Increasing green roof plant drought tolerance through substrate modification and the use of water retention gels

Figures & data

Table 1 Factorial design of drought experiment.

	Brick size
	80% by volume
	Small = 2–5 mm
Species	Large = 4–15 mm	Polyacrylamide gel (1% v/v)	Sedum	Drought treatments (Six replicates of each)	Total number of pots
Festuca ovina	Small	Yes	Yes	No drought & 25 days	12
			No	No drought & 25 days	12
		No	Yes	No drought & 25 days	12
			No	No drought & 25 days	12
	Large	Yes	Yes	No drought & 25 days	12
			No	No drought & 25 days	12
		No	Yes	No drought & 25 days	12
			No	No drought & 25 days	12
Linaria vulgaris	Small	Yes	Yes	No drought & 25 days	12
			No	No drought & 25 days	12
		No	Yes	No drought & 25 days	12
			No	No drought & 25 days	12
	Large	Yes	Yes	No drought & 25 days	12
			No	No drought & 25 days	12
		No	Yes	No drought & 25 days	12
			No	No drought & 25 days	12
Total					192

Table 2 Wilt Index scoring system for F. ovina and L. vulgaris. Adapted from (Engelbrecht et al. 2007).

Wilting index score	F. ovina	L. vulgaris
0	Dead (no resprouting after watering)	Dead (no resprouting after watering)
1	Fully wilted (horizontal form)	Fully wilted and nearly dead (all leaves dead but stem still alive)
2	Severely wilted (only a few vertical grass blades showing)	Severely wilted (strong change in majority of leaf angle/curling up, stem still alive)
3	Half wilted (50% spilt)	Half wilted (some change in leaf angle, some healthy leaves still)
4	Sporadic/slightly wilted (relatively few grass blades wilted)	Sporadic/slightly wilted (some changes to leaf angle but no folding or rolling of leaf)
5	No wilting	No wilting

Table 3 Main factor effects and means (three-way ANOVA) for (a) F. ovina shoot biomass after two drought treatments and (b) L. vulgaris biomass after two drought treatments. Main factor means and the % change are shown between the two levels within that factor (e.g. absence vs. presence of SwellGel). Significant factorial interactions are also shown.

a)

Mean factor means of F. ovina shoot biomass (g) after four drought treatments

Factors

Significant factor interactions

Polyacrylamide gel

Brick Size

Sedum

PG:Brick

PG:Sedum

Brick:Sedum

PG:Brick:Sedum

No

Yes

%

P-value

Small

Large

%

P-value

No

Yes

%

P-Value

P-Value

Drought

Control

2.41

2.8

+16.2

^*

2.98

2.22

− 25.5

^*^*^*

3.21

1.2

− 62.6

^*^*^*

^*

25 Days

2.25

3.02

+34.2

^*^*^*

3.16

2.11

− 33.2

^*^*^*

3.22

2.05

− 36.3

^*^*^*

^*^*

^*

b)

Mean factor means of L. vulgaris shoot biomass (g) after four drought treatments

Factors

Significant factor interactions

PG:Brick

PG:Sedum

Brick:Sedum

PG:Brick:Sedum

Polyacrylamide gel

Brick Size

Sedum

P-Value

No

Yes

%

P-value

Small

Large

%

P-value

No

Yes

%

P-Value

Control

1.33

1.44

+8.3

0.26

1.7

1.07

− 37.1

^*^*^*

1.57

1.2

− 23.6

^*^*^*

Drought

25 Days

0.57

0.82

+42.1

^*^*^*

0.75

0.56

− 25.3

^*^*

0.78

0.53

− 32.1

^*^*^*

Notes: Statistical significances of P-values: ^*p < 0.01, ^**p < 0.001, ^***p < 0.0001. Statistical significances were calculated from three-way ANOVA. Abbreviation for factor interactions is SG = SwellGel.

