Effects of light intensity and nitrogen treatments on photosynthetic characteristics of <i>Stewartia sinensis</i> seedlings

ZHANG Tingyu; ZHANG Mingru; LI Qingxiang; ZHAO Nannan; WAN Qi; CAI Yihang

doi:10.11833/j.issn.2095-0756.20210771

Volume 39 Issue 6

Nov. 2022

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ZHANG Tingyu, ZHANG Mingru, LI Qingxiang, ZHAO Nannan, WAN Qi, CAI Yihang. Effects of light intensity and nitrogen treatments on photosynthetic characteristics of Stewartia sinensis seedlings[J]. Journal of Zhejiang A&F University, 2022, 39(6): 1247-1256. doi: 10.11833/j.issn.2095-0756.20210771

Citation:

ZHANG Tingyu, ZHANG Mingru, LI Qingxiang, ZHAO Nannan, WAN Qi, CAI Yihang. Effects of light intensity and nitrogen treatments on photosynthetic characteristics of Stewartia sinensis seedlings[J]. Journal of Zhejiang A&F University, 2022, 39(6): 1247-1256. doi: 10.11833/j.issn.2095-0756.20210771

Effects of light intensity and nitrogen treatments on photosynthetic characteristics of Stewartia sinensis seedlings

doi: 10.11833/j.issn.2095-0756.20210771

College of Landscape and Architecture, Zhejiang A&F University, Hangzhou 311300, Zhejiang, China

Received Date: 2021-11-26
Accepted Date: 2022-03-29
Rev Recd Date: 2022-03-29

Available Online: 2022-11-21

Publish Date: 2022-12-20

Abstract

Objective This study, with an investigation of the differences in the hotosynthetic hysiological and ecological adaptation of Stewartia sinensis seedlings to different light intensity and soil nitrogen content, is aimed to select combinations of light intensity and nitrogen treatment that are conducive to the cultivation of S. sinensis seedlings. Method With S. sinensis seedlings used as test materials, efforts were made to set four light intensity gradients, namely full light (L₀), light light (44.79±0.51)% (L₁), moderate light (19.60±0.23)% (L₂), high light (7.25±0.10)% (L₃), and three nitrogen application rates, i.e, low nitrogen 0.2 g·kg⁻¹ (N₁), high nitrogen 0.6 g·kg⁻¹ (N₂) and no nitrogen application (N₀) before the light response process, photosynthetic pigment mass fraction and chlorophyll fluorescence parameters were recorded and measured upon 90 d treatment. Result Light intensity, nitrogen and their interaction displayed significant effects on photosynthetic pigment and chlorophyll fluorescence of S. sinensis seedlings (P＜0.05). Chl a/b values of S. sinensis seedlings varied from 2.0 to 2.5, the L_SP of S. sinensis seedlings varied from 571.3 to 931.4 μmol·m⁻²·s⁻¹ whereas L_CP varied from 4.8 to 26.0 μmol·m⁻²·s⁻¹. With the increase of shading degree, the Chl, Car, F_o, F_m, A_QY of S. sinensis seedlings tended to increase, the Chl a/b and Car/Chl tended to decrease while the P_max, F_v/F_o, F_v/F_mand P_IABS increased first and then decreased. With the increase of nitrogen application rate, the P_IABSof S. sinensis seedlings tended to increase, the Chl a/b, F_o and F_m tended to decrease and the Chl, Car, P_max and A_QY increased first and then decreased; With L₁ and L₂ light intensity, P_n, G_s and T_r of S. sinensis seedlings were higher and C_i was lower and the application of low nitrogen significantly promoted P_n, G_s and T_r. The P_max of L₁N₁ increased by 45.21% compared with L₁N₀ with the P_n and P_max of L₂N₁ being the highest. Conclusion S. sinensis seedlings had the best photosynthetic capacity when grown with moderate shading and low nitrogen treatment whereas photosynthetic inhibition occurred with full light or high nitrogen treatment. [Ch, 1 fig. 6 tab. 34 ref.]
- Stewartia sinensis,
- light intensity,
- nitrogen,
- gouble factor control experiment,
- light response,
- photosynthetic pigment,
- chlorophyll fluorescence

