The Influence of Water Stress on the Photosynthesis of a Simulated Sward of Perennial Ryegrass

The rate of canopy photosynthesis, single leaf photosynthesis,leaf resistance to gaseous exchange, and leaf water potentialof simulated swards of perennial ryegrass (Lolium perenne cv.S24) in a controlled environment, were determined during a periodof increasing water stress and recovery from that stress. Canopyphotosynthesis did not decline immediately water was withheldbut continued at an undiminished rate for several days; thereafterit fell rapidly, particularly at first. As water stress increasedsuccessive relationships between canopy photosynthesis and irradiancebecame more curved, indicating that the effect of water stressincreased with increasing irradiance. After the swards werere-watered canopy photosynthesis rose, most rapidly during thefirst 24 h. In general, the pattern of change of leaf waterpotential was similar to that of canopy photosynthesis, althougha more detailed examination of this relationship showed it tobe hysteresial; in particular, the fall in leaf water potentialpreceded that of canopy photosynthesis. Single leaf photosynthesisappeared to be the main agent through which water stress influencedcanopy photosynthesis although in the more severely stressedswards (leaf water potentials of about—15 bars) some leaftissue died and so limited the recovery of canopy photosynthesis.The leaf resistance to gaseous diffusion increased with increasingwater stress, as did the CO₂ compensation point, thereby influencingsingle-leaf photosynthesis and through it canopy photosynthesis.     

The Effect of Drought and Vapour Pressure Deficit on Gas Exchange of Young Kiwifruit (Actinidia deliciosavar.deliciosa) Vines

To evaluate the effect of drought and vapour pressure deficit (VPD) on stomatal behaviour and gas exchange parameters, young kiwifruit vines (Actinidia deliciosavar.deliciosacv. Hayward) were exposed to alternating periods of drought and drought-relief over two growing seasons. Vines were grown either in the field or in containers. Stomatal conductance of fully-expanded leaves rapidly decreased as pre-dawn leaf water potential was reduced below a threshold value of -0.3MPa. Stomatal conductance reached minimum values of 10–20mmol m^-2s^-1. Transpiration rate was similarly sensitive to changes in leaf water status, whereas more severe drought levels were necessary to affect photosynthesis significantly. Net daily carbon gains were estimated at 4.7 and 2.7gm^-2for irrigated and droughted vines, respectively. Gas exchange parameters recovered to values of irrigated vines within a few hours after relief of stress. Rate of recovery depended on the level of stress reached during the previous drought period. There was a steady decline in stomatal conductance when VPD was increased from 0.8 to 2.5kPa in both irrigated and droughted vines. The VPD at which stomatal conductance reached 50% of maximum values was 2.1–2.2kPa for both treatments. We conclude that stomata were highly sensitive to changes in soil water status and that midday depression of photosynthesis measured in kiwifruit vines was related to water deficits arising in the leaf because of both transpirational losses and to the direct effect of increasing VPD.     

Drought effects on photosynthesis and fluorescence in hard wheat cultivars grown in the field

Net photosynthesis of the flag leaf of hard wheat ( Triticum durum L. evs Valforte, Produra, Adamello, Karel, Appulo and El Amel from the collection of the Instituto di Cerealicultura. Foggia, Italy) of different water potential has been studied on three consecutive years. Net photosynthesis was measured in natural conditions with a LI-COR portable instrument and in saturating CO₂ with an oxygen electrode. Net photosynthesis and stomatal conductance were significantly lower in the unirrigated leaves. However, the ratio of intercellular CO₂, concentration (C₁) to ambient CO₃ concentration (C_a) around the stressed plants was similar to the irrigated control. The maximal rate of photosynthesis in saturating CO₂, (P_nmax). measured in the second year of the experiment, was quite close to photosynthesis under natural conditions, indicating that CO₂ supply was not limiting. These results suggest that altered mesophyll photosynthetic capacity, rather than stomatal closure, causes the observed reduction in photosynthesis in the unirrigated plants. The variable fluorescence yield (_v/F_m) in predarkened leaves measured for two consecutive years, did not show differences between treatments or between cultivars. However, the analysis of the slow transients, measured the last year of the experiment, showed a linear relation between the fluorescence decline from the maximum initial level (P) and maximum photosynthesis (P_nmax).     

