Zixu Yin·Shaohui Fan·Wen Xia·Yang Zhou·Xiao Zhou·Xuan Zhang·Chengji Li·Fengying Guan
Abstract The effects of long-day photoperiod on growth,photosynthetic fluorescence,carbon and nitrogen metabolism,and yield of Dendrocalamopsis oldhami and the compensation effects of fertilization were investigated.A completely randomized design was used with two light factors (bamboo culms cultivated in solar greenhouse under long-day [Ls] and short-day [Ln] treatments);two organic nitrogen fertilizer levels (application of organic fertilizer [OF] and no organic fertilizer [NF]);and three nitrogen fertilizer levels (Low [N0],medium [N1] and high nitrogen [N2]).Leaf chlorophyll and fluorescence parameters (φ Po,PIABS, and ETo/CSm) decreased and DIo/CSm increased in Ls compared to Ln.Indole acetic acid (IAA) and gibberellic acid (GA3) levels decreased,whereas abscisic acid (ABA) increased.Leaf area decreased and leaf dry mass increased.The contents of carbon and nitrogen metabolism-related enzymes (nitrate reductase,glutamine synthetase,amylase,and sucrose synthase) and products (total nitrogen,organic carbon,soluble sugar,and starch) increased.Single bamboo shoot weight and diameter at breast height decreased,whereas shoot quantity and total yield increased.Fertilizer application significantly affected physiological growth and yield in the two light treatments,thus promoting carbon and nitrogen metabolism.The φ Po,PIABS,IAA,and GA3 contents increased slightly,whereas ABA levels decreased.Shoot quantity,individual weight,and total yield improved.IAA,soluble sugar,and total yield to organic manure and light were lower than those of nitrogen levels (FN ＞ FL,FO) .Other indicators showed lower responses to different fertilization treatments than the light factor (FL ＞ FN,FO) .The ability of D.oldhami to alter its morphological and physiobiochemical traits and yield in response to variations in light applications may translate into high phenotypic plasticity.Fertilization significantly improved photoplasticity of D.oldhami.Under Ls,D.oldhami had high metabolic rates,was easily inhibited by light,and showed accelerated leaf senescence,and shoot quantity and total output increased.However,the quality of individual shoots decreased.Different fertilization treatments affected D.oldhami differently under the two light intensities.Ls sensitivity to nitrogen was higher.Fertilization could delay leaf dormancy and senescence under Ls treatment.Organic fertilizer addition could improve yield more effectively,with OFN1 being the optimal fertilization level.
Keywords Light·Nitrogen·Organic fertilizer·Fluorescence parameters·Carbon and nitrogen metabolism·Yield
Global warming is expected to lead to the development of agricultural production at higher latitudes.The rise in temperature in the middle and low latitudes prolongs the planting time of crops.Under circumstances of environmental impact and artificial plant breeding,crops grown at low latitudes will expand to high latitudes.Tropical resources at low latitudes contain abundant high-quality germplasm demonstrating pest and drought resistance and infertility tolerance.Using tropical germplasm resources in temperate regions is an important means to cope with a warming climate and address food security.However,the introduction of germplasm from the south to the north needs to focus on the growth period of short-day crops caused by the sharp increase in photoperiod and a change in crop sensitivity to photoperiodism (day length recognition) (Upadhyaya et al.2018;Aki et al.2019;Lindsey et al.2020).In summer in northern regions,shading is often used to shorten sunshine hours so that soybean,perilla,sorghum,and other shortday crops can blossom smoothly (Wada et al.2010;Clerget et al.2021;Lin et al.2021).Inadequate light limits photosynthetic efficiency;however,excessive light intensity can lead to photoinhibition of photosynthesis under light stress,especially at low temperatures,drought,or other adverse environmental factors.When weak light abruptly intensif ies,plants are exposed to strong light which can cause irreversible damage to the photosynthetic mechanism (Danielle and Robert 2012;Wu et al.2017).Numerous studies have shown that long day-length environments increase seedling growth,plant height,stem diameter,total leaf area,induce and promote vegetative growth,inhibit reproductive growth and tuber formation,and affect yields.The effects of prolonged sunlight on indole acetic acid (IAA),gibberellins (GA3),and abscisic acid (ABA) contents differ in different plants,and it is believed that plant dormancy is caused by the combined effects of low temperature and long sunshine (Ceccato et al.2015;Aohara et al.2016;Liang et al.2018;Izadi et al.2020;Sahari et al.2020;Li et al.2021).Bamboo is characterized by clonal growth and adaptability to heterogeneous habitats,with high phenotypic plasticity and generally adopts morphological plasticity regulation (Shi et al.2014),mainly manifested in compositional morphology,resource absorption structure,and dry mass allocation (Liu et al.2015).Studies on the physiological response of bamboo to light factors have focused on canopy and understory vegetation (Yang et al.2014;Xie et al.2016) and potted seedlings with different shading gradients (Du et al.2019;Fan et al.2021).To our knowledge,there have been no studies on the physiological responses of bamboo to long-day light stress.Little is known about the phenotypic plasticity of bamboo to light,which is crucial for understanding survival and performance in heterogeneous habitats.Understanding the physiological mechanisms of bamboo in such habitats will help increase the feasibility of their large-scale introduction and planting at high latitudes.
