Effects of low nitrogen supply on relationships between photosynthesis and nitrogen status at different leaf position in wheat seedlings

Hydroponic experiments were conducted to investigate the effects of low nitrogen (N) nutrition on photosynthesis and its relationships with N status in wheat (Triticum aestivum L.). Two wheat cultivars, Zaoyangmai and Yangmai158, differing in low N nutrition tolerances, were used. The results show that under low N nutrition the area of the first top leaf was significantly reduced, while there was no significant difference in the top second and third leaf areas compared with the control for either cultivar. The net photosynthetic rate and chlorophyll content were significantly reduced in the top three leaves of Zaoyangmai, while no significant difference in these factors was observed in the top first and second leaves of Yangmai158 compared with control under N-limited conditions. The effective quantum yield of photosystem II (PSII) photochemistry and the maximal quantum yield of PSII photochemistry were only slightly altered in both cultivars, indicating that PSII was not damaged by low N nutrition. In addition, the non-photochemical quenching coefficient increased significantly in the top three leaves of Zaoyangmai, and only in the top third leaf of Yangmai158 under N-limited conditions. Furthermore, the ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) and soluble protein contents decreased significantly in the top second and third leaves of Zaoyangmai, while no significant difference was observed in the top first and second leaves of Yangmai158 between low N nutrition and control. We concluded that in Yangmai158, N status changed less, and it maintained almost normal photosynthesis in young leaves, thus Yangmai158 could be more tolerance to low N nutrition.     

Recovery of photosynthesis in sunflower after a period of low leaf water potential

Photosynthesis was studied in sunflower plants subjected to 1 to 2 days of desiccation and then permitted to recover. The leaf water potential to which leaves returned after rewatering was dependent on the severity of desiccation and the evaporative conditions. Under moderately evaporative conditions, leaf water potential returned to predesiccation levels after 3 to 5 hours when desiccation was slight. Leaf water potentials remained below predesiccation levels for several days after rewatering when leaf water potentials decreased to −13 to −19 bars during desiccation. Leaf water potential showed no sign of recovery when leaf water potentials decreased to −20 bars or below during desiccation. The lack of full recovery of leaf water potential was attributable to increased resistance to water transport in the roots and stem. The resistance ultimately became large enough to result in death of the leaves because net water loss continued even after the soil had been rewatered.     

灌水对小麦旗叶光合功能衰退的影响

利用田间小区试验研究了不同灌水对冬小麦旗叶光合功能衰退的影响。研究表明：小麦旗叶光合衰退初期引起光合下降的原因主要是气孔限制,后期则为非气孔限制。灌水可提高旗叶光合速率,并使由气孔限制非气孔转变的时间推后,同时,还可增加叶绿素含量,增强根活力,使小麦旗叶光合功能持续期延长,过量灌水改善旗叶光合速衰的效果主要表现在后期,对产量提高的意义并不大。     

Acclimation of photosynthesis to low leaf water potentials

Photosynthesis is reduced at low leaf water potentials (Ψ_l) but repeated water deficits can decrease this reduction, resulting in photosynthetic acclimation. The contribution of the stomata and the chloroplasts to this acclimation is unknown. We evaluated stomatal and chloroplast contributions when soil-grown sunflower (Helianthus annuus L.) plants were subjected to water deficit pretreatments for 2 weeks. The relationship between photosynthesis and Ψ_l, determined from gas-exchange and isopiestic thermocouple psychometry, was shifted 3 to 4 bars towards lower Ψ_l, in pretreated plants. Leaf diffusive resistance was similarly affected. Chloroplast activity, demonstrated in situ with measurements of quantum yield and the capacity to fix CO₂ at all partial pressures of CO₂, and in vitro by photosystem II activity of isolated organelles, was inhibited at low Ψ_l but less in pretreated plants than in control plants. The magnitude of this inhibition indicated that decreases in chloroplast activity contributed more than closure of stomata both to losses in photosynthesis and to the acclimation of photosynthesis to low Ψ_l.     