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紫茎Stewartia sinensis属于山茶科Theaceae紫茎属Stewartia的落叶小乔木或乔木树种，为第三纪孑遗植物，分布广泛，在浙江、安徽、福建等12个省均有生长，垂直生长于海拔550~2 200 m的常绿或混交阔叶林^[1-2]。紫茎高可达20 m，枝叶繁茂，树形优美，树干片状剥落呈多种色彩，白花黄蕊清秀淡雅，极具观赏性，宜栽植于庭院。紫茎属植物间断分布于东亚至北美大陆^[2]，对于研究东亚至北美植物区系及古植物地理具有重要的科学意义^[3]。由于自然更新能力差和生长缓慢，加之生境破坏、人类活动等影响，野生种群数量不断减少^[4]，紫茎在种间竞争中处于劣势，已被列入国家三级珍稀濒危植物^[5]和浙江省重点保护野生植物^[3]。幼苗是植物生活史最为脆弱的环节之一，木本植物的天然更新以及幼苗幼树的建成，与森林群落内部至群落外缘光强的异质性以及土壤肥沃与否存在密切的生态关系^[6-7]。通过对野外紫茎分布地段调查发现：群落内紫茎的幼苗和幼树数量极为稀少，但在群落外缘的路旁生长定居一定数量的幼苗幼树。推测春夏期间，常绿落叶阔叶森林群落高度郁闭状况必然影响紫茎幼苗的生长光强，从而影响其定居过程。光合作用是植物幼苗其他生命活动的基础，光合作用与氮素水平存在着直接或间接的关系^[8-9]。森林群落植被密度高，濒危植物自身的生态适应性相较于其他植物弱，有限的氮素成为限制濒危植物生长的因素之一^[10]。森林群落内的紫茎幼苗因光照和养分的竞争限制作用，无法占据适宜的生态位而生长不良，甚至出现幼苗枯死现象。已有紫茎的研究成果主要涉及地理区系分布^[1-2]、人工繁殖技术^[11-12]、群落特征^[13-14]、药用价值^[15]等方面，有关幼苗生长适应性与环境的生态关系尚未展开研究，因此迫切需要研究光照强度和氮素水平的耦合效应对紫茎幼苗光合特性的影响，以期在濒危植物紫茎的保护和扩繁过程，为紫茎幼苗阶段的光照和氮素调控提供理论依据。为此，本研究选择紫茎幼苗为材料，设置光强和氮素双因素控制试验，模拟常绿落叶阔叶林中到林缘光环境和土壤肥力的梯度变化，测定不同处理组合紫茎幼苗的光响应、光合色素以及叶绿素荧光，筛选适宜紫茎幼苗生长的光强氮素组合处理。

1. 材料与方法

1.1. 材料与试验设计

2021年3月从清凉峰国家级自然保护区苗圃(30°09′28″N, 118°86′37″E)选取长势基本一致的1年生紫茎实生幼苗[平均苗高(32.41±4.26) cm，地径(0.49±0.06) cm]移至浙江农林大学东湖校区试验地。试验地海拔为42 m，全年降水量为1 628.6 mm，年平均气温为16.4 ℃，全年无霜期约237 d。幼苗栽植于口径24 cm、高20 cm、底径14 cm的花盆内，每盆1株。盆栽基质为当地黄壤[全氮(1.53±0.01) g·kg⁻¹，碱解氮(185.0±10.5) mg·kg⁻¹，有效磷(8.7±0.1) mg·kg⁻¹，速效钾(278.0±14.9) mg·kg⁻¹，有机质(31.6±0.1)%，pH (土水质量比为1.0∶2.5) 5.07±0.01]。盆栽苗放置于2层遮光大棚内，缓苗1个月后，将60盆紫茎随机分成12组，每组5盆。依据紫茎所在常绿或混交阔叶林的郁闭度调查结果，为量化空旷地、疏林、中等郁闭和密林的光照强度，设置3个遮光处理和1个全光对照处理。遮光大棚采用黑色尼龙遮光网覆盖并适当开洞模拟林窗，大棚长30.0 m×宽11.5 m×高3.2 m，通风状况良好，透光率分别为轻度遮光(44.79±0.51)%(L₁)、中度遮光(19.60±0.23)%(L₂)、高度遮光(7.25±0.10)%(L₃)和全光100.00%(L₀)，透光率为在3个晴天正午所测的光量子通量密度(PPFD)，取均值。参考尹丽等^[10]对麻疯树Jatropha curcas幼苗的施氮量，在每个遮光处理下设置3个氮素处理，分别为低氮0.2 g· kg⁻¹ (N₁)、高氮0.6 g·kg⁻¹ (N₂)和不施氮对照 (N₀)，所用氮肥为硫酸铵[(NH₄)₂SO₄]，施氮以盆中干土计，即1 kg土分别施用约1 g (N₁)和3 g (N₂)硫酸铵，采用土表施氮法用适量水溶解稀释后分3次施用，每隔10 d施肥1次，施氮后3 d内不浇水以防流失。处理90 d后测定各项指标。