Nitrogen is one of three nutritional elements required by plants and is a component of proteins,nucleic acids,enzymes,chlorophyll,and numerous endogenous hormones or their precursors (Dong et al.2015;Liu et al.2018).Therefore,the nitrogen status of plants directly affects the photosynthetic rate,growth,metabolism,and ultimately,yield (Kocheva et al.2020;Maswada et al.2021;Mu and Chen 2021;Wu et al.2021).Nitrogen has a significant effect on bamboo yields,and it is well-documented that,of all nutrients,nitrogen demand and uptake are highest during the growing period (Su et al.2013,2019;Yang et al.2022).Compared with a traditional single application of nitrogen,a combined application of organic and chemical fertilizers can prevent or reverse soil acidification caused by chemical nitrogen fertilizer alone (Cai et al.2014),significantly increase soil organic carbon and total nitrogen,and reduce soil carbon and nitrogen loss after fertilization.This combined application is an effective method for maintaining soil physical and chemical environmental stability and nitrogen balance (Su et al.2017),and is among the leading technical measures for achieving fertilizer reduction (Wang et al.2021).Therefore,reasonable fertilization of bamboo forests can supplement the nutrient elements removed by digging and removing bamboo shoots,directly or indirectly improving the soil quality and maintaining and improving stand productivity (Tu et al.2013;Zhang et al.2020a).
The application of nitrogen fertilizer has compensatory effects on the growth of aboveground parts of crops and their ability to resist a variety of environmental factors such as drought,high temperatures,and shade (Walter et al.2016;Tang et al.2019;Cong et al.2020;Yang et al.2021).The input of nitrogen fertilizer can affect photosynthesis,carbon and nitrogen metabolism,and biomass allocation of plants under adverse light environments.Increasing nitrogen applications can promote carbon metabolism,as photosynthesis affects nitrogen metabolism;however,they must be combined with high light exposure.High nitrogen levels increase nitrogen metabolism,aggravate the degree of stand stagnation,are not conducive to carbon assimilation,and cause an imbalance between nitrogen and carbon metabolism (Walter et al.2016;Song et al.2017;Soto et al.2017;Edward 2020).Therefore,only when light and nitrogen are balanced supply can they have the best interactive effect for optimal growth (Wang et al.2012).Therefore,we hypothesized that when short-day plants are transplanted to a long-day environment and suffer injury,nitrogen application may ameliorate the damage to some extent.
The shoots ofDendrocalamopsis oldhamiMunro,often called “fruit shoots” are rich in nutrients,sweet,and tasty,and are of considerable importance in resource developmentand utilization.It is a typical short-day species native to tropical and subtropical areas in Fujian province,and other regions of China.In this study,D.oldhamiwas transplanted to a long-day area in Beijing,northern China,and the differences in leaf growth,photosynthetic fluorescence,carbon and nitrogen metabolism,and yield between the two light treatments were explored using shading to understand the compensation effect of the combined application of nitrogen and organic fertilizer on bamboo under light stress.Morphological,physiological,and biochemical traits were used to evaluate its phenotypic plasticity to light,fertilizer,and their interaction.
The experiment was conducted at the Xiaotangshan solar greenhouse in Beijing (116°23＇ N,40°22＇ E).The average annual temperature outside the greenhouse is 11.8 °C,the lowest is -16.8 °C and the average annual precipitation is 550 mm.The average temperature in the greenhouse was 17.4 °C and the lowest 0.7 °C which generally occurs 06:00-08:00 am in December or January,meeting the lowtemperature tolerance ofD.oldhamiabove -4 °C.The solar greenhouse for the traditional arch structure covers is 400 m2(east-west length of 50 m,north-south width of 8 m,and 3.5 m high back wall).