Nonstomatal inhibition of photosynthesis in sunflower at low leaf water potentials and high light intensities

The inhibition of photosynthesis at low leaf water potentials was studied in soil-grown sunflower to determine the degree to which photosynthesis under high light was affected by stomatal and nonstomatal factors. Below leaf water potentials of −11 to −12 bars, rates of photosynthesis at high light intensities were insensitive to external concentrations of CO₂ between 200 and 400 microliters per liter. Photosynthesis also was largely insensitive to leaf temperature between 10 and 30 C. Changes in CO₂ concentration and temperature had negligible effect on leaf diffusive resistance. The lack of CO₂ and temperature response for both photosynthesis and leaf diffuse resistance indicates that rates of photosynthesis were not limited by either CO₂ diffusion or a photosynthetic enzyme. It was concluded that photosynthesis under high light was probably limited by reduced photochemical activity of the leaves at water potentials below −11 to −12 bars.     

Chloroplast volume: cell water potential relationships and acclimation of photosynthesis to leaf water deficits

Studies were undertaken to determine if there is an association between nonstomatally-mediated acclimation of photosynthesis to low water potential (w) and the maintenance of chloroplast volume during water stress. Spinach plants either kept well watered throughout their growth (non-acclimated), or subjected to water stress such that leaf w dropped to -1.5 megapascals (MPa) and then were rewatered (acclimated) were subjected to drought episodes. During these stress periods, photosynthesis was maintained to a greater extent in acclimated plants as compared to non-acclimated plants at w below -1 MPa.Estimates of internal leaf [CO₂] suggested that photosynthetic acclimation to low w was not primarily due to altered stomatal response. As w dropped from initial values, a decline in steady state levels of ribulose 1,5-bisphosphate (RuBP) occurred in both non-acclimated and acclimated plants. RuBP decline was less severe in acclimated plants.Low w effects on chloroplast volume in non-acclimated and acclimated plants were estimated by measuring the volume of intact chloroplasts isolated from plants in solutions which were made isotonic to declining leaf osmotic potential during the drought episodes. Chloroplast volume was maintained to a greater extent at low w in acclimated, as compared with non-acclimated plants. Although substantial osmotic adjustment occurred in both non-acclimated and acclimated plants, the extent of osmotic adjustment was the same. These data were interpreted as supporting the hypothesis that cellular-level acclimation to low w is associated with chloroplast volume maintenance, and this physiological acclimation is correlated with enhanced photosynthetic capacity of the leaf at low w.Abbreviations [CO₂]_i internal leaf CO₂ concentration - s osmotic potential - RWC relative water content - RuBP ribulose 1,5-bisphosphate - w water potential     

Differing sensitivity of photosynthesis to low leaf water potentials in corn and soybean

Rates of net photosynthesis were studied in soil-grown corn (Zea mays) and soybean (Glycine max) plants having various leaf water potentials. Soybean was unaffected by desiccation until leaf water potentials were below −11 bars. Rates of photosynthesis in corn were inhibited whenever leaf water potentials dropped below −3.5 bars.     

Diurnal changes in leaf water potential of rice,barley and wheat

The diurnal changes in leaf water potential of rice, barley and wheat corresponded to changes in air temperature. The soil moisture did not affect the diurnal changes in leaf water potential of rice and only slightly affected that of barley and wheat.     