Seasonal Changes in Water Potential and Turgor Maintenance in Sorghum and Maize under Water Stress

Leaf water (Ψ) and solute (ψ) potential were measured in field sorghum and maize under well irrigated (I) and dryland (NI) conditions throughout a season. Despite decreases in ψ due to slow soil water depletion and to apparent increases in liquid phase plant resistance, midday leaf turgor (ψ_p) in the NI sorghum was maintained at similar levels as in the I treatment throughout the season due to concomitant decreases in ψ_s. Osmotic adjustment was also observed in maize, although ψ_p was significantly lower in the NI treatment as compared to I during the final stages of grain filling. A seasonal shift in the ψ vs. relative water content relation of NI sorghum leaves was observed, more water being retained by the older leaf at any particular ψ. The major factor for turgor maintenance was a net increase in solutes per unit of tissue. The role played by increases in the proportion of tissue volume occupied by cell wall was also evaluated. No stomatal closure due to water stress was found in NI sorghum even though leaf ψ reached —20 bars late in the season. Under similar conditions, stomata closed at —14 to —16 bars in younger plants where water stress was made to develop much faster.     

Growth response of barley and tomato to nitrogen stress and its control by abscisic acid,water relations and photosynthesis

Barley (Hordeum vulgare L.) and tomato Lycopersicon esculentum Mill.) were grown hydroponically and examined 2, 5, and 10 d after being deprived of nitrogen (N) supply. Leaf elongation rate declined in both species in response to N stress before there was any reduction in rate of dryweight accumulation. Changes in water transport to the shoot could not explain reduced leaf elongation in tomato because leaf water content and water potential were unaffected by N stress at the time leaf elongation began to decline. Tomato maintained its shoot water status in N-stressed plants, despite reduced water absorption per gram root, because the decline in root hydraulic conductance with N stress was matched by a decline in stomatal conductance. In barley the decline in leaf elongation coincided with a small (8%) decline in water content per unit area of young leaves; this decline occurred because root hydraulic conductance was reduced more strongly by N stress than was stomatal conductance. Nitrogen stress caused a rapid decline in tissue NO ₃ ^- pools and in NO ₃ ^- flux to the xylem, particularly in tomato which had smaller tissue NO ₃ ^- reserves. Even in barley, tissue NO ₃ ^- reserves were too small and were mobilized too slowly (60% in 2 d) to support maximal growth for more than a few hours. Organic N mobilized from old leaves provided an additional N source to support continued growth of N-stressed plants. Abscisic acid (ABA) levels increased in leaves of both species within 2 d in response to N stress. Addition of ABA to roots caused an increase in volume of xylem exudate but had no effect upon NO ₃ ^- flux to the xylem. After leaf-elongation rate had been reduced by N stress, photosynthesis declined in both barley and tomato. This decline was associated with increased leaf ABA content, reduced stomatal conductance and a decrease in organic N content. We suggest that N stress reduces growth by several mechanisms operating on different time scales: (1) increased leaf ABA content causing reduced cell-wall extensibility and leaf elongation and (2) a more gradual decline in photosynthesis caused by ABA-induced stomatal closure and by a decrease in leaf organic N.Abbreviation and symbols ABA abscisic acid - c_i leaf internal CO₂ concentration - L_p root hydraulic conductance     

Photosynthetic Response to Water Stress in Phaseolus vulgaris

Water stressed Phaseolus vulgaris L. plants were monitored to detect the relationships between net photosynthesis, transpiration, boundary layer plus stomatal resistance, mesophyll resistance, CO₂ compensation point, ribulose, 1,5-diphosphate carboxylase activity and leaf water potential. At full expansion, the first trifoliate leaves of greenhouse grown bean plants were subjected to water stress by withholding irrigation. Gas exchange and enzyme activity of the central trifoliolate leaflets were monitored as leaf water potential decreased. Although increased stomatal resistance appeared to be the primary causal factor of reduced net photosynthesis, increased mesophyll resistance and decreased ribulose 1,5-diphosphate carboxylase activity further documented the role of non-stomatal factors.     

Gas exchange and resource-use efficiency of Leymus cinereus (Poaceae): diurnal and seasonal responses to naturally declining soil moisture