In April 2017,excellent provenances were introduced from Fujian Province,the origin ofD.oldhami.New bamboo shoots were unearthed in August of the same year and all kept except for diseased ones.In 2018,the dead parent bamboo was removed,the healthy new shoots of the current year used for replanting,and 2-4 shoots of the following year retained.After two years of survival management and protection,the average planting density in the solar greenhouse in March 2019 was 38 stems/ha,the clump spacing was 1.5-2 m,and each had 4-7 stems.The pH and nutrient contents of the greenhouse soil are shown in Table 1.Temperature,lighting conditions,soil temperature,and soil humidity during the experiment are shown in Tables 2 and 3.An automatic meteorological station (FRT-X06A,Fuotong Technology (Beijing) Co.,Ltd.,China) was used for monitoring.
Table 1 Soil pH and nutrient contents in the greenhouse
Table 2 Overview of air temperature,soil temperature,and humidity in the greenhouse from April to August
Table 3 Monthly mean daily light hours and photosynthetic radiation channels from April to August
Experimental design
The greenhouse comprises three parts:the south,east,and west walls,and daylight surfaces.There is a distinct difference in the proportion of solar radiation received at different locations in the greenhouse.Therefore,the distribution of the bamboo clumps was divided into three rows:south,middle,and north.Two rows in the north and south were used as the research materials.The difference in canopy light intensity between clumps was measured using a portable radiation measuring device (Spectrum-3415F,USA).The light intensity of the bamboo canopy (180 m) in the southern and northern rows was measured at 10 measuring points in each row for three time periods (06:00-07:00;13:00-14:00;17:00-18:00),five times in each period,and seven consecutive sunny days were selected to measure the light intensity every month.Changes in bamboo canopy light intensity in the southern and northern rows from April to September are shown in Fig.1.
Fig.1 Diurnal variation of light intensity in the south and north of the two rows from April to August.Ls is long-day treatment,Ln is short-day treatment
The southern bamboo receives sufficient light and is set to a long-day treatment (Ls),whereas the northern bamboo is more shaded and is set to a short-day treatment (Ln).Three inorganic nitrogen fertilizer levels were applied for each light treatment (Zhu 2017):no nitrogen (N0:0 g stalk-1),low nitrogen (N1:108 g stalk-1),and high nitrogen (N2:216 g stalk-1).In addition,no organic fertilizer was used as a control (NFN),and the three nitrogen levels were applied in combination with organic fertilizer (OFN:2.8 kg stalk-1) (Table 4).Organic fertilizer (at least 40% organic matter) and urea (46% N) were applied at the end of March 2019.Organic fertilizer was applied in one go.Urea was applied in two equal doses,with the second in June.During the experiment,watering occurred 3-4 times/week.To stop nutrient flow between treatments,50 cm ditches were dug for each treatment boundary,and plastic sheets buried for isolation.The age structure was two young biennial culms and three young annual culms.Three clumps were selected from each treatment as repetitions.
Table 4 Description of experimental treatments
Leaves and shoot sampling and analyses
Shoots were examined once a week after the experiment started and new shoot quantities recorded during the growth period from May to November.Five healthy shoots(approximately 10 cm long) were excavated from each treatment at the peak of the bamboo shooting period (August).
Diameter at breast height (DBH) of the bamboo shoots was measured with electronic Vernier calipers.Individual shoots were weighed and recorded.The yield was calculated as new shoot quantity in the whole growing period × average individual weight of shoots.
Leaf samples were collected 117 and 162 days after the beginning of the experiment and leaves from approximately 180cm canopy height were measured using the following indices:
Leaf area and dry mass
After 117 days,6-12 intact leaves without disease or insect damage were selected from each treatment on the main branch,and leaf area measured using a leaf area meter (Yaxin-1242,Beijing Yaxin Liyi Technology Co.,Ltd.,Beijing,China).All leaves of annual bamboo culms were collected and dried at 75 °C for 96 h until constant and dry weights were recorded.
