Changes of Photosynthetic Characteristics in Relation to Leaf Senescence in Two Maize Hybrids with Different Senescent Appearance

A field experiment was conducted to investigate the changes in chlorophyll (Chl) and nitrogen (N) contents, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) and phosphoenolpyruvate carboxylase (PEPC) contents and PEPC activity, and the photon-saturated net photosynthetic rate (P _Nsat), and their relationships with leaf senescence in two maize hybrids with different senescent appearance. One stay-green (cv. P3845) and one earlier senescent (cv. Hokkou 55) hybrid were used in this study, and we found that Chl and N contents and the P _Nsat in individual leaves of P3845 were greater than those in corresponding leaves of Hokkou 55 at the successive growth stages. In addition, larger contents of RuBPCO and PEPC, and a greater activity of PEPC were observed in P3845. Due to the lower rates of decrease of Chl, RuBPCO, and PEPC amounts per unit of N, and the lower net C translocation rate per unit of N in the stay-green hybrid, leaf senescence was delayed in comparison to the earlier senescent hybrid.     

Plant Growth Based on Interrelation between Carbon and Nitrogen Translocation from Leaves

In individual leaves, the photon-saturated photosynthetic activity (P _sat, expressed on a dry mass basis) was closely related to the nitrogen content (Nc) as follows: P _sat = Cf Nc + P _sat0, where Cf and P _sat0 are constants. On a whole plant basis, the relative growth rate (RGR) was closely related to Nc in canopy leaf as follows: RGR = DMf Nc + RGR₀, where DMf and RGR₀ are constants. However, the coefficients Cf and DMf were markedly different among plant species. To explain these differences, it is suggested that carbon assimilation (or dry matter production) is controlled by both the Nc in a leaf (or leaves) and by the net N translocation from leaves. This is supported by the finding that P _sat is related to the rate of ³⁵S-methionine translocation from leaves. We propose another estimation method for the net N translocation rate (NFR) from leaves: Nc, after full leafing, is expressed as a function of time: Nc = (Nc₀ – Ncd) exp(–Nft) + Ncd, where Nf is a coefficient, t is the number of days after leaf emergence, Nc₀ is the initial value of Nc, and Ncd is the Nc of the dead leaf. The NFR is then calculated as NFR = Nc/t = –Nf (Nc – Ncd). Thus Nf is the coefficient for the NFR per unit Nc. NFR is a good indicator of net N translocation from leaves because NFR is closely related to the rate of ³⁵S-methionine translocation from leaves. Since P _sat is related to the ¹⁴C-photosynthate translocation rate, Cf (or DMf) corresponds to the coefficient of saccharide translocation rate per unit amount of Nc. Cf (or DMf) is closely related to the Nf of individual leaves (or the Nf of canopy leaf). This indicates that C assimilation and C translocation from leaves are related to Nc and N translocation from leaves (net translocation of N). Cf and Nf are negatively correlated with leaf longevity, which is important because a high or low CO₂ assimilation rate in leaves is accompanied by a correspondingly high or low N translocation in leaf, and the degree of N translocation in leaves decreases or increases leaf longevity. Thus, since a relatively high P _sat (or RGR) is accompanied by a rapid Nc decrease in leaves, it is difficult to maintain a high P _sat (or RGR) for a sustained time period.     

Soil salinity effects on transpiration and net photosynthetic rates,stomatal conductance and Na+ and Cl− contents in durum wheat

Leaf gas exchange, plant growth and leaf ion content were measured in wheat (Triticum durum L. cv. HD 4502) exposed to steady- state salinities (1.6, 12.0 and 16.0 dS nr⁻¹) for 8 weeks. Salinity reduced leaf area and number of tillers, and increased Na⁺ and Cl⁻ concentrations in leaves. Leaf- to- leaf gradients of these ions were observed. The oldest leaf contained 6 to 8 times more Na⁺ and Cl⁻ than the flag leaf. Net photosynthetic rate (P_N), transpiration rate (E) and stomatal conductance (g_S) were the highest in flag leaf, declined in the middle and fully expanded leaves, and were minimum in the oldest leaves. These processes were reduced by salinity with similar leaf- to- leaf gradients. Intercellular CO₂ concentrations in the older leaves were higher than in the flag leaf in non-saline plants, and increased similarly with salinity. Leaf age was the major factor in reducing P_N, and senescence processes were promoted by salinity.     

