Comparative response of wheat and oilseed rape to nitrogen supply: absorption and utilisation efficiency of radiation and nitrogen during the reproductive stages determining yield

We investigated the response of spring wheat and oilseed rape to nitrogen (N) supply, focusing on the critical period for grain number definition and grain filling. Crops were grown in containers under a shelter and treated with five combinations of applied N. Wheat and oilseed rape produced comparable amounts of biomass and yield when corrected for the costs of biomass synthesis (SC). From the responses of biomass and yield to late N applications and the apparent contribution of mobilised biomass to yield, it seems that the yield of oilseed rape was more source-limited during grain filling than that of wheat, particularly at the medium and high N levels. Both species recovered equal amounts of N from the total available N in the soil and had similar N use efficiencies, expressed as yield per unit of N absorbed. However, oilseed rape had higher efficiency to convert absorbed N in biomass, but lower harvest index of N than wheat. Oilseed rape had similar or lower root biomass than wheat, depending on N level, but higher root length per unit soil volume and specific root length. The specific uptake rate of N per unit root dry weight during the critical period for grain number determination was higher in oilseed rape than in wheat. In wheat, N limitation affected growth through a similar or lower reduction in radiation use efficiency corrected for synthesis costs (RUE_SC) than in the cumulative amount of intercepted photosynthetically active radiation (IPARc). In oilseed rape, lower growth due to N shortage was associated more with RUE_SC than IPARc, during flowering while during grain filling both components contributed similarly to decreased growth. RUE_SC and the concentration of N in leaves and inflorescence (LIN%) decreased from flowering to maturity and were curvilinearly related. Oilseed rape tended to have higher RUE_SC than wheat at high N supply during the critical period for grain number determination, and generally lower during grain filling. The reasons for these differences and possibilities to increase yield potential are discussed in terms of the photosynthetic efficiency of the different organs and changes in source–sink ratio during reproductive stages. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of nitrogen deficiency on leaf photosynthetic response of tall fescue to water deficit

Abstract. The objective of the present work was to study the effect of nitrogen deficiency on drought sensitivity of tall fescue plants. The authors compared photosynthetic and stomatal behaviour of plants grown at either high (8 mol m⁻³) or low (0.5 mol m⁻³) nitrogen levels during a drought cycle followed by rehydration. Other processes investigated were stomatal and non-stomatal inhibition of leaf photosynthesis, water use efficiency and leaf rolling. Plants were grown in pots in controlled conditions on expanded clay. A Wescor in situ hygrometer placed on the leaf base outside the assimilation chamber permitted, simultaneously to leaf gas exchange measurements, monitoring of leaf water potential. Drought was imposed by withholding water from the pot. CO₂ uptake and stomatal conductance decreased and leaves started to roll at a lower leaf water potential in the high-N than in the low-N grown plants. Stomatal inhibition of leaf photosynthesis seemed larger in the low-N than in the high-N plants. Water-use efficiency increased more in the high-N than in the low-N grown plants during the drought. The decrease of photosynthesis was largely reversible after rehydration in low-N but not in high-N leaves. The authors suggest that low-N plants avoid water deficit rather than tolerate it.     

Effect of two contrasted N fertilisations on rapeseed growth and nitrate metabolism

Winter oilseed rape was grown under two nitrogen fertilisation conditions. The N1-plants and N5-plants were respectively supplied with 4.5 g N per plant (N-limiting condition) and 22.5 g N per plant (non-N-limiting condition). Growth parameters and nitrate reducing capacity were monitored at five sampling stages interspersed with ammonium nitrate applications. N5-plants showed a higher growth rate producing more leaves and stems, early flower and silique formation and delayed leaf senescence. They also contained more nitrate and a higher nitrate reductase activity (NRA) especially in leaves which represented the main site of nitrate reduction before flowering. However, stems and siliques contributed to NRA especially in nitrogen-limited plants that lost their leaves early. This present study outlines the importance of siliques as individual sinks reducing nitrate essentially in the pod walls. The soluble protein content decreased in senescing leaves which was indicative of the reallocation of proteinic nitrogen towards stems and siliques. In non-limiting conditions, other nitrogen compounds of leaves may account for such a reallocation. Hence, the timing of leaf fall could contribute to the low nitrogen recovery in rapeseed.     

