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  土壤学报  2020, Vol. 57 Issue (1): 195-205  DOI: 10.11766/trxb201903250643
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引用本文  

刘智强, CAO Yuying, 赵正雄. 田间烤烟叶片缺钾症状与钾积累及土壤供钾水平关系. 土壤学报, 2020, 57(1): 195-205.
LIU Zhiqiang, CAO Yuying, ZHAO Zhengxiong. Relationships of Potassium Deficiency Symptoms and Potassium Accumulation in Flue-cured Tobacco Leaves with Soil Potassium Supply Capacity. Acta Pedologica Sinica, 2020, 57(1): 195-205.

基金项目

红塔集团科技计划项目(20181961)和云南农业大学科研发展基金项目(KX900187)资助

通讯作者Corresponding author

赵正雄, E-mail:zhaozx0801@163.com

作者简介

刘智强(1982-), 男, 黑龙江双鸭山人, 博士研究生, 助理研究员, 主要从事植物营养与作物品种生态等方面的研究工作。E-mail:liuzhiqiang128@sina.com
田间烤烟叶片缺钾症状与钾积累及土壤供钾水平关系
刘智强1,2 , CAO Yuying3 , 赵正雄1     
1. 云南农业大学农学与生物技术学院, 昆明 650201;
2. 大连市农业科学研究院, 辽宁大连 116036;
3. Smeal College, Pennsylvania State University, PA16802, USA
摘要:基于田间正常施肥烟株生长中期中、上部叶通常出现缺钾症状的现象,以烤烟品种K326为材料在砂质壤土上进行了田间试验,定期观察无钾(即K0,仅按常规施用氮磷肥,K2O用量为0 kg·hm-2)、常规施肥但中后期出现缺钾症状(即CF,按常规施用氮磷钾肥,K2O用量为364 kg·hm-2但中后期仍有缺钾症状)、常规施肥且正常生长(即CK,按常规施用氮磷钾肥,K2O用量为364 kg·hm-2但中后期无缺钾症状)3处理烟株移栽后生长情况(特别是叶片缺钾症状的出现情况),并及时采集和测定了相应烟株的干物质、钾含量及根区土壤速效钾含量,以阐明正常施肥烤烟中、上部叶出现缺钾症状的可能原因。结果表明:(1)CF和K0烟株在移栽后33 d前叶片均未出现缺钾症状;但根区土壤速效钾含量移栽后42 d低于99.86 mg·kg-1时,K0烟株第8~第15叶陆续表现缺钾;CF烟株根区土壤速效钾含量移栽后57 d低于131.1 mg·kg-1时,第12~第16叶也相继缺钾,而其他叶位叶片正常。(2)从移栽至42 d,CK和K0烟株干物质和钾素累积量虽皆明显增加,但K0烟株总量偏低趋势愈发显著;至移栽后57 d期间,CK和K0烟株前述两个指标继续增加,但此期CF干物质累积有相似的增加趋势,而钾素累积与出现缺钾症状前相比(即移栽后42 d时CK)则略有减少;(3)移栽后42 d至57 d,CF烟株上、中部叶和K0烟株上部叶钾净输出明显,而茎中净输入显著,其他器官或部位总体持平或略有增加;CK烟株上部叶钾素输入、输出维持平衡,其他器官有净增加。(4)生长期间,CK烟叶钾含量皆表现出随叶位上升而呈总体下降,K0烟叶前期有相似规律,但移栽后42 d、57 d呈"上升-下降-上升",CF于移栽后57 d为"下降-上升"趋势。上述结果说明,K0烟株移栽后42 d中部叶开始缺钾是钾整体吸收不足所致,后期则兼有上部叶钾净输出原因;CF烟株出现中、上部叶缺钾而下部叶正常则有3个原因,一是后期体内干物质累积持续增加导致的稀释;二是此期整株钾累积不仅未增加,且有下降;三是CF烟株中、上部叶钾净输出增加。
关键词烤烟    叶片缺钾症状    干物质    钾含量    钾分配    土壤速效钾    