1.2. 测定指标与方法

1.2.1. 光响应进程测定

使用Li-6800光合仪，于2021年5月25日至6月4日(测定期间为晴天) 8:00—11:30，使用Li-6800光合仪测定紫茎的光合参数。对待测功能叶片用1200 μmol·m⁻²·s⁻¹的光强进行20 min光诱导，设置光合有效辐射(P_AR)梯度为2 000、1 500、1 200、1 000、800、500、300、100、50、30、10和0 μmol·m⁻²·s⁻¹。叶室控制温度为(25.0±0.5) ℃，相对湿度为(40±5)%，叶室二氧化碳(CO₂)摩尔分数为400 μmol·mol⁻¹，气体流速为500 μmol·s⁻¹。每个处理随机取3株，测定净光合速率(P_n)、气孔导度(G_s)、蒸腾速率(T_r)和胞间CO₂摩尔分数(C_i)等参数；用直角双曲线修正模型^[16]拟合光响应模型，根据模型计算最大净光合速率(P_max)、光饱和点(L_SP)、光补偿点(L_CP)、表观量子效率(A_QY)和暗呼吸速率(R_d)。

1.2.2. 光合色素质量分数测定

采用乙醇浸提法测定光合色素质量分数^[17]。选取生长良好且成熟的紫茎叶片，剪成2 mm宽的条状碎片，称取0.2 g置于试管中，加入体积分数为95%的乙醇10 mL，置于黑暗处浸提至叶组织完全变白，将提取液倒入光径1 cm的比色杯内，用UV-2700型紫外分光光度计在665、649和470 nm的波长下读取吸光度，并计算总叶绿素(Chl)、叶绿素a(Chl a)、叶绿素b(Chl b)和类胡萝卜素(Car)的质量分数。

1.2.3. 叶绿素荧光动力曲线及参数测定

于2021年5月20—23日，每个处理选3株紫茎，每株选3片叶片，对待测叶片进行40 min的暗适应处理，使用YZQ-500动态荧光仪(翼鬃麒科技公司)测定叶片荧光参数初始荧光(F_o)和最大荧光(F_m)，并采用李鹏民等^[18]的方法计算暗适应下光系统Ⅱ(PSⅡ)潜在活性(F_v/F_o)、PSⅡ最大光能转化效率(F_v/F_m)和以吸收光能为基础的性能指数(P_IABS)。

1.3. 数据处理

用WPS 2019对原始数据进行整理，SPSS 26进行单因素方差分析(one-way ANOVA)和最小显著差异分析(LSD)，Origin 2018制作光响应进程变化图，双因素方差分析法(two-way AVOVA)检验光强与氮素对紫茎幼苗光响应参数、光合色素质量分数和叶绿素荧光参数的交互影响效应。

3. 讨论

3.1. 光强与氮素对紫茎幼苗光响应的影响

大部分研究认为光照过强对植物叶片光系统造成损伤，光合能力降低^[19-20]；氮素供应过量反而会导致叶片碳同化能力降低，最终植株生长不良^[21-22]。本研究中，L₂N₁的紫茎幼苗P_n、G_s和T_r较高，说明中度遮光和低氮处理后，紫茎幼苗的光合机构运转良好，光合能力较强，研究结果与闽楠Phoebe bournei^[7]、高山杜鹃Rhododendron lapponicum等相似^[23]。