灌水模式对冬小麦光合特性、水分利用效率和产量的影响

试验于2013—2014和2014—2015年连续2个生长季在自动控制干旱棚内的隔离池中进行,拔节期设3个灌水梯度,灌水量分别为0(J_0)、37.5(J_1)、75 mm(J_2),扬花期设3个灌水梯度,灌水量分别为0(F_0)、37.5(F_1)、75 mm(F_2),灌浆期所有处理均按75 mm灌溉,共9个处理,研究不同灌溉模式对小麦中后期不同生育阶段植株生长、耗水量、水分利用效率、光合特性和产量构成因素的影响.结果表明:拔节期干旱(0和37.5 mm)显著降低了小麦扬花期的净光合速率和拔节后的叶面积,扬花期的灌水量直接影响扬花期后的旗叶净光合速率;拔节期干旱扬花期补水和扬花期干旱灌浆期补水都可以有效提高植株的干物质量;拔节期灌水量越多,全生育期耗水量越大;除J_1F_2外,全生育期灌水量越大,耗水量越大,产量也越高;J_1F_2处理产量和水分利用效率最高.扬花期充足的灌水量使J_1F_2处理具有较高的花后旗叶净光合速率,此期补偿性灌溉加快了干物质积累,也保证了较高的穗粒数,使其最终产量高于J_2F_2处理或与之持平,同时J_1F_2拔节期较低的灌水量降低了小麦生育中后期的耗水量,其水分利用效率也显著高于其他处理.综上,J_1F_2是小麦生育中期理想的水分处理组合.     

Significance and limits in the use of predawn leaf water potential for tree irrigation

Research in estimating the water status of crops is increasingly based on plant responses to water stress. Several indicators can now be used to estimate this response, the most widely available of which is leaf water potential (Ψ_LWP) as measured with a pressure chamber. For many annual crops, the predawn leaf water potential (Ψ_PLWP), assumed to represent the mean soil water potential next to the roots, is closely correlated to the relative transpiration rate, RT. A similar correlation also holds for young fruit trees grown in containers. However, exceptions to this rule are common when soil water content is markedly heterogeneous. Two experimental conditions were chosen to assess the validity of this correlation for heterogeneous soil water content: 1) young walnut trees in split-root containers. The heterogeneity was created by two unequal compartments (20% and 80% of total volume), of which only the smaller was irrigated and kept at a moisture content higher than field capacity (permanent drainage). 2) adult walnut trees in an orchard. In this case, soil water heterogeneity was achieved by limiting the amount of localised irrigation (20% of the irrigated control) which was applied every evening. Values of sap flux and of minimum and predawn leaf water potentials with homogeneous and heterogeneous soil water content were compared for trees grown in the orchard and in containers. In spite of intense drought reflected by very low RT or stem water potential, Ψ_PLWP of trees under heterogeneous moisture conditions remained high (between -0.2 and -0.4 MPa) both in the orchard and in containers. These values were higher than those usually considered critical under homogeneous soil conditions. A semi-quantitative model, based on the application of Ohm's analogy to split-root conditions, is proposed to explain the apparently conflicting results in the literature on the relation between Ψ_PLWP and soil water potential. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of saline irrigation on plant water traits,photosynthesis and ionic balance in durum wheat genotypes

In the era of climate change, decreased precipitation and increased evapo-transpiration hampers the yield of several cereal crops along with the soil salinity and poor ground water resource. Wheat being the moderately tolerant crop face many challenges in the arid and semi-arid regions under irrigated agriculture. In view of this, the study was planned to explore the potential of durum wheat genotypes under salinity on the basis of physiological traits. Experiment was designed as RBD in three replications to evaluate 15 wheat genotypes with moderate saline irrigation (EC_iw – 6 dS m⁻¹) and extreme saline irrigation (EC_iw – 10 dS m⁻¹) along with one set of control (Best available water). Different physiological traits such as water potential (ψ_p), osmotic potential (ψ_s), relative water content (RWC), Na⁺ and K⁺ content were recorded in roots as well as shoots at the reproductive stage whereas photosynthetic rate and chlorophyll content were measured in the flag leaves. A significant variability (p < 0.001) was noted among the genotypes under different stress environments and it was observed that durum genotype HI 8728 and HI 8737 showed less reduction in plant water traits (RWC, ψ_p and ψ_s) than the salinity tolerant checks of bread wheat KRL 99 and KRL 3–4. HD 4728 and HI 8708 maintained higher photosynthetic rate as well as higher chlorophyll content under the extreme salinity level of EC_iw – 10 dSm⁻¹. No significant differences were found in root Na⁺ in genotypes KRL 99 (3.17^g), KRL 3–4 (3.34^g) and HI 8737 (3.41^g) while in shoots, lowest accumulation was seen in KRL 99, MACS 3949 and KRL 3–4 at EC_iw – 10 dSm⁻¹. The mean range of K⁺ content was 7.60–9.74% in roots and 4.21–6.61% in shoots under control environment which decreased to 50.77% in roots and 46.05% in shoots under extreme salinity condition of EC_iw – 10 dSm⁻¹. At EC_iw – 10 dSm⁻¹, KRL 99 maintained highest K^+/Na⁺ in both root and shoot followed by KRL 3–4, HI 8737, MACS 3949, HD 4728 in roots and MACS 3949, KRL 3–4, MACS 4020, HD 4758, MACS 3972 and HI 8713 in shoots. The differential response of durum wheat genotypes under salinity particularly for physiological traits, confer their adaptability towards stress environments and exhibit their potential as genetic sources in breeding programs for improving salt stress tolerance.     