The Effects of N Nutrition on the Water Relations and Gas Exchange Characteristics of Wheat (Triticum aestivum L.)

The purpose of this study was to characterize leaf photosynthetic and stomatal responses of wheat (Triticum aestivum L.) plants grown under two N-nutritional regimes. High- and low-N regimes were imposed on growth-chamber-grown plants by fertilizing with nutrient solutions containing 12 or 1 millimolar nitrogen, respectively. Gas-exchange measurements indicated not only greater photosynthetic capacity of high-N plants under well-watered conditions, but also a greater sensitivity of CO₂ exchange rate and leaf conductance to CO₂ and leaf water potential compared to low-N plants. Increased sensitivity of high-N plants was associated with greater tissue elasticity, lower values of leaf osmotic pressure and greater aboveground biomass. These N-nutritional-related changes resulted in greater desiccation (lowered relative water content) of high-N plants as leaf water potential fell, and were implicated as being important in causing greater sensitivity of high-N leaf gas exchange to reductions in water potential. Water use efficiency of leaves, calculated as CO₂ exchange rate/transpiration, increased from 9.1 to 13 millimoles per mole and 7.9 to 9.1 millimoles per mole for high- and low-N plants as water became limiting. Stomatal oscillations were commonly observed in the low-N treatment at low leaf water potentials and ambient CO₂ concentrations, but disappeared as CO₂ was lowered and stomata opened.     

Photosynthesis impairment in cassava leaves in response to nitrogen deficiency

Plants of cassava (Manihot esculenta Crantz cv. Cigana Preta) grown in a sand root medium were watered with nutrient solutions containing either 3 mM nitrate (low N) or 12 mM nitrate (high N). Chlorophyll concentration, chlorophyll a/b ratio, stomatal conductance, photorespiration rate and net carbon assimilation rate (on an area and a mass basis, but not on a chlorophyll basis) all decreased in low-N plants as compared with high-N ones. By contrast, photosynthetic nitrogen-use efficiency increased in low-N plants. As indicated by chlorophyll a fluorescence data, these plants exhibited increases in both excitation pressure on Photosystem II and thermal energy dissipation, with a corresponding decrease in quantum yield of electron transport, when contrasted with high-N plants. This decrease paralleled an unchanged maximal Photosystem II photochemical efficiency, suggesting a down-regulation of the Photosystem II photochemistry. It is proposed that decline in biochemical capacity for carboxylation, rather than stomatal limitation or electron transport, were the major constraints associated to the reduced photosynthetic rates induced by nitrogen deficiency in cassava plants.     

Seasonal relationships between leaf nitrogen content (photosynthetic capacity) and leaf canopy light exposure in peach (Prunus persica)

Abstract Field gas exchange measurements on intact peach (Prunus persica (L.) Batsch) leaves indicate that leaf nitrogen content (N_L) and leaf weight per unit leaf area (W_a) are highly correlated with CO₂ assimilation rate (A) and mesophyll conductance (g_m). Therefore, N_L and W_a were used to study seasonal relationships between leaf carboxylation capacity and natural light exposure in tree canopies. From mid-season onwards, N_L and W_a were linearly correlated with light exposure expressed as the amount of time during a clear day that a leaf was exposed to a photosynthetic photon flux density (Q) of ≥ 100 μmol m^?2 s^?1. The data support the hypothesis that whole-tree photosynthesis is optimized by partitioning of photosynthetic capacity among leaves in deciduous tree canopies with respect to natural light exposure.     

Effects of triazole growth retardants on oilseed rape; photosynthesis of single leaves

Rates of CO₂ exchange were measured for single leaves on oilseed rape plants after application of triapenthenol and BAS111. For leaves which expanded after treatment, there was no detectable change in the rate of photosynthesis per unit leaf area compared with the control for either chemical about 3 wk after application. For fully expanded leaves at the time of application a reduction in the rate of photosynthesis per unit leaf area occurred and was sustained for at least 2 wk. The reduction in the rate of photosynthesis is explained by decreased stomatal conductance.     