We examined factors that limit diurnal and seasonal photosynthesis in Leymus cinereus, a robust tussock grass from shrub-steppes of western North America. Data from plants in a natural stand and in experimental field plots indicate that this bunchgrass has 1) a high photosynthetic capacity, 2) high leaf nitrogen content and high nitrogen-use efficiency, 3) a steep leaf-to-air diffusion gradient for carbon dioxide, which enhances intrinsic water-use efficiency, and 4) photosynthetic tissues that tolerate severe water stress and recover quickly from moderate water stress. Midday depressions of CO₂ assimilation (A) and stomatal conductance were slight in plants with plentiful water, but marked in plants subject to moderate water stress. Midday stomatal closure in moderately stressed plants reduced intercellular carbon dioxide concentration (c_i) by ≈40 μl liter^-1. The maximum rate of A achieved during the day for severely stressed plants (predawn water potential = -4 MPa) was one-third and daily carbon gain per unit leaf area was about one-fourth that of well-watered plants. For plants in the natural stand, CO₂-saturated photosynthesis declined almost linearly with decreasing soil water availability over the growing season, whereas there was little effect on A at CO₂ ambient levels or on carboxylation efficiency until predawn water potentials reached -1.8 MPa. Nitrogen-use efficiency declined with diminishing soil moisture, but there was no seasonal change in stomatal limitation or instantaneous water-use efficiency as estimated from A vs. c_i curves at optimal leaf temperature and moderate atmospheric evaporative demand. Thus, reduced stomatal conductance in response to increased evaporative demand may increase stomatal limitation diumally, but over the growing season, stomatal limitation estimated from A vs. c_i curves is relatively constant because maximum stomatal conductance is closely tuned to the CO₂ assimilatory capacity of the mesophyll.     

Leaf Response to Water Deficits in Soybeans

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

Long term water stress inactivates Rubisco in subterranean clover

In long-term field experiments, during consecutive years, microswards of subterranean clover were irrigated to minimise water deficits or subjected to progressively increasing drought over 30 days. Both leaf water potential and relative water content steadily decreased during the experiments. Plants affected by drought grew more slowly and photosynthesis was decreased. Photosynthetic rate (A) and Rubisco were analysed in relation to midday water potentials and relative water contents. The difference in A between draughted and irrigated plants increased progressively, in part as a result of decreased stomatal conductance and CO₂ concentration within leaf (Ci). However, A-Ci curves suggest that the photosynthetic capacity in plants experiencing long-term stress was reduced by 50% when compared with irrigated plants. Drought decreased both the initial and the total Rubisco activity per unit area in a similar way but did not reduce the amount of Rubisco protein per unit leaf area. Thus, the specific activity of Rubisco, rather than its activation state, decreased suggesting that under water stress the active sites were blocked by inhibitors.     

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

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.     

Variations among woody plants in stomatal conductance and photosynthesis during and after drought

Acer saccharum, Fraxinus americana, Juglans nigra, Acer rubrum, Cornus amomum, and Ulmus americana seedlings were subjected to a soil drying cycle and then rewatered. At frequent intervals during the drying cycle and following rewatering, determinations were made of equilibrium photosynthesis rates, leaf conductances and leaf water potentials. As the drying cycle progressed, leaf water potentials decreased, stomata closed, and rates of transpiration and photosynthesis were reduced. Stomata of the two Acer species initially were more sensitive to water stress than were those of the other species. At low leaf water potentials, stomata of Juglans and Cornus were more open than those of the other species. Photosynthesis of Acer saccharum, Fraxinus and Juglans was significantly reduced by plant water stress, while photosynthetic water use efficiency of Cornus and Juglans was most unfavourable. Photosynthesis/leaf conductance ratios in water stressed leaves were higher in Fraxinus than in the other species. Immediately after rewatering, only limited stomatal opening occurred in Acer saccharum and Cornus with recovery of stomatal opening most protracted in Fraxinus and Ulmus. There was extended reduction of photosynthesis of all species as a result of the soil drying treatment. This effect was most significant in Acer saccharum and Juglans. Survival of plants on moist and dry sites is discussed in relation to stomatal control of transpiration and metabolic responses to water stress. Research supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison and the International Shade Tree Conference. The cooperation of the Wisconsin Department of Natural Resources is acknowledged. Research supported by the College of Agricultural and Life Sciences, University of Wisconsin-Madison and the International Shade Tree Conference. The cooperation of the Wisconsin Department of Natural Resources is acknowledged.     

Leaf water potential, stomatal resistance, and photosynthetic response to water stress in peach seedlings

Individual groups of peach (Prunus persica [L.] Batsch) seedlings stressed to −17, −26 and −36 bars recovered to control levels within 1, 3, and 4 days, respectively. Stomatal resistance was significantly correlated with both leaf water potential and net photosynthesis. In seedlings stressed to −52 bars, leaf water potential and stomatal resistance recovered sooner than net photosynthesis, despite recovery of 0₂ evolution at a rate similar to leaf water potential. Therefore, some nonstomatal factor other than reduction in photochemical activity must be responsible for the lag in recovery of CO₂ assimilation following irrigation.     