Chlorophyll,fluorescence parameters,and Chl afluorescence transients OJIP
After 117 days,another 6-12 intact leaves free of disease or insect damage were collected on the main branch.Three points were measured on each leaf and the average value recorded.A hand-held chlorophyll meter CL-01 (Handy PEA fluorometer,Hansatech Instruments Ltd.,King’s Lynn,Norfolk,UK) was used to measure chlorophyll content (Chl) in the field.An additional three points were measured in each leaf,and the average recorded.Chl a fluorescence transient OJIP curves and fluorescence parameters were measured with a plant efficiency analyzer (Handy PEA fluorometer,Hansatech Instruments Ltd.,King’s Lynn,Norfolk,UK) as described by Strasser et al.(2004).Fluorescence parameters involved in this paper are as follows:φPoreflects the maximum photochemical efficiency after dark adaptation.PIABSis a performance index based on absorption.Some studies have shown that PIABSis more sensitive to stress thanφPoand better reflect the effect of stress on the photosynthetic apparatus (Appenroth et al.2001;Heerden et al.2003,2004); DIo/CSmis the heat dissipation per unit area when t= tfm,and ETo/CSmthe quantum yield of electron transport per unit area when t= tfm.
Enzymes and products related to carbon and nitrogen metabolism
After 117 days,several intact leaves on the main branch were selected,wrapped in tinfoil,placed in a black Ziplock bag,and stored at -80 °C for testing.Nitrate reductase (NR),glutamine synthetase (GS),amylase (AMY) and sucrose synthase (SS),soluble sugars,and starch contents were determined using an assay kit (Shanghai Jianglai Biotechnology Co.,LTD.,China).The leaf samples were cleaned at 105 °C and dried at 85 °C to constant weight.Total nitrogen and organic carbon content were determined by the Kjeldahl method and the K2C r2O7-H2S O4volumetric method (Bao 2000).
Content of plant growth hormones
After 162 days,several intact leaves free of disease or insect pests on main branches were selected,wrapped in tinfoil,placed in black Ziplock bags,and stored at -80 °C for testing.IAA,GA3,and ABA levels were determined by highperformance liquid chromatography (HPLC,Waters2695).
Statistical analysis
The data were analyzed by ANOVA,and comparisons between treatment means were carried out using theFtest at 5% probability.Generalized linear model (GML) analysis evaluated the effects of light factor,N application,organic fertilizer application,and their interaction.Statistical analyses were performed using SPSS16.0.Figures were prepared using Microsoft Excel 2013.
Phenotypic plasticity index
The phenotypic plasticity index (PI) from 0 to 1 was calculated for each variable as the difference between the maximum and minimum means divided by the maximum mean (per trait per treatment combination).
Leaf growth
Long-day treatment bamboo had lower leaf areas,IAA,and GA3,and higher leaf dry mass and ABA than those in the shade treatment,regardless of fertilization application (Figs.2,3).Fertilization significantly affected leaf area,IAA,GA3,and ABA for both light treatments,but only affected leaf dry mass in the Ls treatment (P＜ 0.01).Compared with no fertilization,N alone had no significant effect on leaf area,dry mass,and ABA content of the Ls treatment leaves,whereas IAA and GA3levels increased with increasing nitrogen levels (Figs.2,3).Compared with N-application alone,IAA and GA3contents increased,whereas ABA decreased when inorganic N was combined with organic fertilizer.However,leaf area and dry mass increased only at the N1and N2levels (Figs.2,3).
Fig.2 Differences in leaf area and dry mass among different light treatments and fertilization levels.Means followed by the same letter do not differ significantly between fertilization levels within the same light treatment.A significant difference at the fertilization levels is indicated by different capital letters for Ls and by different lowercase letters for Ln (F test,P ＜ 0.05).Vertical bars show ± std.dev.Ls is long-day treatment,Ln is short-day treatment.OFN is combined application of inorganic nitrogen fertilizer and organic fertilizer,and NFN is single application of inorganic nitrogen fertilizer.N0,N1 and N2 refer to nitrogen application levels,which were no nitrogen,low nitrogen,and high nitrogen,respectively
Fig.3 Differences in indole acetic acid (IAA),gibberellic acid (GA3),and abscisic acid (ABA) contents among different light treatments and six fertilization levels.Statistics as in Fig.2
Fluorescence rise kinetics OJIP
The fluorescence rise kinetics of the Ln treatment exhibited a typical O-J-I-P shape (Fig.4).Light stress induced several changes.Compared with Ln,the Ls treatment resulted in a substantial increase in the J-step relative to the other components of chlorophyll fluorescence,which was considered indicative of an accumulation of reduced QAbecause of a slowdown in electron transport beyond QA(Strasser and Govindjee 1992;Chen et al.2007).Fertilizer application significantly affected the value of the J-step under Ls,and the OFN1treatment was the largest followed by NFN0＞ OFN0＞ NFN1＞ OFN2＞ NFN2.