Gas Exchange and Epidermal Characteristics of Miscanthus Populations in Taiwan Varying with Habitats and Nitrogen Application

Seventeen clones of C₄ grass Miscanthus spp. collected from different climatic regions and elevations of Taiwan were transplanted in pots. 15–16 months after collection the plants received 0, 1, and 2 g of nitrogen fertiliser (N₀, N₁, and N₂, respectively) per pot. All the measurements were done 10–12 d after N application. The relationships between net photosynthetic rate (P _N) and photon flux density (PFD) showed a saturated curve, with PFD saturation at about 1 000 µmol m^–2 s^–1. The ranges of PFD saturated P _N (P _sat) for all the tested clones with N₀, N₁, and N₂ were 8–16, 11–18, and 12–21 µmol m^–2 s^–1, respectively. The clones from southern Taiwan, a tropical region, showed the highest P _sat, followed by the clones from northern Taiwan, a subtropical region, while those from mountainous area showed the lowest P _sat. The clones collected from southern Taiwan showed the highest frequency of stomata on the adaxial surface, and those collected from the high mountainous area showed the lowest frequency. Also the adaxial surface of leaves from the higher mountainous area had more wax deposited than the leaves from the lowland. Thus the low P _sat in mountain clones is limited by both stomatal and non-stomatal factors. Further, the lower leaf conductance and different epidermal characteristics of mountain clones might prevent excessive loss of heat through transpiration and provide production against ultraviolet-B radiation.     

Acclimation response of spring wheat in a free-air CO2 enrichment (FACE) atmosphere with variable soil nitrogen regimes. 1. Leaf position and phenology determine acclimation response

We have examined the photosynthetic acclimation of wheat leaves grown at an elevated CO₂ concentration, and ample and limiting N supplies, within a field experiment using free-air CO₂ enrichment (FACE). To understand how leaf age and developmental stage affected any acclimation response, measurements were made on a vertical profile of leaves every week from tillering until maturity. The response of assimilation (A) to internal CO₂ concentration (C_i) was used to estimate the in vivo carboxylation capacity (Vc_max) and maximum rate of ribulose-1,5-bisphosphate limited photosynthesis (A _sat). The total activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), and leaf content of Rubisco and the Light Harvesting Chlorophyll a/b protein associated with Photosystem II (LHC II), were determined. Elevated CO₂ did not alter Vc_max in the flag leaf at either low or high N. In the older shaded leaves lower in the canopy, acclimatory decline in Vc_max and A _sat was observed, and was found to correlate with reduced Rubisco activity and content. The dependency of acclimation on N supply was different at each developmental stage. With adequate N supply, acclimation to elevated CO₂ was also accompanied by an increased LHC II/Rubisco ratio. At low N supply, contents of Rubisco and LHC II were reduced in all leaves, although an increased LHC II/Rubisco ratio under elevated CO₂ was still observed. These results underscore the importance of leaf position, leaf age and crop developmental stage in understanding the acclimation of photosynthesis to elevated CO₂ and nutrient stress. This revised version was published online in June 2006 with corrections to the Cover Date.     

Diurnal Changes in Photosynthesis,Sugars, and Nitrogen of Wheat and Mungbean Grown Under Elevated CO2 Concentration

The diurnal changes in leaf net photosynthetic rate (P _N) and sugar and nitrogen contents in wheat [Triticum aestivum (L.) cv. HD 2285] and mungbean [Vigna radiata (L.) Wilczek cv. PS 16] were analysed under ambient, AC [350±25 µmol mol^–1] and elevated, EC [600±50 µmol mol^–1] CO₂ concentrations. In both mungbean and wheat P _N of AC- and EC-grown plants compared at the same CO₂ concentration showed that P _N was higher under EC. However, increased P _N in EC-plants declined in the afternoon and approached P _N of AC-plants. Depression in P _N, however, was less in mungbean compared with the large depression in wheat. Greater down regulation of P _N in wheat was associated with the accumulation of large amount of sugars and low nitrogen content in wheat leaves. Mungbean leaves accumulated mostly starch under EC and the difference in N content in AC- and EC-plants was relatively less than in wheat.     