太岳山典型阔叶乔木冠层叶片性状的分布格局

以太岳山4种阔叶乔木不同冠层高度的叶片为研究对象,用LI-3000A叶面积仪和Li-6400便携式光合作用测定系统分别测定了这4种乔木不同冠层高度叶片的叶面积大小和单位面积的叶光饱和速率(Aarea);同时测定了其叶氮含量;计算了其比叶面积(SLA)、单位面积叶氮含量(Narea)、单位重量叶氮含量(Nmass)、单位重量的叶光饱和速率(Amass)和光合氮素利用效率(PNUE),对植株不同冠层高度叶片的SLA、叶氮和光合特性的空间分布格局进行了比较研究,结果表明:Aarea、Amass、Nmass、PNUE、SLA和Narea在树冠上层、中层和下层的差异均达到了极显著水平(P<0.001),表明树冠不同高度的叶片性状参数差异较大;在相同SLA下,Nmass和Narea在冠层中的分布均表现为中层>上层>下层,并出现平行位移现象;Aarea和Nmass都以中层值最大,表明冠层光合能力分布格局以中层相对较高。     

Light effects on leaf development and photosynthetic capacity of Hydrocotyle bonariensis Lam.

Rates of net CO₂ uptake were examined in developing leaves of Hydrocotyle bonariensis. Leaves that developed under high photosynthetically active radiation (48 mol m^-2 day^-1 PAR) were smaller, thicker, and reached maximum size sooner than did leaves that developed under low PAR (4.8 mol m^-2 day^-1). Maximum net CO₂ uptake rates were reached after 5 to 6 days expansion for both the low and the high PAR leaves. Leaves grown at high PAR had higher maximum photosynthetic rates and a higher PAR required for light saturation but showed a more rapid decline in rate with age than did low PAR leaves. To assess the basis for the difference observed in photosynthetic rates, CO₂ diffusion conductances and the mesophyll surface available for CO₂ absorption were examined for mature leaves. Stomatal conductance was the largest conductance in all treatments and did not vary appreciably with growth PAR. Mesophyll conductance progressively increased with growth PAR (up to 48 mol m^-2 day^-1) as did the mesophyll surface area per unit leaf area, but the cellular conductance exhibited most of its increase at low PAR (up to 4.8 mol m^-2 day^-1).     

Potential gene flow of two herbicide-tolerant transgenes from oilseed rape to wild B. juncea var. gracilis

Four successive reciprocal backcrosses between F₁ (obtained from wild Brassica juncea as maternal plants and transgenic glyphosate- or glufosinate-tolerant oilseed rape, B. napus, as paternal plants) or subsequent herbicide-tolerant backcross progenies and wild B. juncea were achieved by hand pollination to assess potential transgene flow. The third and forth reciprocal backcrosses produced a number of seeds per silique similar to that of self-pollinated wild B. juncea, except in plants with glufosinate-tolerant backcross progeny used as maternal plants and wild B. juncea as paternal plants, which produced fewer seeds per silique than did self-pollinated wild B. juncea. Germination percentages of reciprocal backcross progenies were high and equivalent to those of wild B. juncea. The herbicide-tolerant first reciprocal backcross progenies produced fewer siliques per plant than did wild B. juncea, but the herbicide-tolerant second or third reciprocal backcross progenies did not differ from the wild B. juncea in siliques per plant. The herbicide-tolerant second and third reciprocal backcross progenies produced an amount of seeds per silique similar to that of wild B. juncea except for with the glufosinate-tolerant first and second backcross progeny used as maternal plants and wild B. juncea as paternal plants. In the presence of herbicide selection pressure, inheritance of the glyphosate-tolerant transgene was stable across the second and third backcross generation, whereas the glufosinate-tolerant transgene was maintained, despite a lack of stabilized introgression. The occurrence of fertile, transgenic weed-like plants after only three crosses (F₁, first backcross, second backcross) suggests a potential rapid spread of transgenes from oilseed rape into its wild relative wild B. juncea. Transgene flow from glyphosate-tolerant oilseed rape might be easier than that from glufosinate-tolerant oilseed rape to wild B. juncea. The original insertion site of the transgene could affect introgression.     