钾是植物生长发育必需的元素之一。但钾在植物体内不形成稳定的化合物,流动性强,再利用程度高[1-3],当钾供应不足时,能及时迅速地通过韧皮部从植物基部或老叶中转移至上部幼嫩的器官中[4],因此,植物生长早期不易观察到缺钾症状,通常在植物生长中、后期才表现出来,首先植株下部老叶的叶缘、叶尖黄化或有褐色的斑点;随后逐渐坏死,并且从老叶至新叶症状的严重程度有明显的梯度[4-5]

烤烟作为收获叶片为主的经济作物,其生长过程中对钾的吸收远多于其他营养元素[6],叶片含钾量是评判其品质优劣的重要指标[7-8]。近年来,在烟草田间生产中,叶片缺钾现象较为普遍,且常常中、上部叶首先出现缺钾症状,下部叶并不表现缺钾[9-11]。关于上述与矿质营养再利用理论不一致的现象存在多种假说:胡国松等[10]认为烟草生长处于雨季,雨水更易于淋湿上部叶片,从而导致上部叶出现缺钾症状;杨绍聪等[11]推测,低温和缺少光照造成吸钾量减少,而下部叶来不及转移,首先由上部叶供应生长点生长,但他们并未做进一步研究。本课题组长期田间观测表明,生产中种植于不同地块的不同品种烤烟中、上部叶发生缺钾症状的现象普遍存在,且很难消除,并证明此现象确为缺钾所致[12]

叶片出现缺钾症状一方面取决于土壤养分的供应水平,另一方面取决于烟株钾素含量的高低和分配情况[13-16]。为此本研究从出现缺钾症状烤烟的土壤养分情况、干物质和叶片钾含量及不同器官钾素累积着手,阐明烤烟中、上部叶首先出现缺钾症状的原因,为进一步揭示此现象作用机理提供基础理论依据,更为烟株合理施肥提供指导。

1 材料与方法 1.1 试验设计

试验于2016年5月至7月在辽宁省大连市农业科学研究院内试验地进行。土壤为砂质壤土,0~25 cm土壤养分状况见表 1,按照第二次全国土壤普查标准,速效钾含量为中等水平。供试材料为烤烟K326。

表 1 试验地种植前0~25 cm土壤基础养分状况 Table 1 Initial nutrient status of the topsoil(0~25 cm)of the experimental fields

试验设3个处理,即常规施肥且正常生长(CK)、常规施肥但中后期出现缺钾症状(CF)、无钾(K0)。其中CK、CF两个处理移栽、施肥、管理等一致,移栽后根据田间烟株是否出现缺钾症状予以区分和标记。种植时选取长势均匀一致且无病害的烟苗进行移栽,株行距为60 cm × 110 cm,每小区100株;随机排列,3次重复。田间施肥量为常规施肥N 120 kg·hm-2、P2O5 162 kg·hm-2和K2O 364 kg·hm-2,移栽前分别开沟(深度10 cm)条施15-15-18的复合肥400 kg·hm-2、过磷酸钙(P2O5160 g·kg-1)637.5 kg·hm-2和硫酸钾(K2O 500 g·kg-1)292.8 kg·hm-2,移栽后25 d和35 d均距烟株10 cm开沟(深度5 cm)环施尿素(N 460 g·kg-1)65.2 kg·hm-2和硫酸钾(K2O 500 g·kg-1)145.6 kg·hm-2;无钾处理除不施钾外,N、P2O5的施用均同常规施肥处理。其他栽培措施按照优质烤烟管理方式进行。

1.2 测定项目与方法

烟株田间缺钾症状的实时观察:从烟苗移栽后开始,每天均认真查看各小区烟株是否有缺钾现象的出现,及其可能出现的相应症状。

烟株及土壤样品的采集和测定:试验分3个时期获取样品,采样时烟株相关生长信息如表 2所示。植株样品的采集是每个小区取有代表性的烟株3株,按叶位自下而上获取所有叶片、茎和根,105 ℃杀青1 h,70℃烘干至恒重后称重和粉碎,用1 mol·L-1 HCl浸提后通过火焰光度计(FP6410,仪电,上海)测定各叶位叶片、茎和根钾含量[17]。获取整株样品的同时,通过抖落法获取相应烟株根区土壤样品,风干过筛,用1 mol·L-1 NH4OAc浸提后通过火焰光度计测定速效钾含量[17]