Chain Correlation between Variables of Gas Exchange and Yield Potential in Different Winter Wheat Cultivars

Variables of gas exchange of flag leaves and grain yield potentials of five representative winter wheat (Triticum aestivum L.) cultivars varied greatly across different development stages under the same management and irrigation. The cultivars with high yield potential had higher net photosynthetic rate (P _N), PPFD (photosynthetic photon flux density) saturated photosynthetic rate (P _sat), stomatal conductance (g _s), and maximum apparent quantum yield of CO₂ fixation (_m,app) than those with low grain yield, but their dark respiration rate (R _D) and compensation irradiance (I _c) were remarkably lower. Compared with overall increase of yield potential of 71 % from low yield cultivars to high yield ones, P _N, P _sat, _m,app, and g _s were 13, 19, 57, and 32 % higher, respectively; but R _D and I _c decreased by 19 and 76 %, respectively. Such difference was evidently large during anthesis stage (e.g., P _N by 33 %), which indicated that this period could be the best for assisting further selection for better cultivars. However, transpiration rate (E) and water use efficiency (WUE) differed only little. At different development stages, especially at anthesis, P _N and P _sat were positively correlated with _m,app, g _s, and yield potential, and negatively correlated with R _D and I _c. Thus the high-yield-potential winter wheat cultivars possess many better characters in photosynthesis and associated parameters than the low-yield cultivars.     

Differences in nitrogen use efficiency between leaves from canopy and subcanopy trees

We examined the effects of increasing light availability along a vertical gradient within a forest community on the efficiency of leaf nitrogen (N) use in individual trees. The N contents of green and senescent leaves in canopy and subcanopy trees of an evergreen coniferous species, Podocarpus nagi, and an evergreen hardwood species, Neolitsea aciculata, were analyzed in a mixed forest community at Mt Mikasa, Nara City, Japan. The inverse of N concentration (N_C) in senescent leaves was used as an index of N use efficiency (NUE) at the leaf-level. The leaf-level NUE was higher in canopy trees than in subcanopy trees in both P.nagi and N.aciculata, although soil N mineralization rates around canopy and subcanopy trees did not differ significantly. The N_C in green leaves was lower in canopy trees than in subcanopy trees. The ratio of resorbed N in senescent leaves to the N content in green leaves was higher in canopy trees than in subcanopy trees. The higher leaf-level NUE of canopy trees was partly a result of lower N_C in living tissues and partly because of greater N resorption during senescence. The present study suggested that the leaf-level NUE could be increased in response to an imbalance between soil N and light availability caused by spatial community structure.     

Photosynthetic capacity,irradiance and sequential senescence of sugar beet leaves

In field-grown sugar beet plants (Beta vulgaris L. cv. Dobrovická A), each of66 successive leaves produoed in the course of the vegetation period was different with respect to its photosynthetic capaoity (P_c), life span, duration of leaf area expansion, and longevity after its maximum leaf area (A_max) has developed. The proportionality between the seasonal changes in these characteristics was not the same if the sequential senescence of leaves was taken into account. With aging of individual leaves, P_c increased with the leaf area expansion having attained the peak value between 75% to 100% of A_max The rate of ontogenetic changes in P_c of each leaf was specified by the rate of its growth and development so that even at comparable ages the successive leaves constituted a series of different physiological units. The seasonal changes in quantum irradiance (PAR) were found to be responsible for differences in the growth characteristics between the successive leaves: Leaf expansion period was related with daily integrals of the incoming PAR (Io), while leaf longevity, after the A_max had been attained, was closely linked with PAR intercepted by the canopy (I). P_c expressed per the total leaf area of the plant was significantly correlated withI, while P_c calculated per unit leaf area of the plant was related toI _o Leaf potential to adapt P_c correspondingly to changes in PAR was greatest during leaf blade expansion; after the leaf had ceased to expand, changes in P_c were independent of differences in leaf irradiance. The results stress, at least for field conditions, the inadmissibility of the extrapolation of attributes from one leaf to the other ones sequentially senescing on the plant.     

Effects of nitrogen supply restriction on gas exchange and photosystem 2 function in flag leaves of a traditional low-yield cultivar and a recently improved high-yield cultivar of rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.)

The effects of nitrogen (N) supply restriction on the CO₂ assimilation and photosystem 2 (PS2) function of flag leaves were compared between two contrastive Japanese rice cultivars, a low-yield cultivar released one century ago, cv. Shirobeniya (SRB), and a recently improved high-yield cultivar, cv. Akenohoshi (AKN). Both cultivars were solution-cultured at four N supply levels from N4 (control) to N1 (the lowest). With a reduction in N-supply, contents of N (LNC), ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO), and chlorophyll (Chl) in flag leaves decreased in both cultivars. In parallel with this, the net photosynthetic rate (P _N), mesophyll conductance (g _m), and stomatal conductance (g _s) decreased. P _N was more dominantly restricted by g _m than g _s. The values of P _N, g _m, and RuBPCO content were larger in AKN than SRB at the four N supply levels. The content of Chl greatly decreased with N deficiency, but the reduction in the maximum quantum yield of PS2 was relatively small. Quantum yield of PS2 (Φ_PS2) decreased with N deficiency, and its significant cultivar difference was observed between the two cultivars at N1: a high value was found in AKN. The content ratio of Chl/RuBPCO was also significantly low in AKN. The low Chl/RuBPCO is one of the reasons why AKN maintained a comparatively high P _N and Φ_PS2 at N deficiency. The adequate ratio of N distribution between Chl and RuBPCO is the important prerequisite for the efficient and sustainable photosynthesis in a flag leaf of rice plant under low N-input.     

