3.1 Dynamics of temperature and precipitation in the whole plant growth periods
Although the daily average temperature of Isatis indigotica fluctuates in a certain range during the whole growth period, the fluctuation was similar to the local temperature variation in the past five years (Fig. 1). In 2016, the lowest daily average temperature was 12.8℃, which appeared at the beginning of seedling stage, and the highest daily average temperature was 27.6℃, which appeared at the fleshy root growth stage. The average daily temperature of Isatis indigotica during the whole growth period in 2017 was 3.75℃ at the beginning of seedling stage and 2.425℃ at the later maturity of fleshy roots, respectively. The maximum daily average temperature was 29.8℃ at fleshy root growth stage. In 2017, the total precipitation of Isatis indigotica was 185.8 mm. The precipitation mainly concentrated in the fleshy root growth stage and the maturity, of which the amount reached 72.8 mm and 60.5 mm respectively, accounting for 39.18% and 32.56% of the total precipitation of the whole growth period. The precipitation of Isatis indigotica in 2017 was 196.5mm in the whole growth period, and the precipitation was evenly distributed in each growth stage. The precipitation in the late vegetative period and the fleshy root growth period in 2017 was less than that in 2016.
3.2 Net photosynthetic rate
Net photosynthetic rate of Isatis indigotica changed marginally among treatments at seedling stage and fleshy root maturity stage, but water deficit showed more significant effect on net photosynthetic rate at vegetative stage and fleshy root growth stage (Fig. 2). In 2016, net photosynthetic rate of all treatments was smaller than that of the control group at vegetative stage(p > 0.05). Among them, WD3 and WD9 net photosynthetic rates decreased by 17.1% and 19.1% respectively compared with CK༈p < 0.05༉. The compensatory effect occurred under WD1 and WD4 after the samples were re-watered at fleshy root growth stage. The net photosynthetic rate reached 19.62 µmol·m− 2·s− 1 and 19.84 µmol·m− 2·s− 1 percent respectively, exceeding that of CK, but that of WD3 only reached 16.05 µmol·m− 2·s− 1, which was lower than that of CK. The net photosynthetic rates of WD5 and WD7 of moderate water deficit decreased respectively at fleshy root growth stage, but it recovered after the samples were re-watered, which was just lower than that of CK, reaching 3.82% and 4.75% respectively. In 2017, the net photosynthetic rates of WD1 and WD4 increased by 5.51% and 1.82% respectively compared with CK at vegetative stage; other treatments were reduced from 2.2–15.6%. The net photosynthetic rate of WD1 increased by 8.48% compared with CK at fleshy root growth stage; other treatments were reduced from 0.5–19.4%. The net photosynthetic rate decreased more significantly as the degree of water deficit intensified.
That indicates that though affected by water deficit at vegetative stage and fleshy root growth stage, the net photosynthetic rate recovers after Isatis indigotica is re-watered at fleshy root growth stage and fleshy root maturity, showing a greater water compensation effect.
3.3 Dry Matter Accumulation
Dry matter accumulation in different growth stages of Isatis indigotica under drip irrigation under mulch is shown in Fig. 3. The dry matter accumulation process of Isatis indigotica showed a “S”-shaped curve during the whole growth period. From seedling stage to vegetative stage, the plant growth process and the dry matter accumulation were slow; from vegetative stage to fleshy root growth stage, the plant grew vigorously and the dry matter accumulation was rapid; from fleshy root growth stage to fleshy root maturity, the plant growth was slow while dry matter accumulation increased steadily. In 2016, the highest dry matter accumulation per plant under WD1 was 7.38 (g/plant) in vegetative period, and there was no significant difference between WD1 and CK (p > 0.05) while other treatments were lower than CK. The dry matter under WD3, WD7, WD8 and WD9 treatments was significantly lower than CK, and the reduction ranged from 12.8–17.6%. In fleshy root growth period, there was no significant difference between WD1, WD2, WD4 and CK, but other treatments were inferior to CK in terms of dry matter accumulation. Those of WD1 and WD4 treatments significantly increased 8.1% and 6.1% compared with that of CK at fleshy root maturity, and those of moderate and severe water deficit treatments were significantly lower than that of CK, with a decrease ranging from 12.5–31.1%. In 2017, there was no significant difference between mild and moderate water deficit treatments, and CK in vegetative period. WD3, WD8 and WD9 with severe water deficit significantly decreased by 18.3%, 23.1% and 24.4% compared with CK; WD1, WD2 and WD4 had no significant difference with CK in fleshy root growth stage in terms of dry matter accumulation, while the other treatments significantly decreased compared with CK; WD1 increased by 4.0% compared with CK in fleshy root maturity, but there was no significant difference.