光饱和点(L_SP)和光补偿点(L_CP)可以反映植物的需光性和需光量^[24]，进而表征植物光强生态幅的变化范围。本研究中，紫茎幼苗的L_SP变化范围为571.3~931.4 μmol·m⁻²·s⁻¹，L_CP变化范围为4.8~26.0 μmol·m⁻²·s⁻¹，表明紫茎幼苗具有一定耐荫性。已有研究证实，耐荫性较强的植物P_max随着遮光程度增加仅出现轻微降低^[25-27]，本研究紫茎幼苗经遮光处理紫茎幼苗的P_max高于全光对照，而且通过降低L_CP提高了净光合速率，同时A_QY的升高也说明紫茎幼苗具有较强的利用弱光能力，有助于紫茎幼苗适应森林群落下层遮蔽的生境。

3.2. 光强与氮素对紫茎幼苗光合色素的影响

在弱光环境下，植物叶片的光合色素质量分数趋于增加，在强光环境下光合色素质量分数出现下降^[28]；施氮能够促进光合色素的合成并提高植物的光合能力，但高水平的氮素起到相反作用^[29]。本研究中，紫茎幼苗的光合色素质量分数和比值受光强和氮素的影响均为极显著(P＜0.001)。与闽楠^[7]、石灰花楸Sorbus folgneri^[30]、风吹楠Horsfieldia glabra^[31]等植物一样，紫茎幼苗叶片Chl质量分数和Car质量分数经过一段时间的遮光生长后显著增加，Car/Chl下降，说明紫茎幼苗通过提高叶绿素的相对含量来适应弱光生境，而Car质量分数提高增加有助于对蓝紫光的吸收，Chl a/b下降，推测在弱光下需要更多Chl b维持光能的捕获传递。通常，阳性植物叶片的Chl a/b约3.0，阴性植物叶片的Chl a/b约2.3^[32]，本研究中未施氮紫茎幼苗叶片的Chl a/b为2.0~2.5，表明紫茎在幼苗阶段的需光性更偏向于阴性树种，具有较强的弱光适应性。

施氮提高紫茎幼苗叶片Chl质量分数，尤其是Chl b质量分数增加最为显著。因此，施氮增强了叶片对光能的捕获率。本研究结果与黄檗Phellodendron amurense幼苗^[33]相同，紫茎幼苗Car质量分数随着施氮的增加先升高后降低，类胡萝卜素可以通过吸收过剩光能保护紫茎幼苗的光合器官。

3.3. 光强与氮素对紫茎幼苗叶绿素荧光的影响

研究表明：强光会破坏植物光合机构，出现光抑制现象，甚至会诱导产生有害物质单线态氧破坏光合反应，F_v/F_m因强光胁迫明显下降被视为植物产生光抑制的敏感指标^[34]，PSⅡ性能指数P_IABS相比F_v/F_m更能准确地反映植物光合机构对环境胁迫的响应^[18]。

本研究中，全光下紫茎幼苗的叶绿素荧光强度最弱，F_v/F_m明显下降且P_IABS也较小，紫茎幼苗的荧光强度随着光强的减弱而上升，F_v/F_m和P_IABS随着光强减弱先升高后降低，均说明紫茎幼苗在全光下出现光抑制，相反经遮光处理具有更大的光合潜能，适度遮光可以提高PSⅡ反应中心开放度，轻度和中度遮光改善了紫茎幼苗的光能吸收状态，结果PSⅡ性能指数较高。还需要指出的是，中度遮光下紫茎幼苗的F_v/F_m相较其他光强下高，但仍低于0.75~0.85，推测幼苗光合机构性能还可能受到高温、湿度等因素的制约。

施氮提高紫茎幼苗的P_IABS，对F_o有一定降低作用，说明施氮提高了紫茎光合机构的光合能力，因而PSⅡ受体侧接受电子的能力增加。全光下经施氮处理后，紫茎幼苗F_v/F_m明显降低，推测施氮加剧了全光下紫茎幼苗的胁迫程度。

3.4. 就地保护建议

野外调查中观察到紫茎多散生于常绿落叶阔叶林和沟谷地带，在紫茎群落下层几乎未见实生幼苗；当垂直带谱更替为常绿阔叶林或落叶阔叶林，群落内的紫茎植株更为稀少。紫茎群落外缘的土壤裸露地段可以看到绝大部分的紫茎实生幼苗，此地段侧方受光且土壤贫瘠，此时幼苗存活率成为决定紫茎天然更新能力的关键问题。可见，野外幼苗幼树的就地保护工作亟待加强。阔叶落叶林在春夏期间郁闭度较高，可在群落竞争评估基础上对密林进行适当修枝或疏伐，形成林窗以促进紫茎幼树生长。对生长林缘的幼苗进行适当遮光和施氮，实现光强和氮素的供需平衡。