Effects of water stress on the chlorophyll content,nitrogen level and photosynthesis of leaves of two maize genotypes

The dynamics of leaf chlorophyll level, nitrogen content, photosynthesis and stomatal conductance were followed in detail in two cultivars of maize (Zea mays) during a short period of water stress, applied at tasseling, and during the subsequent recovery phase. Plants used in the experiment were grown in sand-nutrient solution culture under field weather conditions. Water stress reduced chlorophyll levels, stomatal conductance and photosynthesis, but the nitrogen content of the leaves was not affected. It is concluded that the stress-induced loss of chlorophyll is not mediated by a lack of nitrogen. Considerable differences were observed between genotypes in the rate of post-stress recovery of chlorophyll level. Recovery, upon rewatering, of stomatal conductance and photosynthesis preceded that of chlorophyll level. Losses of up to 40% of leaf chlorophyll content were insufficient to affect rates of photosynthesis measured at mid-day.     

Diurnal variations in leaf water potential and stomatal conductance of pigeon pea (Cajanus cajan (L.) Millsp.) cultivars as affected by irrigation levels

Two cultivars of pigeon pea (Cajanus cajan (L.)Millsp.) UPAS-120 and Parbhat were grown in the field under two irrigation treatments: no irrigation and irrigation when cumulative pan evaporation was equal to 50 per cent depletion of available water content in one metre root zone depth. Diurnal changes in leaf water potential and stomatal conductance were recorded on two daysi.e. October 1, 1979 and October 28, 1979 which corresponded to reproductive growth stage of the crop. Plant water potential decreased rapidly during the day up to about 15.00 and increased during evening hours. Higher leaf water potential was recorded in irrigated treatment on both dates. Adaxial and abaxial stomata differed in their response to water stress. Adaxial stomatal conductance started declining during early morning hours, however, abaxial conductance firstly increased up to 09.00 then decreased and increased again in the afternoon except in irrigated crop of cv. UPAS-120 on 28th October, where conductance never increased after 09.00. The reduced rate of stomatal conductance during day hours may be identified as one of the characteristics responsible for drought tolerance in pigeon pea.     

Diffusional conductance to CO2 is the key limitation to photosynthesis in salt‐stressed leaves of rice (Oryza sativa)

Salinity significantly limits leaf photosynthesis but the factors causing the limitation in salt‐stressed leaves remain unclear. In the present work, photosynthetic and biochemical traits were investigated in four rice genotypes under two NaCl concentration (0 and 150 mM) to assess the stomatal, mesophyll and biochemical contributions to reduced photosynthetic rate (A) in salt‐stressed leaves. Our results indicated that salinity led to a decrease in A, leaf osmotic potential, electron transport rate and CO₂ concentrations in the chloroplasts (C_c) of rice leaves. Decreased A in salt‐stressed leaves was mainly attributable to low C_c, which was determined by stomatal and mesophyll conductance. The increased stomatal limitation was mainly related to the low leaf osmotic potential caused by soil salinity. However, the increased mesophyll limitation in salt‐stressed leaves was related to both osmotic stress and ion stress. These findings highlight the importance of considering mesophyll conductance when developing salinity‐tolerant rice cultivars.     

Effect of potassium on growth and nitrate reductase during water stress and recovery in maize

Abstract The effect of potassium (0,50, 100 and 200 mg/pot) was studied on growth characteristics and nitrate reductase activity in maize (Zea mays) seedlings during water stress and subsequent recovery. In irrigated plants K⁺ increased the rate of leaf area expansion, leading to increased leaf area per plant. Increased leaf area was associated with decreased chlorophyll content. Water stress (–15 bars) enhanced the stomatal resistance of leaves which was further accentuated by K⁺ application. Nitrate reductase activity rose in irrigated plants 24 h after K⁺ application. Subsequently, as water stress developed, K⁺ helped to maintain higher NR activity for the first two days. However, K⁺ had no effect on half life of NR in light or darkness. During recovery from stress K⁺ aided to maintain the higher leaf expansion rate, the chlorophyll content and the stomatal resistance. The results above are discussed in relation to the ability of K⁺ to maintain better growth under water stress.     