Anatomical variation of mesophyll conductance under potassium deficiency has a vital role in determining leaf photosynthesis

Leaves exposed to potassium (K) deficiency usually present decreased mesophyll conductance (g_m) and photosynthesis (A). The relative contributions of leaf anatomical traits in determining g_m have been quantified; however, anatomical variabilities related to low g_m under K starvation remain imperfectly known. A one‐dimensional model was used to quantify anatomical controls of the entire CO₂ diffusion pathway resistance within a leaf on two Brassica napus L. cultivars in response to K deficiency. Leaf photosynthesis of both cultivars was significantly decreased under K deficiency in parallel with down‐regulated g_m. The mesophyll conductance limitation contributed to more than one‐half of A decline. The decreased internal air space in K‐starved leaves was associated with the increase of gas‐phase resistance. Potassium deficiency reduced liquid‐phase conductance by decreasing the exposed surface area of chloroplasts per unit leaf area (S_c/S), and enlarging the resistance of the cytoplasm that can be interpreted by the increasing distance of chloroplast from cell wall, and between adjacent chloroplasts. Additionally, the discrepancies of A between two cultivars were in part because of g_m variations, ascribing to an altered S_c/S. These results emphasize the important role of K on the regulation of g_m by enhancing S_c/S and reducing cytoplasm resistance.     

Some effects of leaf posture on photosynthesis and yield in wheat

Canopy enclosures were used to study the photosynthesis of two winter wheat genotypes with contrasting leaf posture (E, erect; L, lax). Over the grain filling period, the net carbon dioxide fixation during the daytime was nearly always greater in E than in L. In general, a greater proportion of the fixation took place in the lower leaves of E than L. Because of the slower senescence of its lower leaves, the leaf area index of E was slightly but consistently higher than that of L and this appeared to account, in part, for the difference between the genotypes in canopy photosynthesis. However, during June and early July, when the leaf area indices were above three, the more uniform distribution of light in the canopy of E than in that of L appeared to be a cause of its higher rate of photosynthesis. Over the grain filling period, more dry matter was lost from the stems of L than from those of E. It is suggested that the shortfall of assimilate for grain filling was met, at least in part, by translocation of materials from the stems and that their greater depletion in L made up for the lower contribution from current assimilation in this genotype.     

Responses of leaf stomatal density to water status and its relationship with photosynthesis in a grass

Responses of plant leaf stomatal conductance and photosynthesis to water deficit have been extensively reported; however, little is known concerning the relationships of stomatal density with regard to water status and gas exchange. The responses of stomatal density to leaf water status were determined, and correlation with specific leaf area (SLA) in a photosynthetic study of a perennial grass, Leymus chinensis, subjected to different soil moisture contents. Moderate water deficits had positive effects on stomatal number, but more severe deficits led to a reduction, described in a quadratic parabolic curve. The stomatal size obviously decreased with water deficit, and stomatal density was positively correlated with stomatal conductance (g(s)), net CO(2) assimilation rate (A(n)), and water use efficiency (WUE). A significantly negative correlation of SLA with stomatal density was also observed, suggesting that the balance between leaf area and its matter may be associated with the guard cell number. The present results indicate that high flexibilities in stomatal density and guard cell size will change in response to water status, and this process may be closely associated with photosynthesis and water use efficiency.     