Thermal acclimation of leaf and root respiration: an investigation comparing inherently fast- and slow-growing plant species

We investigated the extent to which leaf and root respiration (R) differ in their response to short‐ and long‐term changes in temperature in several contrasting plant species (herbs, grasses, shrubs and trees) that differ in inherent relative growth rate (RGR, increase in mass per unit starting mass and time). Two experiments were conducted using hydroponically grown plants. In the long‐term (LT) acclimation experiment, 16 species were grown at constant 18, 23 and 28 °C. In the short‐term (ST) acclimation experiment, 9 of those species were grown at 25/20 °C (day/night) and then shifted to a 15/10 °C for 7 days. Short‐term Q₁₀ values (proportional change in R per 10 °C) and the degree of acclimation to longer‐term changes in temperature were compared. The effect of growth temperature on root and leaf soluble sugar and nitrogen concentrations was examined. Light‐saturated photosynthesis (A_sat) was also measured in the LT acclimation experiment. Our results show that Q₁₀ values and the degree of acclimation are highly variable amongst species and that roots exhibit lower Q₁₀ values than leaves over the 15–25 °C measurement temperature range. Differences in RGR or concentrations of soluble sugars/nitrogen could not account for the inter‐specific differences in the Q₁₀ or degree of acclimation. There were no systematic differences in the ability of roots and leaves to acclimate when plants developed under contrasting temperatures (LT acclimation). However, acclimation was greater in both leaves and roots that developed at the growth temperature (LT acclimation) than in pre‐existing leaves and roots shifted from one temperature to another (ST acclimation). The balance between leaf R and A_sat was maintained in plants grown at different temperatures, regardless of their inherent relative growth rate. We conclude that there is tight coupling between the respiratory acclimation and the temperature under which leaves and roots developed and that acclimation plays an important role in determining the relationship between respiration and photosynthesis.     

Induction of leaf senescence by low nitrogen nutrition in sunflower (Helianthus annuus) plants

Different parameters which vary during the leaf development in sunflower plants grown with nitrate (2 or 20 mM) for a 42‐day period have been determined. The plants grown with 20 mM nitrate (N+) showed greater leaf area and specific leaf mass than the plants grown with 2 mM nitrate (N?). The total chlorophyll content decreased with leaf senescence, like the photosynthetic rate. This decline of photosynthetic activity was greater in plants grown with low nitrogen level (N?), showing more pronounced senescence symptoms than with high nitrogen (N+). In both treatments, soluble sugars increased with aging, while starch content decreased. A significant increase of hexose to sucrose ratio was observed at the beginning of senescence, and this raise was higher in N? plants than in N+ plants. These results show that sugar senescence regulation is dependent on nitrogen, supporting the hypothesis that leaf senescence is regulated by the C/N balance. In N+ and N? plants, ammonium and free amino acid concentrations were high in young leaves and decreased progressively in the senescent leaves. In both treatments, asparagine, and in a lower extent glutamine, increased after senescence start. The drop in the (Glu+Asp)/(Gln+Asn) ratio associated with the leaf development level suggests a greater nitrogen mobilization. Besides, the decline in this ratio occurred earlier and more rapidly in N? plants than in N+ plants, suggesting that the N? remobilization rate correlates with leaf senescence severity. In both N+ and N? plants, an important oxidative stress was generated in vivo during sunflower leaf senescence, as revealed by lipid peroxidation and hydrogen peroxide accumulation. In senescent leaves, the increase in hydrogen peroxide levels occurred in parallel with a decline in the activity of antioxidant enzymes. In N+ plants, the activities of catalase and ascorbate peroxidase (APX) increased to reach their highest values at 28 days, and later decreased during senescence, whereas in N? plants these activities started to decrease earlier, APX after 16 days and catalase after 22 days, suggesting that senescence is accelerated in N‐leaves. It is probable that systemic signals, such as a deficit in amino acids or other metabolites associated with the nitrogen metabolism produced in plants grown with low nitrogen, lead to an early senescence and a higher oxidation state of the cells of these plant leaves.     

