Root sap flow and hydraulic redistribution of Populus tomentosa

doi:10.17521/cjpe.2021.0492

Abstract

Abstract:

Aims To determine whether there is hydraulic redistribution in the root system of Populus tomentosa, and to explore its characteristics and influencing factors.
Methods The heat ratio method was used to monitor the long-term sap flow of 7 lateral roots (R1-R7) of four-year P. tomentosa trees, and the soil moisture and meteorological factors were measured simultaneously.
Important findings This study showed two patterns of hydraulic redistribution of P. tomentosa, namely, drought-induced hydraulic lift and rainfall-induced hydraulic descent. The occurrence and characteristic of hydraulic redistribution were affected by the distribution depth and diameter size of the lateral roots. In general, the magnitude of hydraulic redistribution was relatively low. In the growing season, the amount of water redistributed by P. tomentosa roots was low; however, under extreme drought conditions, the amount of water redistributed by the lateral roots could reach 64.6% of its total daily sap flow, indicating that hydraulic redistribution would provide plenty of water for dry lateral roots. This study showed that the root water uptake was significantly related to the meteorology-soil coupling factors (solar radiation (R_s) × soil water content (SWC), vapor pressure deficit (VPD) × SWC, reference evapotranspiration (ET_o) × SWC), but not to the hydraulic redistribution. In addition, this study found a unique daytime reverse sap flow occurred in shallow lateral roots. The reverse sap flow could account for up to 79.2% (R1) and 90.7% (R2) of the total daily sap flow, which could play an essential role in the drought resistance of shallow roots.

Key words: Populus tomentosa, root sap flow, hydraulic redistribution, hydraulic lift

LIU Yang, MA Xu, DI Nan, ZENG Zi-Hang, FU Hai-Man, LI Xin, XI Ben-Ye. Root sap flow and hydraulic redistribution of Populus tomentosa[J]. Chin J Plant Ecol, 2023, 47(1): 123-133.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2021.0492

https://www.plant-ecology.com/EN/Y2023/V47/I1/123

Figures/Tables 9

Fig. 1 Schematic diagrams of the typical hydraulic redistribution. A, Hydraulic lift, water moves from the deep to the shallow layer during drought. B, Hydraulic descent, water moves from the shallow to the deep layer after raining. C, Horizontal hydraulic redistribution, water moves from wet to dry soil after soil uneven wetting.

Table 1 Information of the roots for measuring sap flow of Populus tomentosa

侧根编号 Lateral root number	直径 Diameter (cm)	平均分布深度 Average distribution depth (m)	最大分布深度 Maximum distribution depth (m)
R1	4.13	8.7	11.5
R2	2.89	11.3	13.2
R3	4.54	12.5	21.1
R4	3.26	21.6	42.6
R5	4.86	>44.6	>70.0
R6	3.73	9.0	20.7
R7	4.80	10.6	19.1

Fig. 2 Changing dynamics of roots sap flow velocity of Populus tomentosa and meteorological factors during drought process. In B-E, the gray shade indicates the sap flow velocity (Vs) <0, that is, the reverse sap flow. In F, SWC0.1 m, SWC0.5 m and SWC2.0 m are the soil water content (SWC) at 0.1, 0.5 and 2.0 m depths; the gray bars represent rainfall. Rs, solar radiation; VPD, vapor pressure deficit; R1-R5, the lateral roots of different numbers.

Fig. 3 Changing dynamics of roots sap flow of Populus tomentosa and meteorological factors after rainfall. In B-E, the gray shade indicates the sap flow velocity (Vs) < 0, that is, the reverse sap flow. In F, SWC0.1 m, SWC0.5 m and SWC2.0 m are the soil water content (SWC) at 0.1, 0.5 and 2.0 m depths; the gray bars represent rainfall. Rs, solar radiation; VPD, vapor pressure deficit; R1-R5, the lateral roots of different numbers.

Fig. 4 Changing dynamics of daytime reverse sap flow of Populus tomentosa and meteorological factors. In B and C, The gray shade indicates the sap flow velocity (Vs) < 0, that is, the reverse sap flow. In D, SWC0.1 m, SWC0.5 m and SWC2.0 m are the soil water content (SWC) at 0.1, 0.5 and 2.0 m depths; the gray bars represent rainfall. Rs, solar radiation; VPD, vapor pressure deficit; R1-R2, the lateral roots of different numbers.

Fig. 5 Seasonal dynamics of meteorological factors of Populus tomentosa plantation in the North China Plain. SWC0.1 m, SWC0.5 m and SWC2.0 m are the soil water content (SWC) at 0.1, 0.5 and 2.0 m depths.

Fig. 6 Seasonal dynamics of daily sap flow (Q) of Populus tomentosa roots. R1-R7, the lateral roots of different numbers.

Table 2 Types and proportions of hydraulic redistribution (HR) and their proportions of Populus tomentosa

侧根编号 Lateral root number	干旱驱动的HR Drought induced HR		降雨驱动的HR Rainfall induced HR		总R_HRTotal R_HR (%)
侧根编号 Lateral root number	时间 Time (d)	最高日R_HR Max daily R_HR (%)	时间 Time (d)	最高日R_HR Max daily R_HR (%)	总R_HRTotal R_HR (%)
R1	26	50.8	0	0.0	1.2
R2	46	64.6	0	0.0	3.5
R3	18	6.7	7	3.1	0.4
R4	16	4.9	7	12.3	0.6
R5	16	1.8	3	0.6	0.1
R6	15	2.0	11	4.7	0.3
R7	21	4.4	8	5.8	0.4

Fig. 7 Response of root sap flow of Populus tomentosa to meteorological factors. A-C, the relation between R2 sap flow (Q) and the product of solar radiation (Rs), vapor pressure deficit (VPD), reference evapotranspiration (ETo) and soil water content (SWC). D-F, the relation between Q of R4 and the product of Rs, VPD, ETo with SWC. The red points represent the positive sap flow (QP), and the blue points represent the reverse sap flow (QR); the red and blue lines are the linear fitting of QP and QR to environmental factors, where the solid lines represent the significant correlation (p < 0.05) and the dashed lines represent the uncorrelated (p > 0.05). R2 and R4 are lateral root number, see datails in Table 1.

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