The dry matter accumulation processes of Isatis indigotica in 2016 and 2017 were similar. There was no significant difference in dry matter accumulation of Isatis indigotica during the growth stage due to no water deficit treatment at seedling stage (p > 0.05). However, the dry matter accumulations of Isatis indigotica in vegetative stage and fleshy root growth stage under water deficit treatment were significantly different; There were significant differences in the growth stage (p < 0.05), because the effect of water on the physiological growth of crops had been formed in the fleshy root maturity stage. The dry matter accumulation of Isatis indigotica showed a “S”-shaped curve with time passing by during the whole growth period, but severe water deficit would significantly reduce the dry matter accumulation of Isatis indigotica, while timely and appropriate mild water deficit would not significantly affect the dry matter accumulation, and was conducive to improving the water use efficiency of crops.
3.4 Yield
In 2016, yield of Isatis indigotica of CK was the highest under the condition of full irrigation (up to 8,315.58kg /hm2) in the whole growth period, and yield of Isatis indigotica under other water deficit treatments was reduced (Table 2). WD1 and WD4 showed no significant difference compared with CK (p > 0.05), reaching 8,239.56kg/hm2 and 8,215.52 kg/hm2 respectively. WD3, WD7, WD8 and WD9 treated with moderate and severe water deficit showed significant difference compared with CK (p < 0.05), decreased respectively by 17.09 %, 16.23 %, 36.13 % and 37.12 %.
In 2017, yield of Isatis indigotica was the highest under the condition of full irrigation WD1 (up to 8,390.80kg /hm2), and yield of Isatis indigotica under other water deficit treatments was reduced. There was no significant difference between yield of Isatis indigotica under WD4 and CK, reaching 8,235.32kg /hm2. WD3, WD7, WD8 and WD9 treated with moderate and severe water deficit showed significant difference compared with CK, decreased respectively by 18.29%%, 18.05%, 31.67% and 33.43%. The effects of water deficit on yield of Isatis indigotica were similar in 2016 and 2017. WD1 and WD4 had no significant difference compared with CK. However, moderate and severe water deficit significantly reduced the yield compared with CK.
3.5 Water use efficiency and irrigation water use efficiency
Water use efficiency (WUE) of WD1 and WD4 under the condition of mild water deficit was significantly different (p > 0.05). In 2016, WUE of WD1 and WD4 showed the most significance (p < 0.05), which was 8.00% and 8.45% higher than CK. WUE of other treatments was reduced. WD2, WD3, WD7, WD8 and WD9 were significantly lower than CK in terms of WUE, which decreased by 8.01%, 7.69%, 7.45%, 24.38% and 25.41%, respectively. In 2017, WUE of WD1 treatment was the highest (23.62kg/hm2•mm), 8.35% higher than that under CK, followed by WD4, 6.74% higher than CK. The effect of mild water deficit on WUE of Isatis was significant.
The irrigation water use efficiency of WD5, WD8 and WD9 treatment decreased significantly, with a rate of 4.19%, 3.91% and 3.73%, respectively. Mild water regulation deficit during Vegetative stage significantly improved the irrigation water use efficiency, while moderate and severe water regulation deficit during Vegetative stage significantly reduced the irrigation water use efficiency.