Ozone sensitivity in Triticum durum and T. aestivum with respect to leaf injury, photosynthetic activity and free radical content

Sensitivity to ozone is highly variable in cultivars of different wheat species, leading to differences in leaf injury and yield. Not much is known about the physiological background of these differences. The objective of this study was to compare the effects of ozone on photosynthetic parameters in Triticum aestivum L. (spring wheat cv. Nandu, winter wheat cv. Perlo) and Triticum durum Desf. (cv. Extradur). Plants cultivated in pots were exposed to 80 nmol mol^?1 ozone, or were used as control plants in a greenhouse. Stages of growth and senescence of single leaves were recorded. Light-saturated net photosynthesis, leaf conductance for water vapour, and chlorophyll fluorescence were measured. Stomatal limitation was calculated from CO₂ response curves, and the free radical content of whole leaves measured by EPR spectroscopy. Senescence of single leaves was enhanced by the ozone-treatment in all three cultivars, in the order Nandu > Perlo > Extradur. Development of whole plants was slightly delayed in Perlo and Nandu, but was accelerated significantly in Extradur. The rate of net photosynthesis under light saturation (A_sat) decreased significantly in older, ozone-fumigated leaves of Perlo and Nandu but not of Extradur. Leaf conductance (g₁) showed a similar behaviour, but stomatal limitation (l) was similar between ozone-treated and control plants. Thus, an ozone-induced closure of stomata was not the reason for the observed difference in A_sat. Perlo and Nandu showed a significant, only partly reversible decrease in F_v/F_m in ozone-fumigated leaves, whereas in Extradur the decrease was fully reversible only in older leaves. Whole leaves of Extradur, in contrast to Perlo and Nandu, showed no increase in EPR free radical signals. The higher ozone tolerance of Extradur was thus not caused by decreased ozone uptake via the stomata, but by a better ability of photosynthetically active mesophyll cells to cope with photooxidative stress.     

Boron Deficiency Induced Changes in Translocation of 14CO2-Photosynthate Into Primary Metabolites in Relation to Essential Oil and Curcumin Accumulation in Turmeric (Curcuma longa L.)

Changes in leaf growth, net photosynthetic rate (P _N), incorporation pattern of photosynthetically fixed ¹⁴CO₂ in leaves 1–4 from top, roots, and rhizome, and in essential oil and curcumin contents were studied in turmeric plants grown in nutrient solution at boron (B) concentrations of 0 and 0.5 g m^-3. B deficiency resulted in decrease in leaf area, fresh and dry mass, chlorophyll (Chl) content, and P _N and total ¹⁴CO₂ incorporated at all leaf positions, the maximum effect being in young growing leaves. The incorporation of ¹⁴CO₂ declined with leaf position being maximal in the youngest leaf. B deficiency resulted in reduced accumulation of sugars, amino acids, and organic acids at all leaf positions. Translocation of the metabolites towards rhizome and roots decreased. In rhizome, the amount of amino acids increased but content of organic acids did not show any change, whereas in roots there was decrease in contents of these metabolites as a result of B deficiency. Photoassimilate partitioning to essential oil in leaf and to curcumin in rhizome decreased. Although the curcumin content of rhizome increased due to B deficiency, the overall rhizome yield and curcumin yield decreased. The influence of B deficiency on leaf area, fresh and dry masses, CO₂ exchange rate, oil content, and rhizome and curcumin yields can be ascribed to reduced photosynthate formation and translocation.     

拟南芥莲座叶片发育过程中P1基因的表达特性

该实验对CDF1类似蛋白基因(P1)在拟南芥叶片发育不同阶段的定量PCR结果显示,P1基因在拟南芥叶片发育的所有时期均可表达,但在茎生叶和衰老叶中的表达水平明显高于成熟叶和幼叶。GUS报告基因表达的组织化学染色结果显示,P1启动子在拟南芥叶片中有较高的驱动活性;在营养生长阶段的幼苗和植株(4~5周)的所有叶片中均能检测到GUS表达,但在植株转入生殖生长阶段后(6周及以后),GUS表达主要集中在逐渐衰老的叶中,并随着叶片衰老程度加剧GUS染色程度也越深,这一结果与GUS荧光定量检测结果一致。通过分析P1基因启动子上可能存在的顺式调控元件,发现茉莉酸甲酯、热压、干旱和水杨酸等均能够引起叶片衰老调控元件的响应,证实P1的表达受到这些因素的调控。研究表明,P1在拟南芥莲座叶片中很可能参与了对上游衰老信号的响应,该研究结果为进一步探究P1在叶片衰老过程中的分子功能验证奠定了基础。     