表 2 土壤与植株取样时间 Table 2 Soil and plant sampling times
1.3 数据处理

试验数据采用Excel 2010整理,SPSS 17.0软件统计分析,各处理间的差异显著性使用新复极差法(Duncan法)进行分析。

2 结果 2.1 烤烟不同生育期叶片缺钾症状

本试验无钾、常规施肥烟株均有缺钾症状的出现,但发生时间、发生进程、发生数量和空间分布有所不同。常规施肥烟株和不施钾烟株在移栽后33 d前叶片均未出现缺钾症状(图 1a),图 1b));但至移栽后42 d时,不施钾烟株(K0)第9叶即表现出缺钾(图 1d)),之后至移栽后57 d时有约85%的烟株缺钾,且其第8~第15叶均表现出缺钾,而第1~第7叶与第16~第19叶皆未见缺钾症状(图 1g))。常规施肥(CK)烟株至移栽后42 d时未出现缺钾症状,至49 d时大部分烟株生长仍然正常(即未出现缺钾症状,图 1e)),但也有少数烟株第12~第16叶出现缺钾,而第1~第11叶和第17~第19叶均正常生长(即CF,图 1f));至57 d时常规施肥烟株三个小区分别有11%、20%株和19%的烟株有缺钾表现。常规施肥烟株叶片出现的缺钾症状与本课题组2014年于田间观察现象一致[12],且出现缺钾叶片均表现为首先叶尖、叶缘由绿转黄,随后失绿坏死反卷,叶脉间出现坏死斑点,并逐步发展至全叶。可见,烤烟叶片缺钾症状的发生发展比较复杂,不施钾较常规施肥缺钾症状出现的要早、叶片部位更低,但缺钾中心始终向上移动。

注:CK为常规施肥且正常生长烟株,K0为不施钾肥烟株,CF为常规施肥但中后期出现缺钾症状烟株;为便于拍照,获取植株后放盆中固定。下同   Note:CK:plants applied with NPK at the conventional rate, and free of potassium deficiency symptoms, K0:plants applied with NP only at the same rate, and CF:sharing the same plants with CK, but only those exhibiting potassium deficiency symptoms during the middle and late stages; In order to facilitate photographing, the sample plants collected from the field were placed and fixed in pots. The same below 图 1 不同生育期烟株叶片缺钾症状 Fig. 1 Potassium deficiency symptoms of flue-cured tobacco leaves relative to growth stage
2.2 植烟根区土壤速效钾含量变化

烟株移栽后33 d、42 d和57 d时的根区土壤速效钾含量检测结果(表 3)表明,随着烤烟生育期的推进,所有处理土壤速效钾含量均逐步下降,且常规施肥处理(包括CK以及移栽后57 d时出现缺钾症状的CF)皆显著高于不施钾处理(即K0);其中移栽后57 d时的CF处理虽明显高于K0处理,但也显著低于CK。与移栽后33 d时相比,57 d时CK、CF和K0 3个处理根区土壤速效钾含量分别下降了22.14%、39.19%和40.78%。

表 3 根区土速效钾含量 Table 3 Content of readily available potassium in the root zone soil /(mg·kg-1)
2.3 烤烟不同生育期叶片干物质变化

试验结果(图 2)表明,移栽后33 d、42 d和57 d时的各叶位叶片干物质累积量均呈现出随叶位上升而增加,至中间叶位叶片达到最大值之后而下降的现象。其中移栽后33 d时,常规施肥烟株和不施钾烟株同一叶位叶片间干物质累积无明显差异;至42 d常规施肥烟株和不施钾烟株第6~第13叶位间干物质积累差异显著,尤其是第9~第11叶分别减少27.18%、25.95%和26.72%;57 d时3个处理各叶位干物质累积均表现为:CK > CF > K0,其中对照和不施钾所有叶位间、对照和常规施肥但中后期出现缺钾症状烟株第10~16叶位间以及不施钾与常规施肥但中后期出现缺钾症状烟株第1~第11叶和第19叶叶片干物质差异显著。

