosynthetic rate was manipulated independently of PFD and, to some extent, also of transpiration, by subjecting the leaf to different CO₂ concentrations. Transpiration rate was changed independently of PFD and photosynthetic rate by subjecting the leaf to different vapour pressure differences (VPD). A reduced partial pressure of CO₂ reduced specific leaf mass (SLM) as did a decreased PFD, but did not change leaf N per unit area (N_LA) and light saturated rate of photosynthesis (A_max). A reduced VPD caused several effects consistent with the effect of PFD. It decreased N_LA and A_max and increased the chlorophyll to N ratio in old and young leaves. Furthermore, it decreased the chlorophyll a to b ratio and inhibited leaf growth in young leaves. The transpiration stream is partitioned among the leaves of a plant according to their transpiration rates. The results suggest that relative rates of import of xylem sap into leaves of a plant play an important role in the perception of partial shading of a plant, a situation normally found in dense vegetations. The possible role of cytokinin influx into leaves as controlled by transpiration rate, is discussed.     

Comparing model predictions and experimental data for the response of stomatal conductance and guard cell turgor to manipulations of cuticular conductance, leaf-to-air vapour pressure difference and temperature: feedback mechanisms are able to account for all observations

Stomata respond to increasing leaf-to-air vapour pressure difference (LAVPD) ( D ) by closing. The mechanism by which this occurs is debated. A role for feedback and peristomatal transpiration has been proposed. In this paper, we apply a recent mechanistic model of stomatal behaviour, and compare model and experimental data for the influence of increasing D on stomatal conductance.
We manipulated cuticular conductance ( g _c) by three independent methods. First, we increased g _c by using a solvent mixture applied to both leaf surfaces prior to determining stomatal responses to D ; second, we increased g _c by increasing leaf temperature at constant D ; and third, we coated a small area of leaf with a light oil to decrease g _c. In all three experiments, experimental data and model outputs showed very close agreement.
We conclude, from the close agreement between model and experimental data and the fact that manipulations of g _c, and hence cuticular transpiration, influenced g _s in ways consistent with a feedback mechanism, that feedback is central in determining stomatal responses to D .     

Selection for higher yields and heat resistance in Pima cotton has caused genetically determined changes in stomatal conductances

Gas exchange analysis was used to characterize photosynthetic and stomatal responses to key environmental stimuli in five commercial lines of Pima cotton ( Gossypium barbadense L.) which represent a gradient of selection for higher yields and heat resistance, and a primitive, uncultivated G. barbadense. At constant light and vapor pressure deficit, stomatal conductance increased linearly with air temperature in the 23 to 36$C range in all five commercial lines, and conductance at each temperature increased as a function of selection. In contrast, photosynthetic rates had a low sensitivity to temperature in the 23 to 36$C range, particularly in the advanced lines. In a segregating F₂ population from a cross between the advanced line, Pima S-6, and the primitive cotton, B368, the slope of the stomatal response to temperature in each F₂ plant was positively correlated with the stomatal conductance measured at 40$C. An analysis of the frequency distribution of stomatal conductance in F₁ and F₂ progeny of the cross showed that the differences in stomatal conductance between lines were genetically determined. These data indicate that selection for higher yield and heat resistance in Pima cotton has caused genetically determined changes in stomatal properties. Characterization of the relationship between the altered stomatal properties and the attained increases in heat resistance and yields could make it possible to use these physiological traits as selection criteria in future breeding programs.