Table 2
Effect of Different Water Deficit on Yield and WUE of Isatis indigotica
Year
|
Treatment
|
Precipitation/mm
|
Total Water Consumption/mm
|
Yield/kg·hm− 2
|
IWUE
kg/hm2·mm
|
WUE/kg·hm− 2·mm− 1
|
2016
|
CK
|
185.8
|
374.04
|
8315.58a
|
50.94b
|
22.23b
|
WD1
|
185.8
|
343.28
|
8239.56a
|
54.04a
|
24.01a
|
WD2
|
185.8
|
353.05
|
7219.67b
|
49.03c
|
20.45d
|
WD3
|
185.8
|
335.92
|
6894.60d
|
51.03b
|
20.52d
|
WD4
|
185.8
|
340.85
|
8215.52a
|
54.67a
|
24.11a
|
WD5
|
185.8
|
346.06
|
7164.91bc
|
49.32c
|
20.70cd
|
WD6
|
185.8
|
338.38
|
7083.69c
|
49.68c
|
20.93c
|
WD7
|
185.8
|
338.56
|
6965.85d
|
50.57b
|
20.57d
|
WD8
|
185.8
|
316.03
|
5311.57e
|
46.10d
|
16.81e
|
WD9
|
185.8
|
315.27
|
5228.54e
|
46.48d
|
16.58e
|
2017
|
CK
|
196.5
|
381.75
|
8322.25a
|
50.36bc
|
21.80b
|
WD1
|
196.5
|
355.25
|
8390.80a
|
54.57a
|
23.62a
|
WD2
|
196.5
|
366.06
|
7462.24b
|
49.89c
|
20.39c
|
WD3
|
196.5
|
343.62
|
6800.36e
|
51.47b
|
19.79d
|
WD4
|
196.5
|
353.93
|
8235.32a
|
54.03a
|
23.27a
|
WD5
|
196.5
|
357.65
|
7051.11c
|
48.25d
|
19.72d
|
WD6
|
196.5
|
348.66
|
6981.71cd
|
49.11cd
|
20.02cd
|
WD7
|
196.5
|
347.35
|
6819.79de
|
50.20bc
|
19.63d
|
WD8
|
196.5
|
329.02
|
5686.71f
|
48.39d
|
17.28e
|
WD9
|
196.5
|
327.78
|
5539.79f
|
48.48d
|
16.90e
|
Note: Within each column different small letters mean significant difference at p < 0.05. |
3.5 Quality
Indigo, indirubin and (R, S)-goitrin content of Isatis indigotica were affected by water deficit during the vegetative stage and fleshy root growth stage (Table 3). Combining the two-year (2016 and 2017) experimental data, the water deficit treatment can increase the content of active ingredients and improve the quality of Isatis indigotica. However, the severe water deficit is not conducive to accumulation of active ingredients. Due to the impact of (R, S)-goitrin under water deficit treatment, the content (R, S)-goitrin of WD3 and WD8, WD9 decreased significantly by 8.59–13.23% compared with CK (p < 0.05). In addition, the content (R,S)-goitrin was increased in fleshy root growth stage and vegetative stage under water deficit. WD4 and WD5 significantly increased by 5.43–7.94%. Because of impact of indirubin on water deficit treatment, the mild and moderate continuous deficit is beneficial to the increase of indirubin content. The differences between WD4, WD5, WD6 and WD7 compared with CK were significant, with the increase of 3.6–9.9%. The content of effective components was increased and the quality improved under water deficit treatments. The content of effective components under each treatment reached the pharmacopoeia standard. The quality of Isatis indigotica was improved as water deficit intensifies. However, severe water deficit is not conducive to the accumulation of effective components.
Table 3
Impact of Different Water Deficit Treatments on Quality of Isatis indigotica
|
2016
|
2017
|
Treatment
|
Indigo
(mg·kg− 1)
|
Indirubin
(mg·kg− 1)
|
(R, S)-goitrin
(mg·g− 1)
|
Indigo
(mg·kg− 1)
|
Indirubin
(mg·kg− 1)
|
(R, S)-goitrin
(mg·g− 1)
|
CK
|
6.117cd
|
9.663c
|
0.237bc
|
6.121d
|
9.687c
|
0.239c
|
WD1
|
6.153c
|
9.653c
|
0.230c
|
6.139d
|
9.616cd
|
0.234cd
|
WD2
|
6.093d
|
9.510d
|
0.231bc
|
6.109d
|
9.594d
|
0.232d
|
WD3
|
5.737e
|
8.487e
|
0.216d
|
5.722e
|
8.474e
|
0.212e
|
WD4
|
6.463b
|
9.690c
|
0.251a
|
6.458b
|
9.788b
|
0.252b
|
WD5
|
6.670a
|
10.173a
|
0.253a
|
6.733a
|
10.195a
|
0.258a
|
WD6
|
6.443b
|
9.813b
|
0.240b
|
6.415bc
|
9.854b
|
0.249b
|
WD7
|
6.410b
|
9.807b
|
0.239bc
|
6.344c
|
9.666cd
|
0.238cd
|
WD8
|
5.733e
|
8.440e
|
0.208de
|
5.741e
|
8.463e
|
0.210e
|
WD9
|
5.713e
|
8.463e
|
0.205e
|
5.715e
|
8.412e
|
0.208e
|
Note: Within each column different small letters mean significant difference at p < 0.05. |