4. 结论

光强、氮素及其交互作用对紫茎幼苗的光合色素质量分数和叶绿素荧光参数的影响达到显著水平(P＜0.05)。紫茎在幼苗阶段对弱光有较强适应性，随着遮光程度的增加，光合色素质量分数和叶绿素荧光强度均提高，光合能力先增强后减弱，在全光下出现典型的光抑制；施氮显著提高了紫茎幼苗光合色素质量分数，低氮对紫茎幼苗光合能力的提升作用较显著，高氮的促进效应降低甚至产生胁迫抑制。本研究中，经中度遮光和低氮处理后，紫茎幼苗的光合参数处于最适宜范围。

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处理	R_d/(μmol·m⁻²·s⁻¹)	P_max/(μmol·m⁻²·s⁻¹)	L_SP/(μmol·m⁻²·s⁻¹)	L_CP/(μmol·m⁻²·s⁻¹)	A_QY/(μmol·mol⁻¹)
L₀N₀	0.69±0.08 ab	2.09±0.03 def	791.4±79.6 ab	20.0±0.7 abc	0.027±0.003 b
L₀N₁	0.55±0.09 ab	1.92±0.15 def	717.7±103.1 ab	19.1±4.4 abc	0.031±0.006 b
L₀N₂	0.34±0.12 b	1.54±0.17 ef	638.2±63.0 ab	9.5±2.2 bc	0.032±0.007 b
L₁N₀	0.51±.0.09 b	2.61±0.42 cde	741.2±78.9 ab	10.5±1.3 bc	0.036±0.007 ab
L₁N₁	1.00±.0.27 a	3.79±0.32 abc	931.4±66.3 a	23.6±5.6 ab	0.047±0.017 ab
L₁N₂	0.72±0.35 ab	1.12±0.06 f	877.2±92.0 ab	26.0±17.3 a	0.034±0.014 ab
L₂N₀	0.62±0.21 ab	3.95±0.12 ab	797.1±70.8 ab	11.9±4.4 abc	0.043±0.007 ab
L₂N₁	0.52±0.13 ab	4.63±0.65 a	758.7±101.5 ab	8.3±2.0 bc	0.041±0.002 ab
L₂N₂	0.52±0.11 ab	2.81±0.43 bcd	571.3±49.6 b	11.3±2.7 abc	0.032±0.002 b
L₃N₀	0.30±0.08 b	2.93±0.26 bcd	778.0±185.8 ab	4.8±1.3 c	0.043±0.002 ab
L₃N₁	0.49±0.15 b	3.01±0.18 bcd	699.4±98.1 ab	9.6±4.2 bc	0.048±0.005 ab
L₃N₂	0.55±0.05 ab	2.56±0.48 cde	733.6±101.4 ab	8.3±1.3 bc	0.061±0.019 a
说明：数值为平均值±标准误，同一参数不同小写字母表示不同处理间差异显著(P＜0.05)

参数	光强	氮素	光强×氮素
R_d	2.028 ns	0.759 ns	1.783 ns
P_max	13.462 ***	11.209 ***	1.948 ns
L_SP	1.362 ns	0.642 ns	0.985 ns
L_CP	4.105 *	0.559 ns	1.943 ns
A_QY	3.133 *	0.283 ns	0.596 ns
说明：、、**、ns分别表示影响效应的显著性为P＜　　　　　0.05、P＜0.01、P＜0.001、P＞0.05