Fig.4 Chlorophyll a fluorescence induction kinetics of Dendrocalamopsis oldhami leaves treated with different light intensities and six fertilization levels
Chlorophyll and fluorescence parameters
Regardless of fertilization,Ls significantly decreased leaf chlorophyll,ETo/CSmratio,φPo,and PIABS,whereas DIo/CSmincreased (Fig.5).Fertilization significantly affected Chl,ETo/CSm,φPo,and PIABSin both light treatments,but only DIo/CSmin the Ls treatment (P＜ 0.01 or 0.01 ＜P＜ 0.05).In addition,compared with NFN0,Chl,ETo/CSm,φPo,and PIABSincreased with increasing nitrogen,whereas DIo/CSmdecreased (Fig.5).Compared with NF,Chl,ETo/CSm,φPo,and PIABSdecreased during OFN1treatment,but increased or had similar levels under OFN0and OFN2treatments (Fig.5).
Fig.5 Chlorophyll contents and fluorescence parameters between light treatments and among six fertilization levels;statistics as in Fig.2
Carbon and nitrogen metabolism-related enzymes and products
Leaf nitrate reductase (NR),glutamine synthesis (GS),amylase (AMY),and sucrose synthesis (SS) in Ls were higher than in the Ln treatment regardless of fertilization (Fig.6) However,fertilization significantly affected NR,GS,AMY,and SS activities under both light regimes (P＜ 0.01).Compared with no fertilization (NFN0),nitrogen increased NR,GS,AMY,and SS which increased with increasing nitrogen (Fig.6).NR,GS,AMY,and SS increased during OFN1treatment compared to NF but decreased after OFN2treatment (Fig.6).
Long-day plants had higher leaf total N and lower organic carbon,soluble sugars,and starch compared with those in the shade treatment,regardless of fertilization (Fig.6).Fertilization significantly affected leaf total N,organic carbon,soluble sugar,and starch in both light treatments (P＜ 0.01).Compared with NFN0,the application of nitrogen alone increased organic carbon at the N2level;however,it decreased at the N1level.Moreover,total N increased with increasing nitrogen,whereas soluble sugar and starch decreased (Fig.6).Compared with NF,the combined application of organic fertilizer increased soluble sugar and starch content at the N1and N2levels (Fig.6).
Fig.6 Differences in enzymes activities related to carbon and nitrogen metabolism (NR:nitrate reductase,GS:glutamine synthase,AMY:amylase,and SS:sucrose synthase),soluble sugar,starch,total nitrogen,organic carbon content among different light treatments and six fertilization levels.Statistics as in Fig.2
Yield
Regardless of fertilization application,DBH and single shoot weight decreased under short-day conditions while shoot quantity and total yield increased (Fig.7).Fertilization significantly affected DBH,single shoot weight,quantity,and total yield under both light treatments (P＜ 0.01).Compared with NFN0,applying nitrogen alone had no significant effect on DBH and single shoot weight under short-day treatment while quantity and total yield increased with increasing nitrogen (Fig.7).Compared with NF,the combined application of organic and chemical fertilizers increased DBH,but there was no difference in nitrogen levels (OFN0,OFN1,and OFN2) .Additionally,the combined fertilizers increased single shoot weight,quantity,and total yield,reaching maximum values in the OFN2,OFN1,and OFN2treatments,respectively.
Fig.7 Differences in DBH,single shoot weight,shoot quantity,total yield among different light treatments and six fertilization levels.Statistics as in Fig.2
Proportion of explained variance and phenotypic plasticity
All variables analyzed were significantly affected by light,and few variables (including E To/CSm) were not signif ciantly affected by fertilizer application (Table 5).All variables (except IAA,AMY,soluble sugar,and total yield) responded more to light than fertilizer factors.However,all variables displayed weak or negligible responses to the interaction of light and fertilizer factors (Table 5).
The average PI relative to the nitrogen fertilizer factor (PINitrogen) for all variables was 20.9% lower than that relative to the light factor (PILight) (PINitrogen=0.32; PILight=0.41),while the average PI relative to the organic fertilizer factors (PIOrganic) was 38.0% lower than that relative to the light factor (PILight) (PIOrganic=0.26; PILight=0.41) (Table 5).Average PI for all trait groups (growth,photosynthetic fluorescent,carbon and nitrogen metabolism and yield) relative to the fertilizer factor was lower than that relative to the light factor (PINitrogen=0.19-0.45; PIOrganic=0.10-0.39; PILight=0.21-0.57) (Table 5).