Table 4 Main factor effects and means (three-way ANOVA) for (a) F. ovina control pot root biomass (g), (b) L. vulgaris control pot root biomass (g), (c) F. ovina root:shoot ratio, (d) L. vulgaris root:shoot ratio. Main factor means and the % change are shown between the two levels within that factor (e.g. absence vs. presence of SwellGel). Significant factorial interactions are also shown when present.

				Factor means of F. ovina root biomass (g) from control drought pots
				Polyacrylamide gel		Brick Size		Sedum
Factor	Df	F-value	P-value	No	Yes	Small	Large	No	Yes	% Change ( ± SE, n = 64)
a)
Polyacrylamide gel	1	31.6	^*^*^*	2.40	1.53					− 36.3 ± 9.3
Brick Size	1	12.8	^*^*^*			2.23	1.66			− 25.6 ± 12.6
Sedum	1	3.2	0.08					2.09	1.81	− 13.4 ± 8.0
PG:Brick	1	8.2	^*^*
b)				Factor means of L. vulgaris root biomass (g) from control drought pots
Polyacrylamide gel	1	31.6	^*^*^*	1.18	0.75					− 36.4 ± 6.8
Brick Size	1	0.8	0.39			1.00	0.93			− 7.0 ± 4.8
Sedum	1	0.02	0.90					0.96	0.97	+1.0 ± 7.3
c)				Factor means of F. ovina root:shoot ratio from control drought pots
Polyacrylamide gel	1	62.0	^*^*^*	1.04	0.58					− 44.2 ± 4.5
Brick Size	1	1.2	0.27			0.78	0.83			+6.4 ± 12.5
Sedum	1	21.7	^*^*^*					0.68	0.94	+38.2 ± 13.2
PG:Brick:Sedum	1	5.4	^*
d)				Factor means of L. vulgaris root:shoot ratio from control drought pots
Polyacrylamide gel	1	49.6	^*^*^*	0.98	0.56					− 42.9 ± 4.7
Brick Size	1	27.2	^*^*^*			0.61	0.92			+50.8 ± 9.8
Sedum	1	18.2	^*^*^*					0.64	0.89	+39.1 ± 10.3
PG:Brick	1	5.4	^*
Brick:Sedum	1	4.9	^*
PG:Brick:Sedum	1	7.2	^*

Notes: Statistical significances of P-values: ^* p < 0.01, ^**p < 0.001, ^*** p < 0.0001. Statistical significances were calculated from three-way ANOVA. Abbreviation for factor interactions is SG = SwellGel.

Figure 1 (a) Mean Fv/Fm values for F. ovina during long drought (25 days). Black dashed line indicates end of drought and start of two week watered recovery period. Error bars are ±  one standard error. For polyacrylamide (PG) and Sedum, “N” and “Y” refer to “yes” and “no”. For Brick, “L” and “S” are “large” and “small” particle size. (b) Mean Fv/Fm values for L. vulgaris during long drought (25 days). Black dashed line indicates end of drought and start of two week watered recovery period. Error bars are ±  one standard error. For polyacrylamide (PG) and Sedum, “N” and “Y” refer to “yes” and “no”. For Brick, “L” and “S” are “large” and “small” particle size.

Figure 2 (a) Mean wilt index values for F. ovina during long drought (25 days). Black dashed line indicates end of drought and start of two week watered recovery period. Error bars are ±  one standard error. For polyacrylamide (PG) and Sedum, “N” and “Y” refer to “yes” and “no”. For Brick, “L” and “S” are “large” and “small” particle size. (b) Mean Fv/Fm values for L. vulgaris during long drought (25 days). Black dashed line indicates end of drought and start of two week watered recovery period. Error bars are ±  one standard error. For polyacrylamide (PG) and Sedum, “N” and “Y” refer to “yes” and “no”. For Brick, “L” and “S” are “large” and “small” particle size.

Engelbrecht, B.M.J., Tyree, M.T., and Kursar, T.A., 2007. Visual assessment of wilting as a measure of leaf water potential and seedling drought survival. Journal of Tropical Ecology, 23, 497–500.

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