注:不同字母表示相同叶位不同处理间差异达5%显著水平。下同。   Note:Different letters indicate significant
difference(at 5% level)between leaves the same in leaf position but different in treatment. The same below
图 2 烤烟不同生育期叶片干物质 Fig. 2 Dry matter of flue-cured tobacco leaves relative to growth stage
2.4 烤烟不同生育期叶片钾含量变化

图 3所示,随生育期的推移,移栽后33 d至57 d对照的叶片钾含量表现为先升高后降低的趋势,常规施肥但中后期出现缺钾症状烟株在移栽后57 d叶片钾含量出现明显下降,第12~第14叶降幅尤为明显,均在13 mg·kg-1以上;不施钾烟株在移栽后42 d至57 d均呈下降趋势,其中移栽后42 d以第1~第4叶降幅较大,均在8 mg·kg-1以上,移栽后57 d以第12~第15叶下降较多,均在5 mg·kg-1以上。

图 3 烤烟不同生育期叶片钾含量 Fig. 3 K+ concentration in flue-cured tobacco leaves relative to growth stage

移栽后33 d,对照(CK)与不施钾(K0)烟株叶片钾含量均表现为下部叶明显高于上部叶,由下至上基本呈下降趋势,其中对照的钾含量在42.6~33.5 mg·kg-1之间,不施钾烟株在34.0~25.5 mg·kg-1之间,且相同叶位对照均显著高于不施钾处理。移栽后42 d,对照各叶位钾含量仍表现为由下至上依次减少,在46.6~34.7 mg·kg-1之间;不施钾烟株则呈下部叶先升高后下降而后再上升的趋势,第1~第8叶呈上升趋势,在21.4~25.6 mg·kg-1之间,但至第9叶突然降至最低(19.2 mg·kg-1),从第10叶开始又逐步上升,至最幼嫩的第15叶达最高(27.9 mg·kg-1)。移栽后57 d,对照烟株各叶位钾随着叶位的升高逐步降低,钾含量均显著高于出现缺钾症状的2个处理,在41.2~31.5 mg·kg-1之间;常规施肥但中后期出现缺钾症状烟株以第2叶钾含量最高(32.0 mg·kg-1),其他叶位烟叶钾含量随着叶位的升高逐渐降低,至第14片叶达最低(17.0 mg·kg-1),随后又不断上升,至顶第19片叶时达26.7 mg·kg-1;不施钾烟株与移栽后42 d规律基本一致,由底叶开始钾含量逐步上升,至第7叶达23.5 mg·kg-1后,呈下降趋势,至第12叶最低(16.3 mg·kg-1),而后逐步升高,至第18叶为最高,达24.3 mg·kg-1。出现缺钾症状烟株比较,常规施肥但中后期出现缺钾症状烟株的钾含量均高于不施钾烟株(第14叶除外),第1~第11叶皆达到显著差异。

2.5 烤烟不同生育期体内钾素积累变化

对照与不施钾处理整株钾积累量随生长时间的推移均显著增加,但不施钾烟株较低,且随生育期的延长差距愈发显著,差异幅度从29.78%增至58.61%,而常规施肥但中后期出现缺钾症状烟株与出现缺钾症状前相比(即移栽后42 d时CK)略有减少(表 4)。从烟株各部位来看,移栽后0 d至42 d,对照与不施钾烟株各部位钾均显著增加;移栽后42 d至57 d,不施钾烟株上部叶钾净输出显著,降幅达29.93%,其他部位钾仍为净输入,其中下部叶和茎的净输入显著,增幅达29.76%和132.1%;常规施肥但中后期出现缺钾症状烟株与出现缺钾症状前相比,其上部叶和中部叶钾净输出显著,降幅分别为54.04%和26.41%,而下部叶、茎和根的钾仍呈净输入状态,其中茎的净输入量增幅达221.2%;对照烟株上部叶钾输入输出维持平衡,其他部位钾净输入显著,由高至低依次为茎、下部叶、根、中部叶、上部叶,其中茎与下部叶的净输入增幅达182.8%和48.97%。

表 4 烟株生长期间不同器官钾素累积动态 Table 4 Potassium accumulation dynamics in tobacco plant during the growth season relative to organ of the plant/(K+ g·plant-1)
3 讨论 3.1 烤烟缺钾症状与土壤速效钾水平

