Responses of root vessel anatomical structures to drought exposure for two Kobresia species in an alpine meadow habitat in Southeast Tibet

doi:10.11686/cyxb2020530

Abstract

Abstract:

Xylem vessels in the vascular tissue of roots are the channels by which plants transport water， and the plasticity of their structure is an important mechanism for plants to adapt to the habitat， which will also impact the efficiency and safety of water transport. Kobresia humilis and Kobresia macrantha are two Cyperaceae species that differ in their soil moisture preferences， K. humilis being more tolerant of drier environments. This research investigated whether there is any difference in water transport efficiency and safety between these two Kobresia species. To answer this question， K. humilis and K. macrantha root samples were collected at six sample plots along a soil water gradient in an alpine meadow of Southeast Tibet. Collected root samples were embedded in paraffin， sectioned by microtome. The vessel diameter， lumen area and wall thickness of the two species of Kobresia were measured by microscopy and image analysis and a cell wall coefficient of reinforcement （cwr） for the vessels was derived. From these data an overall vessel density and hydraulic diameter were calculated， and histograms of diameter class distributions for the sampled roots were prepared. Correlation analysis， one-way ANOVA and principal component analysis were used to analyze and interpret the data. In K. humilis vessel wall thickness was significantly negatively and vessel density significantly positively （P<0.01） correlated with soil water content； In K. macrantha vessel wall thickness and cwr were significantly negatively and vessel lumen area and average diameter significantly positively （P<0.01） correlated with soil water content. Vessel wall thickness of both species was increased in dry soil conditions and decreased in wet soil conditions. Comparing K. humilis and K. macrantha， vessel lumen area and average diameter， stele hydraulic diameter were all very significantly less， while cwr and vessel density were very significantly greater in K. humilis than in K. macrantha （P<0.01）. K. macrantha is a species adapted to continuously wet environments， while K. humilis is adapted to drier soil conditions. The ratio of narrow vessel and middle vessel of K. humilis is similar， which has a strong ability to adjust efficiency and safety of water transport， while the ratio of middle vessel of K. macrantha is always the highest， with weak balance ability for efficiency and safety of water transport. A mechanistic hypothesis from these data for confirmation in future research， is that K. macrantha has greater average xylem vessel area and diameter in the root stele than K. humilis， which would be expected to decrease resistance to water flow. By contrast， narrower diameter， more reinforced xylem vessels in the root stele of K. humilis than K. macrantha would be better adapted to withstanding negative water potentials involved in extracting water from drier soils.

Key words: vessel structure, diameter class structure, drought tolerance, efficiency of water transport, safety of water transport

Chun-jiao YANG, Yu-zhen HAN, Zhong-kui LI, Da-cai ZHANG, Hong-bin WANG, Hong-lin LI. Responses of root vessel anatomical structures to drought exposure for two Kobresia species in an alpine meadow habitat in Southeast Tibet[J]. Acta Prataculturae Sinica, 2022, 31(2): 76-87.

Figures/Tables 10

Table 1 Information of sample plots

样方编号 No. of sample	土壤含水率 Soil water content （%）	海拔 Altitude （m）	地形 Landform	地理坐标 Longitude and latitude	优势种 Dominant species
S₁	22.4	4803	陡坡Steep slope	29°71′94″ N， 98°04′53″ E	矮生嵩草 K. humilis
S₂	30.3	4773	陡坡Steep slope	29°72′61″ N， 98°04′51″ E	矮生嵩草 K. humilis
S₃	37.6	4766	陡坡Steep slope	29°71′08″ N， 98°04′48″ E	矮生嵩草 K. humilis
S₄	42.9	4762	缓坡Gentle slope	29°71′58″ N， 98°04′24″ E	矮生嵩草 K. humilis
S₅	49.9	4759	平坦Flat	29°72′95″ N， 98°04′08″ E	矮生嵩草 K. humilis 大花嵩草 K. macrantha
S₆	58.7	4758	溪边By the stream	29°72′81″ N， 98°04′37″ E	大花嵩草 K. macrantha

Table 2 Design of experiment

水分梯度 Water gradient	土壤含水率 Soil water content（%）	各实验对象及重复的样本量Sample sizes of each subjects and replicates						合计样本量 Total sample size
		矮生嵩草K. humilis			大花嵩草K. macrantha
		1	2	3	1	2	3
1	22.4	5	5	5	5	5	5	30
2	30.3	5	5	5	5	5	5	30
3	37.6	5	5	5	5	5	5	30
4	42.9	5	5	5	5	5	5	30
5	49.9	5	5	5	5	5	5	30
6	58.7	5	5	5	5	5	5	30

Fig.1 Anatomic graph of the middle column， root cross section of K. humilis and K. macrantha