Photosynthetically versatile thin shade leaves: A paradox of irradiance-response curves

Thick sun leaves have a larger construction cost per unit leaf area than thin shade leaves. To re-evaluate the adaptive roles of sun and shade leaves, we compared the photosynthetic benefits relative to the construction cost of the leaves. We drew photosynthetically active radiation (PAR)-response curves using the leaf-mass-based photosynthetic rate to reflect the cost. The dark respiration rates of the sun and shade leaves of mulberry (Morus bombycis Koidzumi) seedlings did not differ significantly. At irradiances below 250 μmol m⁻² s⁻¹, the shade leaves tended to have a significantly larger net photosynthetic rate (P _N) than the sun leaves. At irradiances above 250 μmol m⁻² s⁻¹, the P _N did not differ significantly. The curves indicate that plants with thin shade leaves have a larger daily CO₂ assimilation rate per construction cost than those with thick sun leaves, even in an open habitat. These results are consistently explained by a simple model of PAR extinction in a leaf. We must target factors other than the effective assimilation when we consider the adaptive roles of thick sun leaves.     

Photosynthetic nitrogen-use efficiency of species that differ inherently in specific leaf area

Factors that contribute to interspecific variation in photosynthetic nitrogen-use efficiency (PNUE, the ratio of CO₂ assimilation rate to leaf organic nitrogen content) were investigated, comparing ten dicotyledonous species that differ inherently in specific leaf area (SLA, leaf area:leaf dry mass). Plants were grown hydroponically in controlled environment cabinets at two irradiances (200 and 1000 μmol m^–2 s^–1). CO₂ and irradiance response curves of photosynthesis were measured followed by analysis of the chlorophyll, Rubisco, nitrate and total nitrogen contents of the leaves. At both irradiances, SLA ranged more than twofold across species. High-SLA species had higher in situ rates of photosynthesis per unit leaf mass, but similar rates on an area basis. The organic N content per unit leaf area was lower for the high-SLA species and consequently PNUE at ambient light conditions (PNUE_amb) was higher in those plants. Differences were somewhat smaller, but still present, when PNUE was determined at saturating irradiances (PNUE_max). An assessment was made of the relative importance of the various factors that underlay interspecific variation in PNUE. For plants grown under low irradiance, PNUE_amb of high-SLA species was higher primarily due to their lower N content per unit leaf area. Low-SLA species clearly had an overinvestment in photosynthetic N under these conditions. In addition, high SLA-species allocated a larger fraction of organic nitrogen to thylakoids and Rubisco, which further increased PNUE_amb. High-SLA species grown under high irradiance showed higher PNUE_amb mainly due to a higher Rubisco specific activity. Other factors that contributed were again their lower contents of N_org per unit leaf area and a higher fraction of photosynthetic N in electron transport and Rubisco. For PNUE_max, differences between species in organic leaf nitrogen content per se were no longer important and higher PNUE_max of the high SLA species was due to a higher fraction of N in␣photosynthetic compounds (for low-light plants) and a higher Rubisco specific activity (for high-light grown plants). Received: 11 October 1997 / Accepted: 9 April 1998     