Leaf growth
Leaf growth rate reflects the ability of plants to obtain and utilize light energy,water,and nutrients.Bamboo morphological traits and growth respond significantly to heterogeneous habitats (Zheng et al.2021).For example,different canopy environments have a significant effect on the morphological traits of bamboo growing in the understory (Song et al.2007;Xie et al.2010;Cristian and Ernesto 2011;Yang et al.2014).Different canopy light environments and fertilization treatments have different effects on leaf growth and functional traits (Xu et al.2021;Liu et al.2022).D.oldhamileaf area,dry mass accumulation,and growth hormones (ABA,GA3) to light factor (FL) were the largest and much higher than the response to organic fertilizer (FO) and nitrogen (FN) (Table 5).Under shade conditions,plants resorted to a series of shade-tolerant mechanisms,and leaf growth is one of the indicators that most directly reflects their growth status.For example,an increase in leaf area (Yuan et al.2021) and a decrease in leaf dry mass accumulation affect the endogenous hormone levels of leaves (Kurepin et al.2007;Zhang et al.2021),while long-day or strong light treatment has the opposite effect and accelerates senescence dormancy of leaves (Qiao et al.2008;Liu et al.2019;Flores et al.2021;Wang et al.2022).During long-day treatment (Ls),leaf dry mass accumulation was greater,but leaf area,IAA,and GA3decreased significantly,whereas ABA increased significantly (Figs.2,3).Nitrogen is one of the elements that is a component of plant hormones,and it significantly affects the content of source hormones in plants.Zeatin,GA,and IAA are growth-promoting hormones,and nitrogen promotes their accumulation.ABA,a growth-inhibiting hormone,initiates and promotes plant aging or dormancy (Wang et al.1994;Sebastián et al.2019).The effects of fertilization on growth hormones,leaf area and dry mass ofD.oldhamidiffered under the different light conditions.IAA and GA3levels under Ln reached the maximum value at the NFN2level for weak light,and in the Ls treatment,they maximized at the OFN2level for long-day treatment (Figs.2,3).This is related to the sensitivity of plants to nitrogen under different light conditions,and the sensitivity of plants to nitrogen under weak light conditions is lower.Compared with chemical fertilizer alone,combined with organic fertilizer had benef icial effects on soil nutrient release,plant uptake,and yield (Wang et al.2016;Choudhary et al.2020).This study also showed that nitrogen application alone could not meet the nutrient requirements ofD.oldhamiunder long-day treatment,and the combined application of organic fertilizer could increase leaf area and dry mass accumulation and slow leaf senescence;the higher the nitrogen application level,the more pronounced the effect.
Chlorophyll content and fluorescence parameters
Chlorophyll fluorescence parameters reflect the absorption,transfer,and dissipation of light energy,and the effects of stress on photosynthesis can also be reflected by chlorophyll fluorescence (Baker and Eva 2004;Westhuizen et al.2020).The chlorophyll and fluorescence parameters ofD.oldhamihad the greatest response to light,followed by nitrogen and organic fertilizers (FL＞FN＞FO) (Table 5).The most direct physiological response to changes in light condition is a change in photosynthetic capacity.The relative chlorophyll content ofD.oldhamileaves was higher under shade (Fig.5),which is an adaptation to a low-light environment.This is consistent with the results of studies on changes in chlorophyll content of shade plants (Yan et al.2016;Fan et al.2019).With an increase in nitrogen application,thechlorophyll content ofD.oldhamiincreased under the weak light treatment,but the opposite was found under the long-day treatment at the OFN1level (Fig.5).This may be because the growth and metabolism rate of the species was faster under sufficient light,especially in the shooting stage (Dong 1995),and the production of new leaves and the total number of leaves were high.With the increase in dry mass and leaf area,dysplasia of leaf palisade tissue also slows down the growth of mature leaves,resulting in a relative decrease in chlorophyll content in individual leaves under limited nutrient supply (Xu et al.2013).Physiological changes in plants,such as senescence (Dai et al.2004;Lu and Zhang 2010) or light stress (Kalaji et al.2012),can directly or indirectly affect the function of photosystem II (PSII).When environmental conditions change,chlorophyll fluorescence changes reflect the influence of environmental factors on plants to a certain extent (Krause and Weis 1991;Maxwell and Johnson 2000;Jiang et al.2003).In the mechanism of photosynthesis,while capturing light energy,some energy is dissipated in the form of heat and fluorescence,which is a competitive relationship among the three,and the change of anyone will lead to the change of the other two.In general,leafφPois relatively stable;however,under stress and inhibition,this parameter is significantly reduced (Maxwell and Johnson 2000).For the Ls treatment,leafφPoand PIABSwere reduced under the long-day treatment,indicating that light suppression ofD.oldhamiwas caused by longday conditions,and the function of PSII was destroyed.The substantial increase in the J-step in the OJIP curve conf irms this result (Figs.4 and 5).In addition,the Ls treatment also reduced ETo/CSmand increased DIo/CSm(Fig.5),indicating that photoinhibition ofD.oldhamiwas due to the obstruction of photosynthetic electron transfer in the leaves by long-day treatment and increased heat dissipation.Shangguan et al.(2000) found that the application of nitrogen fertilizers under environmental stress can increaseφPo.Ciompi et al.(1996) suggested thatφPois not affected by nitrogen def iciency,i.e.,photosynthetic electron transport is independent of nitrogen content.φPo,ETo/CSm,and PIABSincreased with an increase in nitrogen,but DIo/CSmresults were the opposite (Fig.5),indicating that nitrogen application could alleviate photoinhibition ofD.oldhamileaves.However,the fluorescence parameters showed opposite results for the OFN1treatment,consistent with the change in chlorophyll content.