Table 3 Measurement method and data quantity of vessel structures variables

指标 Indices	单位 Unit	测量与计算方法 Methods of measurement and calculation	每个生境数据量 Date volume per habitat	总数据量 Total data
管腔面积 Lumen area （s）	μm²	测量根横切面上每个导管的内腔面积。The lumen area of each vessel was measured.	公式1 Formula 1	1055
导管平均直径 Average diameter of vessel （ $d -$ ）	μm	测量每个导管互相垂直方向上的两组直径（d₁， d₂），以平均值作为每个导管的直径。Two groups of diameters（d₁， d₂） of each vessel perpendicular to each other were measured， and the average value was calculated as the diameter of each vessel.	公式1 Formula 1	1055
水力直径^［9］ Hydraulic diameter （dh）	μm	同一条根所有导管计算1个水力直径，n代表每条根中导管数量，计算方法见公式2。One hydraulic diameter was calculated for all vessels in the same root， and “n” represents the number of vessels in each root. The calculation formula was as Formula 2.	15	90
管壁厚度 Wall thickness （th）	μm	每个导管随机测两组壁厚（th₁， th₂），以平均值作为该导管的管壁厚度。 The wall thickness（th₁， th₂） of each catheter was measured randomly， and the average value was calculated as the wall thickness of the vessel.	公式1 Formula 1	1055
加固系数^［21］ Coefficient reinforcement （cwr）		公式3。Formula 3.	公式1 Formula 1	1055
导管密度 Vessel density （p）	n·mm^-2	测量根横切面面积（a），计数根横切面上所有导管个数（n），根据公式4计算导管密度。The area of root cross section（a） was measured， and the number of vessel on root cross section（n） was counted. The formula for calculating vessel density is as Formula 4.	15	90

Table 3 Measurement method and data quantity of vessel structures variables

指标 Indices	单位 Unit	测量与计算方法 Methods of measurement and calculation	每个生境数据量 Date volume per habitat	总数据量 Total data
管腔面积 Lumen area （s）	μm²	测量根横切面上每个导管的内腔面积。The lumen area of each vessel was measured.	公式1 Formula 1	1055
导管平均直径 Average diameter of vessel （ $d -$ ）	μm	测量每个导管互相垂直方向上的两组直径（d₁， d₂），以平均值作为每个导管的直径。Two groups of diameters（d₁， d₂） of each vessel perpendicular to each other were measured， and the average value was calculated as the diameter of each vessel.	公式1 Formula 1	1055
水力直径^［9］ Hydraulic diameter （dh）	μm	同一条根所有导管计算1个水力直径，n代表每条根中导管数量，计算方法见公式2。One hydraulic diameter was calculated for all vessels in the same root， and “n” represents the number of vessels in each root. The calculation formula was as Formula 2.	15	90
管壁厚度 Wall thickness （th）	μm	每个导管随机测两组壁厚（th₁， th₂），以平均值作为该导管的管壁厚度。 The wall thickness（th₁， th₂） of each catheter was measured randomly， and the average value was calculated as the wall thickness of the vessel.	公式1 Formula 1	1055
加固系数^［21］ Coefficient reinforcement （cwr）		公式3。Formula 3.	公式1 Formula 1	1055
导管密度 Vessel density （p）	n·mm^-2	测量根横切面面积（a），计数根横切面上所有导管个数（n），根据公式4计算导管密度。The area of root cross section（a） was measured， and the number of vessel on root cross section（n） was counted. The formula for calculating vessel density is as Formula 4.	15	90

Fig.2 The variation of coverage of two Kobresia species along a gradient of soil water content

Table 4 Correlation between root vessel structure and community coverage or soil water content for two Kobresia species

指标 Indices	相关系数 Correlation coefficient （R）
	矮生嵩草K. humilis		大花嵩草K. macrantha
	土壤含水率Soil water content	盖度Coverage	土壤含水率Soil water content	盖度Coverage
管壁厚度 Wall thickness	-0.131**	-0.218**	-0.185**	-0.170**
加固系数 Coefficient reinforcement	-0.038	-0.082**	-0.333**	-0.387**
导管密度 Vessel density	0.386**	0.339**	-0.194	-0.167
管腔面积 Lumen area	-0.053	-0.061*	0.162**	0.167**
导管平均直径 Average diameter	-0.053	-0.051	0.180**	0.208**
水力直径 Hydraulic diameter	-0.008	0.021	0.153	0.174

Fig.3 Variations of lumen area， average diameter of vessel and hydraulic diameter of K. humilis and K. macrantha with soil water gradient

Fig.4 Variations of wall thickness， coefficient reinforcement and vessel density of K. humilis and K. macrantha with soil water gradient

Fig.5 Distribution of vessel diameter classes of K. humilis and K. macrantha

Table 5 Eigenvectors and contribution rates of principal components in vessel structures of K. humilis and K. macrantha

指标 Indices	矮生嵩草 K. humilis				大花嵩草 K. macrantha
	因子载荷 Factor loading		F	排序 Rank	因子载荷 Factor loading			F	排序 Rank
	F₁	F₂	F	排序 Rank	F₁	F₂	F₃	F	排序 Rank
管腔面积Lumen area	-0.566	0.796	-0.144	5	0.958	-0.019	-0.063	0.396	3
导管平均直径 Average diameter	0.336	0.920	0.517	4	0.982	0.028	0.004	0.438	1
水力直径 Hydraulic diameter	-0.357	0.010	-0.243	6	-0.054	-0.072	-0.805	-0.250	6
管壁厚度 Wall thickness	0.986	-0.022	0.674	2	0.306	0.933	0.051	0.437	2
加固系数 Coefficient reinforcement	0.903	0.059	0.642	3	-0.492	0.846	0.036	0.058	5
导管密度 Vessel density	0.981	0.022	0.684	1	-0.102	-0.009	0.801	0.156	4
特征值 Characteristic value	3.31	1.49			2.23	1.59	1.30
方差贡献率 Variance contribution rate （%）	55.17	24.76			37.18	26.56	21.61
累计贡献率 Cumulative contribution rate （%）	55.17	79.93			37.18	63.73	85.35

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