高大气CO2浓度下氮素对小麦叶片光能利用的影响

关于氮素对高大气CO₂浓度下C₃植物光合作用适应现象的调节机理已有较为深入的研究, 但对其光合作用适应现象的光合能量转化和分配机制缺乏系统分析。该文以大气CO₂浓度和施氮量为处理手段, 通过测定小麦(Triticum aestivum)抽穗期叶片的光合作用-胞间CO₂浓度响应曲线以及荧光动力学参数来测算光合电子传递速率和分配去向, 研究了长期高大气CO₂浓度下小麦叶片光合电子传递和分配对施氮量的响应。结果表明, 与正常大气CO₂浓度处理相比, 高大气CO₂浓度下小麦叶片较多的激发能以热量的形式耗散, 增施氮素可使更多的激发能向光化学反应方向的分配, 降低光合能量的热耗散速率; 大气CO₂浓度升高后小麦叶片光化学淬灭系数无明显变化, 高氮叶片的非光化学猝灭降低而低氮叶片明显升高, 施氮促进PSII反应中心的开放比例, 降低光能的热耗散; 高大气CO₂浓度下高氮叶片通过PSII反应中心的光合电子传递速率(J_F)较高, 而且参与光呼吸的非环式电子流速率(J₀)显著降低, 较正常大气CO₂浓度处理的高氮叶片下降了88.40%, 光合速率增加46.47%; 高大气CO₂浓度下小麦叶片J_F-J₀升高而J₀/J_F显著下降, 光呼吸耗能被抑制, 更多的光合电子分配至光合还原过程。因此, 大气CO₂浓度增高条件下, 小麦叶片激发能的热耗散速率增加, 但增施氮素后小麦叶片PSII反应中心开放比例提高, 光化学速率增加, 进入PSII反应中心的电子流速率明显升高, 光呼吸作用被抑制, 光合电子较多地进入光化学过程, 这可能是高氮条件下光合作用适应性下调被缓解的一个原因。     

Diaheliotropic leaf movement enhances leaf photosynthetic capacity and photosynthetic light and nitrogen use efficiency via optimising nitrogen partitioning among photosynthetic components in cotton (Gossypium hirsutum L.)

• Phototropic leaf movement of plants is an effective mechanism for adapting to light conditions. Light is the major driver of plant photosynthesis. Leaf N is also an important limiting factor on leaf photosynthetic potential. Cotton (Gossypium hirsutum L.) exhibits diaheliotropic leaf movement. Here, we compared the long‐term photosynthetic acclimation of fixed leaves (restrained) and free leaves (allowed free movement) in cotton.
• The fixed leaves and free leaves were used for determination of PAR, leaf chlorophyll concentration, leaf N content and leaf gas exchange. The measurements were conducted under clear sky conditions at 0, 7, 15 and 30 days after treatment (DAT).
• The results showed that leaf N allocation and partitioning among different components of the photosynthetic apparatus were significantly affected by diaheliotropic leaf movement. Diaheliotropic leaf movement significantly increased light interception per unit leaf area, which in turn affected leaf mass per area (LMA), leaf N content (N_A) and leaf N allocation to photosynthesis (N_P). In addition, cotton leaves optimised leaf N allocation to the photosynthetic apparatus by adjusting leaf mass per area and N_A in response to optimal light interception.
• In the presence of diaheliotropic leaf movement, cotton leaves optimised their structural tissue and photosynthetic characteristics, such as LMA, N_A and leaf N allocation to photosynthesis, so that leaf photosynthetic capacity was maximised to improve the photosynthetic use efficiency of light and N under high light conditions.

     

Leaf senescence and N uptake parameters as selection traits for nitrogen efficiency of oilseed rape cultivars

The cultivation of N-efficient oilseed rape cultivars could contribute to a reduction of the large N balance surpluses of this crop. To facilitate the breeding process of N-efficient cultivars, the identification of secondary plant traits correlating with N efficiency is necessary. The objectives of this study were to investigate leaf senescence and N uptake parameters of oilseed rape cultivars and doubled haploid (DH) lines with contrasting N efficiency in a short-term nutrient solution experiment and to relate these results to their performance in field experiments. In the nutrient solution experiment, genotypes differed in leaf senescence of fully expanded leaves and maximum N uptake rate per unit root length under low N supply. A high maximum N uptake rate seemed to have contributed to delayed leaf senescence by enhancing N accumulation in leaves. Also in the field experiments, genotypes differed in leaf senescence after flowering at limiting N supply. Additionally, the most N-efficient DH line was able to adapt leaf photosynthetic capacity to the low-light conditions in the canopy during flowering. N efficiency (grain yield at limiting N supply) was positively correlated with delayed leaf senescence both in nutrient solution and field experiments. It is concluded that important leaf and root traits of N-efficient cultivars are expressed in short-term nutrient solution experiments, which may facilitate the selection of N-efficient cultivars.     