本研究常规施肥K2O的施用量为364 kg·hm-2,为较高水平,且分2次施用,虽根际土壤速效钾含量和烟株各部位叶片钾含量在旺长期均达到了较高水平,但仍有部分烟株于移栽后49 d出现了中、上部叶缺钾症状,这也与不施钾肥形成了鲜明的对比。究其原因,可能与本试验地土壤类型为砂质壤土,局部微域土壤保水保肥能力不强有关。本研究中土壤速效钾含量随着烤烟生育期的推移而下降的规律也与其他文献报道[18-19]相同,说明烤烟生长期间根系吸收速率、土壤淋失速率和固定速率三者超过了缓效钾向速效钾转换的速率[20]。烤烟生长时期正属于降雨量大的季节,土壤速效钾含量迅速下降[21],或是雨水冲刷中、上部叶面[22],也可能是出现此现象的原因。

不同处理各时期的土壤速效钾含量,可以作为评判烤烟是否缺钾的参考。黎妍妍等[23]研究认为,当土壤速效钾含量为150 mg·kg-1时,即已达到土壤供钾丰缺的临界水平。王毅等[24]认为烤烟土壤速效钾含量小于55 mg·kg-1为低,55~110 mg·kg-1为较低,110~160 mg·kg-1为中等,160~230 mg·kg-1为较丰富,大于230 mg·kg-1为丰富。本研究表明,移栽后42 d土壤速效钾低于99.86 mg·kg-1表 3),不施钾烟株的第8~第15叶陆续出现缺钾症状,移栽后57 d常规施肥地块速效钾含量低于131.1 mg·kg-1(属中等水平)时,烟株第12~第16叶也相继出现缺钾,说明此时生长旺盛,根系可能未及时吸收或下部叶未及时转移,即出现缺钾症状,这两组数据可视为烤烟不同生长阶段的土壤钾素缺乏的临界点。所以,考虑植烟土壤是否缺钾,必须综合考虑土壤钾含量与烟株生育期等因素。

3.2 烤烟缺钾症状与叶片钾含量

通过田间试验,进一步验证了烤烟前期不易缺钾,往往在旺长期出现缺钾症状,首先现于中、上部叶的真实性[12],而且发现了前人未曾报道的现象:如田间烤烟叶片缺钾症状视缺钾程度而定,缺钾越严重,出现缺钾症状的时期越早,叶位越偏下;而且发展逐步向上,且发展不连续,中部某些叶片缺钾症状较轻微。这些发现均表明,田间土壤在缺钾条件下烤烟叶片缺钾症状较为复杂,远非经典理论理解的自下部成熟或老叶逐步向上发展。

从叶片钾含量(图 3)而言,缺钾症状与其大致吻合。首先,出现缺钾症状烟株叶片钾含量明显低于正常生长烟株,如移栽后42 d,不施钾烟株叶片钾含量为19.17~27.87 mg·kg-1,常规施肥烟株为34.73~46.60 mg·kg-1;移栽后57 d,不施钾烟株为16.33~24.27 mg·kg-1,常规施肥但中后期出现缺钾症状烟株为17.00~31.80 mg·kg-1,常规施肥正常生长烟株为31.53~41.23 mg·kg-1;其次,出现缺钾症状烟株的叶片钾含量均较同一烟株未出现缺钾症状叶片更低,如旺长期不施钾第9叶为19.17 mg·kg-1,现蕾期不施钾第8~第15叶为16.33~20.60 mg·kg-1,常规施肥但中后期出现缺钾症状烟株第12~第16叶为17.00~23.73 mg·kg-1。但上述数据与前人对烟株叶片出现缺钾症状研究有所不同,王毅等[24]提出,烤烟现蕾初期下部烟叶钾含量小于9 mg·kg-1为缺钾,9~12.5 mg·kg-1为轻度缺钾,12.6~16.0 mg·kg-1为潜在缺钾,大于16.0 mg·kg-1为钾素充足。Krishnamurthy等[25]认为,缺钾烟叶的钾含量在6.2~7.5 mg·kg-1之间。杨绍聪等[11]研究表明,“K326”叶片钾含量为26.75 mg·kg-1即出现缺钾症状,“红花大金元”叶片在15.91 mg·kg-1才出现缺钾。因此,烤烟叶片临界钾含量可能需要针对不同生育期、不同叶位以及不同品种来界定。