推迟拔节水灌溉对宽幅精播麦田冠层温度与叶片水分利用效率的影响

为了探讨冬小麦高效节水灌溉模式,于2012—2013年在山东农业大学试验站采用两种种植模式(宽幅精播种植和常规种植),每种种植模式设3种灌溉处理(全生育期不灌溉、拔节期灌溉60 mm和拔节后10 d灌溉60 mm),研究了推迟拔节水灌溉对宽幅精播麦田冠层温度、光合速率、蒸腾速率、叶片水分利用效率(WUEL)和籽粒产量等的影响。结果表明,推迟拔节水灌溉显著提高了宽幅精播麦田生育后期的冠层温度、旗叶光合速率和蒸腾速率,且在冬小麦生长后期推迟拔节水灌溉显著提高了宽幅精播麦田的WUEL,有利于实现宽幅精播麦田的节水高产;产量构成因素中,推迟拔节水灌溉对两种种植模式的千粒重均没有显著影响,但推迟拔节水灌溉显著提高了宽幅精播麦田的穗粒数和籽粒产量。统筹考虑冬小麦的WUEL和籽粒产量,推迟拔节水灌溉对宽幅精播麦田实现节水高产具有一定的现实意义。     

Adaptation potential of photosynthesis in wheat cultivars with a capability of leaf rolling under high temperature conditions

For the first time, the adaptive role of the rolling leaf trait for tolerance of wheat plants (Triticum aestivum L.) to the main factor of drought, high temperature, was demonstrated. Cv. Otan with high degree of this trait expression was more tolerant to temperature stress (40°C, 4 h during 2 days (2h/day)). Changes in parameters of chlorophyll fluorescence, F _v/F _m and R _Fd690, suggest that cv. Otan was tolerant to inhibition of photochemical activities of photosystem II (PSII) and photosystem I (PSI). Furthermore, high temperature had no effect on the rate of net photosynthesis (P _N) in cv. Otan, although it decreased this parameter in the other wheat cultivars. The main factors, which provid for this tolerance, were adaptation of the photosynthetic pigment system by active accumulation of carotenoids, more stable structural organization of PSII and PSI, and their high photosynthetic activities, as well as efficient stomatal regulation of transpiration and supplying of mesophyll cells with CO₂. It is hypothesized that the physiological role of the rolling leaf trait is the maintenance of adaptation potential by increasing the efficiency of water metabolism in the flag leaves of wheat under high temperature.     

Temperature responses of photosynthesis and leaf hydraulic conductance in rice and wheat

Studies on the temperature (T) responses of photosynthesis and leaf hydraulic conductance (K_leaf) are important to plant gas exchange. In this study, the temperature responses of photosynthesis and K_leaf were studied in Shanyou 63 (Oryza sativa) and Yannong 19 (Triticum aestivum). Leaf water potential (Ψ_leaf) was insensitive to T in Shanyou 63, while it significantly decreased with T in Yannong 19. The differential Ψ_leaf − T relationship partially accounted for the differing g_m–T relationships, where g_m was less sensitive to T in Yannong 19 than in Shanyou 63. With different g_m–T and Ψ_leaf–T relationships, the temperature responses of photosynthetic limitations were surprisingly similar between the two lines, and the photosynthetic rate was highly correlated with g_m. With the increasing T, K_leaf increased in Shanyou 63 while it decreased in Yannong 19. The different K_leaf–T relationships were related to different Ψ_leaf–T relationships. When excluding the effects of water viscosity and Ψ_leaf, K_leaf was insensitive to T in both lines. g_m and K_leaf were generally not coordinated across different temperatures. This study highlights the importance of Ψ_leaf on leaf carbon and water exchanges, and the mechanisms for the g_m–T and K_leaf–T relationships were discussed.     