Nitrogen reallocation and photosynthetic acclimation in response to partial shading in soybean plants

The first trifoliate of soybean was shaded when fully expanded, while the plant remained in high light; a situation representative for plants growing in a closed crop. Leaf mass and respiration rate per unit area declined sharply in the first few days upon shading and remained rather constant during the further 12 days of the shading treatment. Leaf nitrogen per unit area decreased gradually until the leaves were shed. Leaf senescence was enhanced by the shading treatment in contrast to control plants growing in low light. Shaded leaves on plants grown at low nutrient availability senesced earlier than shaded leaves on plants grown at high nutrient availability. The light saturated rate of photosynthesis decreased also gradually during the shading treatment, but somewhat faster than leaf N, whereas chlorophyll contents declined somewhat slower than leaf N.
Partitioning of N in the leaf over main photosynthetic functions was estimated from parameters derived from the response of photosynthesis to CO₂. It appeared that the N exported from the leaf was more at the expense of compounds that make up photosynthetic capacity than of those involved in photon absorption, resulting in a change in partitioning of N within the photosynthetic apparatus. Photosynthetic nitrogen use efficiency increased during the shading treatment, which was for the largest part due to the decrease in leaf N content, to some extent to the decrease in respiration rate and only for a small part to change in partitioning of N within the photosynthetic apparatus.     

Variation in the export of 13C and 15N from soybean leaf: the effects of nitrogen application and sink removal

Translocation of carbon and nitrogen within a single source-sink unit, comprising a trifoliated leaf, the axillary pod and the subtending internode, and from this unit to the rest of the plant was examined in soybean (Glycine max L. cv. Akishirome) plant by feeding ¹³CO₂ and ¹⁵NO₃. The plants were grown at two levels of nitrogen in the basal medium, i.e. low-N (2 g N m^–2) and high-N (35 g N m^–2) and a treatment of depodding was imposed by removing all the pods from the plant, except the pod of the source sink unit, 13 days after flowering. The plants at high-N accumulated more biomass in its organs compared to low-N and pod removal increased the weight of the vegetative organs. When the terminal leaflet of the source-sink unit was fed with ¹³CO₂, almost all of the radioactive materials were retained inside the source-sink unit and translocation to rest of the plants was insignificant under any of the treatments imposed. Out of the¹³C exported by the terminal leaflet, less than half went into the axillary pod, as the lateral leaflets claimed equal share and very little material was deposited in the petiole. Pod removal decreased ¹³C export at high-N , but not at low-N. Similar to ¹³C, the source-sink unit retained all the ¹⁵N fed to the terminal leaflet at high-N. At low-N, the major part of ¹⁵N partitioning occurred in favour of the rest of the plant outside the source-sink unit, but removal of the competitve sinks from the rest of the plants nullified any partitioning outside the unit. Unlike the situation in ¹³C, no partitioning of ¹⁵N occurred in favour of the lateral leaflets from the terminal leaflet inside the unit. It is concluded that sink demand influences partitioning of both C and N and the translocation of carbon is different from that of nitrogen within a source-sink unit. The translocation of the N is more adjustive to a demand from other sink units compared to the C.     

Effect of nitrogen-deficiency on midday photoinhibition in flag leaves of different rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) cultivars

Effects of nitrogen (N)-deficiency on midday photoinhibition in flag leaves were compared between two contrastive Japanese rice cultivars, a traditional japonica cultivar with low yield, cv. Shirobeniya (SRB), and a japonica-indica intermediate type with high yield, cv. Akenohoshi (AKN). Both cultivars were grown under high-N and low-N conditions. At midday, low-N supply resulted in more intensive reductions in net photosynthetic rate, stomatal conductance, maximal quantum yield of photosystem II (PSII) and quantum yield of PSII electron transport in SRB than in AKN, indicating that SRB was more strongly photoinhibited than AKN under low-N condition. At midday, the low-N plants of two cultivars showed higher superoxide dismutase (SOD) activities than the high-N plants. However, ascorbate peroxidase (APX) activity was maintained in AKN but significantly decreased in SRB under low-N condition (N-deficiency). In contrast, hydrogen peroxide (H₂O₂) content in SRB significantly increased under low-N condition, indicating that the susceptibility to midday photoinhibition in the low-N plants of SRB is related to the increased H₂O₂ accumulation. It is suggested that the midday depression in photosynthesis may be a result of oxidative stress occurring in the low-N plants in which antioxidant capacity is not enough to cope with the generation of H₂O₂. Therefore, H₂O₂-scavenging capacity could be an important factor in determining the cultivar difference of midday photoinhibition in flag leaves of rice under low-N condition.     