Excess irradiance causes early symptoms of senescence during leaf expansion in photoautotrophically <Emphasis Type="Italic">in vitro</Emphasis> grown tobacco plants

Photosynthetic parameters, growth, and pigment contents were determined during expansion of the fourth leaf of in vitro photoautotrophically cultured Nicotiana tabacum L. plants at three irradiances [photosynthetically active radiation (400–700 nm): low, LI 60 μmol m⁻² s⁻¹; middle, MI 180 μmol m⁻² s⁻¹; and high, HI 270 μmol m⁻² s⁻¹]. During leaf expansion, several symptoms usually accompanying leaf senescence appeared very early in HI and then in MI plants. Symptoms of senescence in developing leaves were: decreasing chlorophyll (Chl) a+b content and Chl a/b ratio, decreasing both maximum (F_V/F_M) and actual (Φ_PS2) photochemical efficiency of photosystem 2, and increasing non-photochemical quenching. Nevertheless, net photosynthetic oxygen evolution rate (P _N) did not decrease consistently with decrease in Chl content, but exhibited a typical ontogenetic course with gradual increase. P _N reached its maximum before full leaf expansion and then tended to decline. Thus excess irradiance during in vitro cultivation did not cause early start of leaf senescence, but impaired photosynthetic performance and Chl content in leaves and changed their typical ontogenetic course.     

Photosynthesis and yield responses of ozone-polluted winter wheat to drought

Winter wheat (Triticum aestivum L. cv. Jingdong 8) was exposed to short-term high ozone treatment after anthesis and then was either well irrigated with soil water content (SWC) of 80–85 % (O₃+W) or drought treated (SWC 35–40 %, O₃+D). Short-term ozone exposure significantly decreased irradiance-saturated net photosynthetic rate (P _N) of winter wheat. Under good SWC, P _N of the O₃-treated plant was similar to that of control on 2 d after O₃-exposure (6 DAA), but decreased significantly after 13 DAA, indicating that O₃ exposure accelerated leaf senescence. Meanwhile, green flag leaf area was reduced faster than that of control. As a result, grain yield of O₃+W was significantly decreased. P _N of O₃+D was further notably decreased and green flag leaf area was reduced more than that in O₃+W. Consequently, substantial yield loss of O₃+D was observed compared to that of O₃+W. Although P _N was significantly positively correlated with stomatal conductance, it also had notable positive correlation with the maximum photochemical efficiency in the dark adapted leaves (F_v/F_m), electron transport rate (ETR), photochemical quenching (q_P), as well as content of chlorophyll, suggesting that the depression of P _N was mainly caused by non-stomatal limitation. Hence optimal soil water condition should be considered in order to reduce the yield loss caused by O₃ pollution.     

Increased cytokinin levels in transgenic PSAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

We studied the impact of delayed leaf senescence on the functioning of plants growing under conditions of nitrogen remobilization. Interactions between cytokinin metabolism, Rubisco and protein levels, photosynthesis and plant nitrogen partitioning were studied in transgenic tobacco (Nicotiana tabacum L.) plants showing delayed leaf senescence through a novel type of enhanced cytokinin syn‐thesis, i.e. targeted to senescing leaves and negatively auto‐regulated (P_SAG12–IPT), thus preventing developmental abnormalities. Plants were grown with growth‐limiting nitrogen supply. Compared to the wild‐type, endogenous levels of free zeatin (Z)‐ and Z riboside (ZR)‐type cytokinins were increased up to 15‐fold (total ZR up to 100‐fold) in senescing leaves, and twofold in younger leaves of P_SAG12–IPT. In these plants, the senescence‐associated declines in N, protein and Rubisco levels and photosynthesis rates were delayed. Senescing leaves accumulated more (¹⁵N‐labelled) N than younger leaves, associated with reduced shoot N accumulation (–60%) and a partially inverted canopy N profile in P_SAG12–IPT plants. While root N accumulation was not affected, N translocation to non‐senescing leaves was progressively reduced. We discuss potential consequences of these modified sink–source relations, associated with delayed leaf senescence, for plant productivity and the efficiency of utilization of light and minerals.