Effect of low water potential on photosynthesis in intact lichens and their liberated algal components

Earlier experiments (T.D. Brock 1975, Planta124, 13–23) addressed the question whether the fungus of the lichen thallus might enable the algal component to function when moisture stress is such that the algal component would be unable to function under free-living conditions. It was concluded that the liberated phycobiont in ground lichen thalli could not photosynthesize at water potentials as low as those at which the same alga could when it was present within the thallus. However, our experience with lichen photosynthesis has not substantiated this finding. Using instrumentation developed since the mid-1970's to measure photosynthesis and control humidity, we repeated Brock's experiments. When applying “matric” water stress (equilibrium with air of constant relative humidity) we were unable to confirm the earlier results for three lichen species including one of the species,Letharia vulpina, had also been used by Brock. We found no difference between the effects of low water potential on intact lichens and their liberated algal components (ground thallus material and isolated algae) and no indication that the fungal component of the lichen symbiosis protects the phycobiont from the adverse effects of desiccation once equilibrium conditions are reached. The photosynthetic apparatus of the phycobiont alone proved to be highly adapted to water stress as it possesses not only the capability of functioning under extremely low degrees of hydration but also of becoming reactivated solely by water vapor uptake.     

Effect of leaf structure and water status on carbon isotope discrimination in field-grown durum wheat

The relationships between leaf and kernel carbon isotope discriminations (Δ) and several leaf structural parameters that are indicators of photosynthetic capacity were studied in durum wheat grown in the field under three water regimens. A set of 144 genotypes were cultivated in two rain-fed trials, and 125 of these were grown under supplementary irrigation before heading. Total chlorophyll and nitrogen (N) contents, the dry mass per unit leaf area (LDM, the reciprocal of specific leaf area) and carbon isotope discrimination (Δ) were measured in penultimate leaves and Δ of mature kernels was also analysed. Both LDM and N per unit area showed significant (P≤ 0.001) negative correlation (r=–0.60 and r=–0.36, respectively) with leaf Δ in the wettest trial. Little or no correlation was found for any structural parameter and leaf Δ in the rain-fed trials. In contrast, in the two rain-fed trials LDM was the parameter with the strongest positive correlation (P≤ 0.001) with kernel Δ (r= 0.47 and 0.30) and grain yield (r= 0.43 and 0.29), whereas no correlation was found in the irrigation trial. These correlations, rather than representing a causal link between the amount of photosynthetic tissue and Δ, were probably indirect associations caused by a parallel effect of water status and phenology on leaf structure, grain Δ and yield. Correlations across trials (i.e. environments) between leaf structure and either Δ and yield were very high, although also spurious. Our results suggest that LDM should be used to cull segregating population differences in leaf Δ based on the internal photosynthetic capacity only in the absence of drought. Selecting for kernel Δ and grain yield on the basis of LDM is worthwhile for rain-fed trials.     

短期干旱对水稻叶水势、光合作用及干物质分配的影响

采用盆栽水分试验,研究了不同生育期短期干旱处理对水稻叶水势、光合作用和干物质分配的影响．结果表明,干旱胁迫后,水稻叶水势低于对照,午后叶水势回升缓慢。凌晨叶水势随土壤含水量的降低而降低,表现为阈值反应。叶片净光合速率与凌晨叶水势密切相关,低于凌晨叶水势临界值,水稻叶片净光合速率急剧下降在水稻抽穗期和灌浆期叶片净光合速率显著下降的凌晨叶水势临界值为-1.04和-1．13MPa,对应的土壤含水量阈值分别为饱和含水量的61.0％和50．9％,土壤水势分别为-0．133和-0．240MPa干旱胁迫下单叶净光合速率的日变化规律表现为：胁迫较轻时,单叶净光合速率在正午附近出现低谷;胁迫严重时,净光合速率全天低于对照,且不及对照的一半。短期干旱后,水稻叶、根、穗的分配指数均降低,茎鞘的分配指数升高。本研究可为水稻节水灌溉管理和水分限制下水稻的生长模拟提供生理基础和理论依据。