Table 5 Proportion of explained variance by light,organic fertilizer,and nitrogen fertilizer and their interactions (generalized linear models),and the index of phenotypic plasticity of morphological and physio-biochemical traits of Dendrocalamopsis oldhami bamboo
Carbon and nitrogen metabolism
Carbon and nitrogen metabolism-related enzymes (NR,GS,SS,AMY) and products (starch,total N,organic carbon) ofD.oldhamileaves showed the greatest response to light levels and the least response to organic fertilizer (FL,FN＞FO) (Table 5).The relationship between carbon and nitrogen metabolism is very close.Under Ls treatment,NR,GS,AMY,and SS activity and nitrogen assimilation and carbon metabolism were higher (Fig.6),consistent with previous studies (Merlo et al.1993,2010).The response ofD.oldhamileaves to nitrogen was similar for the two light treatments;however,there were differences.Under the long-day treatment,D.oldhamihad a higher nitrogen assimilation at higher nitrogen levels.The maximum nitrogen assimilation was observed in OFN1(Fig.6).When the supply of nutrients continued to increase,nitrogen metabolism was adversely affected.Mooney et al.(1995) suggested that plants grown in low-light environments have fewer unstructured carbohydrate components (Poorter et al.2006).In addition,plants grow relatively slowly under adverse conditions,and their demand for photosynthates is reduced,resulting in the accumulation of large amounts of soluble sugars and starch in the leaves (Munns 2010).The results of this study are consistent with these studies for the contents of organic carbon,soluble sugars,and starch under the two light intensity conditions.Photosynthate C3compounds are used for NH3assimilation into amino acids and proteins and for the synthesis of soluble sugars and starches.During conditions of low nitrogen or lack of nitrogen,nitrogen assimilation was weak,and C3compounds were used to synthesize soluble sugars and starch.Thus,at the N0level,organic carbon,soluble sugars,and starch levels were higher (Fig.6).As nitrogen and nitrogen assimilation increased,C3compounds flowed incrementally to nitrogen metabolism,soluble sugars and starch decreased and did not change at the N2level (Fig.6).This result is consistent with the influence of two light factors and five nitrogen levels on carbon and nitrogen metabolism enzyme activities and products in the study by Guan et al.(2000).Leaf nitrogen content is determined by photosynthetic physiology and nutrient transport.Numerous studies have shown that leaf total nitrogen content also increases with increased light intensity.There has always been a positive correlation between high leaf nitrogen levels and sufficient light (Zedaker 1986;Lambers et al.1998;Rozendaal et al.2006;Niinemets 2010;Goodman et al.2014),but Yoshimura (2010) found that low light intensity did not limit the distribution of leaf nitrogen in some tree species.Long-day treatment was more benef ciial to accumulating total nitrogen inD.oldhamileaves than under shade.The combined application of nitrogen and organic fertilizers under long-day treatment was benef icial to total nitrogen accumulation in leaves,whereas the combined application of low nitrogen (NFN1and OFN1) under shading was benef ciial to total nitrogen accumulation (Fig.6).The effect of fertilization on leaf nitrogen accumulation was consistent with the activity of NR and GS or was related to nitrogen assimilation ability.In addition,the studies of Warren et al.(2003) and Tissue et al.(2010) showed that changes in soil nitrogen levels had a direct impact on leaf nitrogen distribution,i.e.,nitrogen in leaves increased with an increase in soil nitrogen.Therefore,changes in nitrogen content inD.oldhamileaves after fertilization may be related to soil nitrogen content.