3.3 烤烟缺钾症状与干物质和钾素积累

干物质不仅是衡量烤烟生长发育的重要指标,更是烟叶产量与质量形成的物质基础,且干物质积累和烟株对钾的吸收、利用及体内的分配是决定烟叶钾含量的关键[16, 26-27]。本试验中,随着叶片的生长,3个处理烟株的各叶位干物质一直在增加,但与对照比较出现缺钾症状2处理干物质总量偏低,不施钾烟株移栽后33 d至57 d差距越发显著(从10.35%增至27.19%),常规供钾但中后期出现缺钾症状烟株移栽后57 d干物质仅为对照的85.59%(图 2);同时积累重心也不断向上推移,移栽后33 d对照与不施钾烟株均为第5叶干物质累积量最大,移栽后42 d对照为第9叶、不施钾肥为第8叶累积量最大,移栽后57 d对照为第10叶、常规供钾但中后期出现缺钾症状烟株为第9叶、不施钾烟株仍为第8叶累积量最大。可见缺钾使干物质累积放缓,尤其是症状发生叶的干物质降幅明显,如移栽后42 d不施钾烟株第9叶、移栽后57 d常规供钾但中后期出现缺钾症状和不施钾烟株的第10~第13叶,同时相应烟株干物质积累中心未再向上移动(图 2)。这表明钾素缺乏影响叶片干物质的累积,中、上部出现缺钾症状叶片尤为突出。

在移栽后33 d至42 d,对照和不施钾处理烟株钾素积累量与干物质积累量规律类似,均呈明显增加态势,但不施钾烟株总量偏低且较干物质差距更大,从29.78%增至51.58%(图 3);至移栽后57 d,对照与不施钾肥处理钾累积继续增加,但不施钾烟株钾总量差距为58.61%,此期出现缺钾的常规施肥烟株(CF)较出现缺钾症状前(即移栽后42 d时CK)却略有减少。上述表明不施钾处理烟株在整个生育期钾吸收不足,体内钾量难以满足其生长需求,至移栽后42 d即出现缺钾症状;而常规施肥但中后期出现缺钾症状烟株移栽后57 d在干物质持续增加的情况下,其体内的钾含量不升反降,也是造成其中、上部叶片缺钾的主要原因。

由于钾在植物体内主要以离子形式存在,其再利用程度远远高于其他元素[28]。前人研究指出,在盐胁迫[29-30]、被寄生虫感染[31]等情况下钾素均会在植物体内发生转移,甚至处于成熟期的烟株也会发生转移[32]。本试验表明,移栽后42 d至57 d,出现缺钾症状的常规施肥烟株上部叶和中部叶(较出现缺钾症状前)及不施钾烟株上部叶钾净输出明显,分别较移栽后42 d减少54.04%、26.41%和29.93%,而在茎中输入显著,分别增加122.1%和221.2%,其他部位保持不变或略有增加(表 4),对照烟株的上部叶输入、输出维持平衡,其他部位为净输入。烤烟移栽后42 d至57 d中、上部叶应为生长中心,但钾缺乏使中、上部叶中的钾转移至茎部并滞留其中的特殊现象与钾的再分配关系密切,此现象需进一步深入研究其体内生理变化,以明确其发生发展过程,为烤烟合理施肥提供依据。

4 结论

几乎所有的不施钾肥烟株和部分常规施肥烟株在旺盛生长阶段均会出现中、上部叶缺钾现象,不施钾肥烟株较常规施肥烟株出现症状更早、叶位更偏下。不施钾肥烟株缺钾首先由于相应地块土壤速效钾含量相对较低,使其整体吸钾量不足,无法满足烟株进一步生长需求;后期烟株缺钾严重,加之其上部叶钾由之前的净输入转为净输出,加剧其中、上部叶缺钾症状的发生、发展。常规施肥烟株出现中、上部叶缺钾而下部叶正常生长可能有3方面原因:①干物质在后期仍有较大幅度增加,其累积速度明显高于钾,由此产生的稀释效应;②后期整株钾积累量不仅未增加,且有所减少;③烟株中部叶和上部叶钾净输出向茎中转移,降低了其钾含量。