Effects of leaf position and nitrogen supply on the expansion of leaves of field grown sunflower (Helianthus annuus L.)

Leaf growth responses to N supply and leaf position were studied using widely-spaced sunflower plants growing under field conditions. Both N supply (range 0.25 to 11.25 g added N per plant) and leaf position significantly (p=0.001) affected maximum leaf area (LA_max) of target leaves through variations in leaf expansion rate (LER); effects on duration of expansion were small. Specific leaf nitrogen (SLN, g N m^-2) fell quite rapidly during the initial leaf expansion phase (LA < 35% LA_max) but leveled off during the final 65% increase of leaf area. This pattern held across leaf positions and N supply levels. Leaf nitrogen accumulation after 35% LA_max continued up to achievement of LA_max; reductions in the higher SLN characteristic of the initial phase were insufficient to cover the nitrogen requirements for expansion during the final phase. LER in the quasi-linear expansion phase (35 to 100% of LA_max) was strongly associated with SLN above a threshold that varied with leaf position (mean 1.79±0.225 g N m^-2). This contrasts with the response of photosynthesis at high irradiance to SLN, which has previously been shown to have a threshold of 0.3 g N m^-2; in the present work saturation of photosynthetic rate was evident when SLN reached 1.97 g N m^-2. Thus, once the area of a leaf exceeds 35% of LA_max, expansion proceeds provided SLN values are close to the levels required for maximum photosynthesis. However, growth of leaves during the initial expansion phase ensures a minimum production of leaf area even at low N supply levels.     

Relationships between photosynthetic activity and silica accumulation with ages of leaf in Sasa veitchii (Poaceae, Bambusoideae)

BACKGROUND AND AIMS: Bamboos have long-lived, evergreen leaves that continue to accumulate silica throughout their life. Silica accumulation has been suggested to suppress their photosynthetic activity. However, nitrogen content per unit leaf area (N(area)), an important determinant of maximum photosynthetic capacity per unit leaf area (P(max)), decreases as leaves age and senescence. In many species, P(max) decreases in parallel with the leaf nitrogen content. It is hypothesized that if silica accumulation affects photosynthesis, then P(max) would decrease faster than N(area), leading to a decrease in photosynthetic rate per unit leaf nitrogen (photosynthetic nitrogen use efficiency, PNUE) with increasing silica content in leaves. METHODS: The hypothesis was tested in leaves of Sasa veitchii, which have a life span of 2 years and accumulate silica up to 41 % of dry mass. Seasonal changes in P(max), stomatal conductance, N(area) and silica content were measured for leaves of different ages. KEY RESULTS: Although P(max) and PNUE were negatively related with silica content across leaves of different ages, the relationship between PNUE and silica differed depending on leaf age. In second-year leaves, PNUE was almost constant although there was a large increase in silica content, suggesting that leaf nitrogen was a primary factor determining the variation in P(max) and that silica accumulation did not affect photosynthesis. PNUE was strongly and negatively correlated with silica content in third-year leaves, suggesting that silica accumulation affected photosynthesis of older leaves. CONCLUSIONS: Silica accumulation in long-lived leaves of bamboo did not affect photosynthesis when the silica concentration of a leaf was less than 25 % of dry mass. Silica may be actively transported to epidermal cells rather than chlorenchyma cells, avoiding inhibition of CO2 diffusion from the intercellular space to chloroplasts. However, in older leaves with a larger silica content, silica was also deposited in chlorenchyma cells, which may relate to the decrease in PNUE.