Yield
The yield of bamboo culms was determined by diameter at breast height,single shoot weight,quantity,and total yield.The results show that DBH and single shoot weight responded the most to light,followed the response to organic fertilizer and nitrogen (FL＞FO＞FN) .The light intensity also influenced the quantity,followed by the effects of nitrogen and organic fertilizers (FL＞FN＞FO) .Total yield was affected by nitrogen fertilizer,followed by the effects of organic fertilizer and light intensity (FN＞FO＞FL) (Table 5).Compared with the shade environment,photosynthesis was more vigorous under long-day treatment during the growing period.Nitrogen application,especially combined application of organic fertilizer and nitrogen fertilizer,promoted carbon and nitrogen metabolism.(Figs.5,6).This promoted the differentiation of shoot buds,and bamboo shooting increased significantly,as did total leaf dry mass and leaf area (Figs.2,7).As a result,the distribution of carbon and nitrogen compounds in the shoots as well as the single shoot weight and DBH decreased.Organic fertilizer increased DBH,individual weight,yield,and the number of shoots under long-day treatment.Nitrogen increased the number of shoots,and the combined nitrogen and organic fertilizer increased shoot weight but had no significant effect on DBH (Fig.7).In general,fertilization increased the yield of bamboo,and nitrogen fertilizer combined with organic fertilizer increased yields more significantly.High nitrogen applications combined with organic fertilizer (OFN2) resulted in the maximum yield,but evidently the contribution of fertilizer amount to yield under long-day treatment was higher.
Phenotypic plasticity
Morphological plasticity is an important feature of plants to adapt to changing environments,especially light (Poorter et al.2012).This study showed that the ability ofD.oldhamito alter photosynthetic fluorescence and physiological metabolic characteristics was significantly lower than its ability to alter organ growth morphology or dry mass (Table 5).Light intensity has an important influence on various physiological activities and plants adapt to changes in light conditions through morphological and physiological plasticity (Givnish 1988).In our study,the ability ofD.oldhamito alter morphological and physio-biochemical traits and yields in response to variations in light,nitrogen,and organic fertilizer applications may be translated into high phenotypic plasticity,particularly in response to light.Studies have shown that fertilization positively affects the morphology,physio-biochemical traits,and yield of bamboo plants (Kim et al.2018;Su et al.2019;Zhang et al.2020b).Fertilization significantly improve the photoplasticity ofD.oldhamiand contributed to the morphological and physio-biochemical traits and yield ofD.oldhamiat time of shooting.However,the responses of these traits to light-nitrogen interactions and light-organic fertilizer were weak or negligible (Table 5).Because of the complexity of plant plasticity,short-term studies cannot provide comprehensive information;therefore,long-term research is needed to understand the plasticity ofD.oldhamito exotic introduction or under rapid climate change,and to predict and manage the potential impact of climate change on introducedD.oldhami.
Light intensity dominated leaf growth (except levels of IAA),photosynthetic fluorescence,carbon,and nitrogen metabolism (except amylase and soluble sugar),and yield (DBH,individual shoot weight,and quantity) ofD.oldhamicultivated in a greenhouse.However,nitrogen levels were the dominant factor affecting total yield.D.oldhamileaves were subjected to photoinhibition under Ls (long-day) conditions,which could be avoided by shading (Ln).However,Ls accelerated carbon and nitrogen metabolism rates compared with Ln conditions.Although senescence or dormancy increased,the leaves accumulated more total nitrogen owing to rapid growth.Organic carbon,dry mass,and other photosynthetic products are continuously transported underground,promoting the differentiation of shoots and buds and increasing the number of shoots and the total output of the stands.However,as DBH and individual weight of bamboo shoots are generally small,the quality of individual shoots is reduced.Fertilization can compensate for photoinhibition and delay the premature senescence of leaves (N2is the highest),promote carbon and nitrogen metabolism (OFN1is the highest),increase DBH and individual shoot weight (application of organic fertilizer is more efficient),and increase the quantity (OFN1is preferred) and total yield of bamboo shoots (OFN1is preferred).
AcknowledgementsWe would like to thank the Fujian Academy of Forestry for providing quality bamboo material ofD.oldhamiand Chen Guobiao for his guidance and help with the introduction and planting work.