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Relationships of Potassium Deficiency Symptoms and Potassium Accumulation in Flue-cured Tobacco Leaves with Soil Potassium Supply Capacity
LIU Zhiqiang1,2 , CAO Yuying3 , ZHAO Zhengxiong1     
1. College of Agronomy and Biotechnology, Yunnan Agricultural University, Kunming 650201, China;
2. Dalian Academy of Agricultural Sciences, Dalian, Liaoning 116036, China;
3. Smeal College, Pennsylvania State University, PA16802, USA
Abstract: 【Objective】 To explore potential causes of the occurrence of potassium deficiency symptoms in the middle and upper leaves, rather than lower leaves, of normally fertilized tobacco plants in the field during their growth, a field experiment was carried out in this study.【Method】 The field experiment was laid out in a tobacco field of sandy loam planted with flue-cured tobacco of variety K326, designed to have three treatments, i.e. K0 (plants applied with N P at the conventional rate and without K, 0 kg·hm-2 K2O), CF (plants applied with NPK at the conventional rate, 364 kg·hm-2 K2O, and only those among the plants exhibiting potassium deficiency symptoms during the middle and late stages were sorted as Treatment CF), and CK (sharing the same plants with Treatment CF, applied with NPK at the same rate, and only those free from any potassium deficiency symptoms during the middle and late stages were sorted as Treatment CK). Observation of the plants began after their transplantation for growth and possible appearance of potassium deficiency symptoms in leaves. Leaves were sampled timely for determination of dry matter and potassium content, and rhizosphere soils were for analysis of available potassium content.【Result】 (1) The tobacco plants in Treatments CF and K0 did not show any symptoms of potassium deficiency during the first 33 days after transplanting. However, when soil available potassium in the root zone was gradually depleted and dropped below 99.86 mg·kg-1 in content on D42, symptoms appeared one by one on the 8th~15th leaves of the plants in Treatment K0; and when soil available potassium in the root zone was lowered down below 131.1 mg·kg-1 on D57, symptoms appeared one by one on the 12th~16th leaves of the plant in Treatment CF, but not on the other leaves; (2) During the first 42 days after transplanting, dry matter and potassium accumulation significantly increased in Treatments CK and K0, but the trend of the total weight of the tobacco plants in Treatment K0 getting lower became more and more apparent; and the two indexes kept on rising till D57, and during this period of time, the two treatments exhibited a rising trend in dry matter accumulation, similar to that Treatment CF did, but Treatment CF was a bit lower than Treatment CK in potassium during the first 42 days after transplantation; (3) From D42 to D57, net potassium export was significant from the upper and middle leaves of the plants in Treatment CF and the upper leaves in Treatment K0, while net potassium import was in the stems, and potassium in the other organs or parts was generally kept in balance or slightly increased; and in Treatment CK, potassium stayed in balance in the upper leaves of the tobacco plants and slightly increased in the other organs; and (4) During the growth period, potassium content increased with rising position of the leaf but declined in overall in the plants of Treatment CK; similar patterns were observed Treatments K0 and CF in the early stage, but Treatment K0 showed a "rise-decline-rise" trend after D42 and D57, while Treatment CF did a "decline-rise" trend after D57.【Conclusion】 All the findings indicate that the appearance of potassium deficiency symptoms in the middle leaves of the plants in Treatment K0 on D42 was attributed to (1) insufficient potassium absorption as a whole and (2) high net export of potassium in the upper leaves in the later stage. The appearance of potassium deficiency symptoms in the middle and upper leaves rather than lower leaves of the tobacco plants in Treatment CF was because (1) the on-going fast dry matter accumulation diluted potassium in the plants in the later stage; (2) potassium accumulation of the whole plant declined instead of going on in the later stage and (3) the net potassium export increased in middle and upper leaves of the plants.
Key words: Flue-cured tobacco    Symptoms of potassium deficiency in leaves    Dry matter    Potassium content    Potassium distribution    Soil readily available potassium