Mild water stress effects on carbon-reduction-cycle intermediates, ribulose bisphosphate carboxylase activity, and spatial homogeneity of photosynthesis in intact leaves

We have examined the effect of mild water stress on photosynthetic chloroplast reactions of intact Phaseolus vulgaris leaves by measuring two parameters of ribulose bisphosphate (RuBP) carboxylase activity and the pool sizes of RuBP, 3-phosphoglycerate (PGA), triose phosphates, hexose monophosphates, and ATP. We also tested for patchy stomatal closure by feeding ¹⁴CO₂. The k_cat of RuBP carboxylase (moles CO₂ fixed per mole enzyme per second) which could be measured after incubating the enzyme with CO₂ and Mg²⁺ was unchanged by water stress. The ratio of activity before and after incubation with CO₂ and Mg²⁺ (the carbamylation state) was slightly reduced by severe stress but not by mild stress. Likewise, the concentration of RuBP was slightly reduced by severe stress but not by mild stress. The concentration of PGA was markedly reduced by both mild and severe water stress. The concentration of triose phosphates did not decline as much as PGA. We found that photosynthesis in water stressed leaves occurred in patches. The patchiness of photosynthesis during water stress may lead to an underestimation of the effect of stomatal closure. We conclude that reductions in whole leaf photosynthesis caused by mild water stress are primarily the result of stomatal closure and that there is no indication of damage to chloroplast reactions.     

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.     

C4 photosynthesis and water stress

Background

In contrast to C₃ photosynthesis, the response of C₄ photosynthesis to water stress has been less-well studied in spite of the significant contribution of C₄ plants to the global carbon budget and food security. The key feature of C₄ photosynthesis is the operation of a CO₂-concentrating mechanism in the leaves, which serves to saturate photosynthesis and suppress photorespiration in normal air. This article reviews the current state of understanding about the response of C₄ photosynthesis to water stress, including the interaction with elevated CO₂ concentration. Major gaps in our knowledge in this area are identified and further required research is suggested.

Scope

Evidence indicates that C₄ photosynthesis is highly sensitive to water stress. With declining leaf water status, CO₂ assimilation rate and stomatal conductance decrease rapidly and photosynthesis goes through three successive phases. The initial, mainly stomatal phase, may or may not be detected as a decline in assimilation rates depending on environmental conditions. This is because the CO₂-concentrating mechanism is capable of saturating C₄ photosynthesis under relatively low intercellular CO₂ concentrations. In addition, photorespired CO₂ is likely to be refixed before escaping the bundle sheath. This is followed by a mixed stomatal and non-stomatal phase and, finally, a mainly non-stomatal phase. The main non-stomatal factors include reduced activity of photosynthetic enzymes; inhibition of nitrate assimilation, induction of early senescence, and changes to the leaf anatomy and ultrastructure. Results from the literature about CO₂ enrichment indicate that when C₄ plants experience drought in their natural environment, elevated CO₂ concentration alleviates the effect of water stress on plant productivity indirectly via improved soil moisture and plant water status as a result of decreased stomatal conductance and reduced leaf transpiration.

Conclusions

It is suggested that there is a limited capacity for photorespiration or the Mehler reaction to act as significant alternative electron sinks under water stress in C₄ photosynthesis. This may explain why C₄ photosynthesis is equally or even more sensitive to water stress than its C₃ counterpart in spite of the greater capacity and water use efficiency of the C₄ photosynthetic pathway.Key words: C₃ and C₄ photosynthesis, stomatal and non-stomatal limitation, high CO₂, water stress     

Photosynthesis and stomatal conductance of potato leaves—effects of leaf age,irradiance, and leaf water potential

Potatoes (Solanum tuberosum L., cv. Bintje) were grown in a naturally lit glasshouse. Laboratory measurements on leaves at three insertion levels showed a decline with leaf age in photosynthetic capacity and in stomatal conductance at near saturating irradiance. Conductance declined somewhat more with age than photosynthesis, resulting in a smaller internal CO₂ concentration in older relative to younger leaves. Leaves with different insertion number behaved similarly. The changes in photosynthesis rate and in nitrogen content with leaf age were closely correlated. When PAR exceeded circa 100 W m^–2 the rate of photosynthesis and stomatal conductance changed proportionally as indicated by a constant internal CO₂ concentration. The photosynthesis-irradiance data were fitted to an asymptotic exponential model. The parameters of the model are AMAX, the rate of photosynthesis at infinite irradiance, and EFF, the slope at low light levels. AMAX declined strongly with leaf age, as did EFF, but to a smaller extent. During drought stress photosynthetic capacity declined directly with decreasing water potential (range –0.6 to –1.1 MPa). Initially, stomatal conductance declined faster than photosynthetic capacity.Abbreviations LNx leaf number x, counted in acropetal direction - DAP days after planting - DALA days after leaf appearance - C_i CO₂ concentration in the leaf - C_a CO₂ concentration in ambient air - LWP leaf water potential - OP osmotic potential - PAR photosynthetically active radiation