处理	Chl a/(mg·g⁻¹)	Chl b/(mg·g⁻¹)	Chl/(mg·g⁻¹)	Car/(mg·g⁻¹)	Chl a/b	Car/Chl
L₀N₀	0.59±0.04 f	0.26±0.02 g	0.85±0.05 i	0.17±0.01 f	2.28±0.13 b	0.20±0.00 a
L₀N₁	1.88±0.04 b	1.15±0.13 c	3.03±0.15 d	0.46±0.05 a	1.72±0.10 d	0.16±0.02 b
L₀N₂	1.74±0.03 c	0.93±0.04 d	2.66±0.06 e	0.47±0.01 a	1.89±0.06 c	0.18±0.00 b
L₁N₀	1.07±0.04 e	0.43±0.02 f	1.50±0.06 h	0.26±0.01 e	2.50±0.04 a	0.17±0.00 b
L₁N₁	1.32±0.06 d	0.70±0.05 e	2.03±0.11 g	0.36±0.03 bcd	1.90±0.04 c	0.18±0.00 b
L₁N₂	2.09±0.01 a	1.95±0.03 b	4.03±0.02 b	0.32±0.04 cde	1.07±0.02 e	0.08±0.01 d
L₂N₀	1.73±0.02 c	0.68±0.01 e	2.42±0.03 f	0.37±0.00 bcd	2.52±0.01 a	0.15±0.00 b
L₂N₁	2.09±0.01 a	1.25±0.02 c	3.34±0.03 c	0.43±0.03 ab	1.68±0.02 d	0.13±0.01 c
L₂N₂	2.12±0.02 a	1.92±0.05 b	4.03±0.04 b	0.29±0.02 de	1.22±0.03 e	0.07±0.01 d
L₃N₀	2.17±0.02 a	1.11±0.06 c	3.28±0.08 c	0.40±0.01 abc	2.00±0.08 c	0.12±0.01 c
L₃N₁	2.10±0.01 a	2.00±0.07 b	4.10±0.06 b	0.25±0.04 e	1.06±0.04 e	0.06±0.01 d
L₃N₂	1.77±0.02 c	3.19±0.07 a	4.95±0.05 a	0.05±0.01 g	0.56±0.02 f	0.01±0.00 e
说明：数值为平均值±标准误，同一参数不同小写字母表示不同处理间差异显著(P＜0.05)

参数	光强	氮素	光强×氮素
Chl a	316.423***	346.477***	184.931***
Chl b	314.398***	605.842***	48.029***
Chl	434.234***	726.104***	54.300***
Car	15.993***	12.818***	26.129***
Chl a/b	89.690***	373.330***	21.619***
Car/Chl	113.570***	104.247***	12.472***
说明：***表示影响效应的显著性为P＜0.001

处理	F_o	F_m	F_v/F_o	F_v/F_m	P_IABS
L₀N₀	396.0±14.1 d	1081.7±29.7 fg	1.75±0.10 d	0.633±0.014 cd	0.544±0.103 f
L₀N₁	378.0±5.9 d	816.9±50.2 h	1.15±0.11 e	0.526±0.024 e	0.436±0.077 f
L₀N₂	389.3±9.4 d	1013.9±92.0 g	1.58±0.20 d	0.595±0.031 d	0.735±0.182 f
L₁N₀	458.1±8.8 c	1396.7±22.8 d	2.06±0.08 c	0.671±0.009 bc	0.783±0.110 ef
L₁N₁	458.8±9.8 c	1193.9±66.0 ef	1.61±0.14 d	0.606±0.025 d	0.644±0.105 f
L₁N₂	381.9±8.7 d	1298.3±33.0 de	2.40±0.05 ab	0.706±0.004 ab	2.014±0.225 a
L₂N₀	503.7±7.4 b	1798.9±22.0 ab	2.58±0.06 a	0.720±0.005 a	1.381±0.121 cd
L₂N₁	461.2±6.0 c	1627.2±24.6 c	2.53±0.04 a	0.716±0.003 ab	1.800±0.103 ab
L₂N₂	469.2±13.0 c	1673.2±41.7 bc	2.57±0.06 a	0.719±0.004 a	1.686±0.152 abc
L₃N₀	565.3±8.5 a	1786.1±47.9 ab	2.16±0.05 bc	0.682±0.006 ab	1.102±0.052 de
L₃N₁	543.0±12.6 a	1822.4±43.5 a	2.36±0.01 ab	0.702±0.001 ab	1.437±0.063 bcd
L₃N₂	503.2±8.0 b	1711.2±32.4 abc	2.40±0.03 ab	0.706±0.002 ab	1.525±0.069 bc
说明：数值为平均值±标准误，同一参数不同小写字母表示不同处理间差异显著(P＜0.05)