Biostimulant type and application method affect plant growth and mineral nutrient compositions of kale cultivars grown in a high tunnel

Figures (3) Tables (7)

Figure 1.
Daily minimum, average, and maximum air temperature in the high tunnel recorded using a temperature and RH sensor (HOBO S-THB-M002; Onset Computer Corp) connected to a data logger (HOBO Micro Station H21-002; Onset Computer Corp., Bourne, MA, USA) at hourly intervals from November 12, 2024, to March 25, 2025. The daily average air temperature outdoors for Starkville was obtained from the USDA Natural Resources Conservation Service website.
Figure 2.
Daily minimum, average, and maximum RH in the high tunnel recorded using a temperature and RH sensor (HOBO S-THB-M002; Onset Computer Corp) connected to a data logger (HOBO Micro Station H21-002; Onset Computer Corp., Bourne, MA, USA) at hourly intervals from November 12, 2024, to March 25, 2025. The daily average RH outdoors for Starkville was obtained from the USDA Natural Resources Conservation Service website.
Figure 3.
Daily light integral (DLI) inside the high tunnel from November 12, 2024, to March 25, 2025. DLI was calculated by multiplying the daily average of PAR by 0.0864. The PAR under the high tunnel was recorded hourly by a quantum sensor (HOBO S-LIA-M003; Onset Computer Corp.) connected to a data logger (HOBO Micro Station H21-002; Onset Computer Corp., Bourne, MA, USA).

Cultivar	34 DAT Plant height^z (cm)	34 DAT Crown diameter (cm)	35 DAT SPAD	99 DAT Plant height (cm)	99 DAT Crown diameter (cm)	99 DAT SPAD
Black Magic	15.5 bc	31.4 b	77.5 a	32.3 b	46.8 bc	84.8 a
Redbor	15.00 c	27.5 c	47.2 d	31.2 b	45.5 c	56.5 c
Red Russian	18.1 a	44.6 a	56.0 b	39.7 a	61.2 a	62.8 b
Westlandse Winter	15.9 b	32.2 b	50.5 c	32.4 b	48.2 b	52.4 d
p value	< 0.0001	< 0.0001	< 0.0001	< 0.0001	< 0.0001	< 0.0001
^z Different lowercase letters suggest significant difference among means within a column according to Tukey's HSD test at p < 0.05.

Table 1.

Variation of plant height, crown diameter, and leaf SPAD measured on two dates among kale cultivars.

Biostimulant	34 DAT Plant height^z (cm)	34 DAT Crown diameter (cm)	35 DAT SPAD	99 DAT Plant height (cm)	99 DAT Crown diameter (cm)	99 DAT SPAD	Sulfur (mg·g⁻¹)
Control	15.8 b	34.0	57.1	33.4 b	51.4 ab	63.2	4.66 a
Continuum V2	15.6 b	33.7	58.1	33.1 b	49.4 c	63.9	4.42 ab
Dune	15.8 b	33.7	57.6	33.9 ab	49.6 bc	65.1	4.18 b
Kelpak Maxx	17.4 a	34.5	58.6	35.5 a	51.6 a	64.3	4.53 ab
p value	< 0.0001	0.38	0.76	0.0012	0.002	0.40	0.013
^z Different lowercase letters suggest significant difference among means within a column according to Tukey's HSD test at p < 0.05.

Table 2.

Plant height, crown diameter, and leaf SPAD measured on two dates as affected by the main effect of biostimulant type.

Cultivar	L^z	a*	b*
Black Magic	26.00 c	−4.10 b	3.00 c
Redbor	19.15 d	9.28 a	−1.51 d
Red Russian	34.77 b	−6.81 c	8.10 b
Westlandse Winter	37.75 a	−10.15 d	17.84 a
p value	< 0.0001	< 0.0001	< 0.0001
^z Different lowercase letters suggest significant difference among means within a column according to Tukey's HSD test at p < 0.05.

Table 3.

Kale cultivars varied in leaf coloration measured at 120 DAT.

Cultivar	February 2025				March 2025
Cultivar	Total fresh leaf weight^z (g·plant⁻¹)	Total dry leaf weight (g·plant⁻¹)	Individual fresh leaf weight (g·leaf⁻¹)	Individual dry leaf weight (g·leaf⁻¹)	Total fresh weight (g·plant⁻¹)	Total dry weight (g·plant⁻¹)	Individual fresh leaf weight (g·leaf⁻¹)	Individual dry leaf weight (g·leaf⁻¹)
Black Magic	290.0 c	42.0 c	12.4 b	1.76 b	198.4 b	36.2 b	6.50 c	1.21 c
Redbor	264.8 c	39.2 c	12.0 b	1.77 b	233.9 b	39.9 b	6.33 c	1.07 c
Red Russian	596.8 a	80.4 a	24.3 a	3.22 a	374.3 a	61.2 a	10.8 b	1.79 b
Westlandse Winter	502.6 b	68.9 b	23.5 a	3.20 a	329.9 a	60.1 a	15.8 a	2.87 a
p value	< 0.0001	< 0.0001	< 0.0001	< 0.0001	< 0.0001	< 0.0001	< 0.0001	< 0.0001
^z Different lowercase letters suggest significant difference among means within a column according to Tukey's HSD test at p < 0.05.

Table 4.

Kale cultivars varied in total fresh and dry leaf weights and individual fresh and dry leaf weights harvested in February and March 2025.

Cultivars	Nitrogen^z (mg·g⁻¹)	Phosphorus (mg·g⁻¹)	Potassium (mg·g⁻¹)	Calcium (mg·g⁻¹)	Magnesium (mg·g⁻¹)	Sulfur (mg·g⁻¹)
Black Magic	47.3 a	4.63 a	29.3 a	14.84 a	3.10 a	5.36 a
Redbor	41.1 b	4.29 b	25.7 b	13.17 b	2.84 b	3.97 c
Red Russian	33.0 d	3.88 c	29.6 a	9.45 c	1.78 d	3.78 c
Westlandse Winter	36.3 c	4.28 b	29.6 a	12.92 b	2.51 c	4.70 b
p value	< 0.0001	< 0.0001	< 0.0001	< 0.0001	< 0.0001	< 0.0001
^z Different lowercase letters suggest significant difference among means within a column according to Tukey's HSD test at p < 0.05.

Table 5.

Kale cultivars varied in macronutrient concentrations.

Cultivars	Copper^z (µg·g⁻¹)	Zinc (µg·g⁻¹)	Manganese (µg·g⁻¹)	Iron (µg·g⁻¹)	Boron (µg·g⁻¹)
Black Magic	1.82 a	63.0 a	83.3 b	142.3 a	41.4 a
Redbor	1.76 a	67.6 a	98.7 a	134.5 a	40.0 a
Red Russian	1.01 b	44.7 b	40.2 d	48.8 c	26.8 b
Westlandse Winter	1.24 b	43.6 b	47.9 c	79.5 b	27.9 b
p value	< 0.0001	< 0.0001	< 0.0001	< 0.0001	< 0.0001
^z Different lowercase letters suggest significant difference among means within a column according to Tukey's HSD test at p < 0.05.

Table 6.

Kale cultivars varied in macronutrient concentrations.

Biostimulant	Application method^{z, y}	Iron (µg·g⁻¹)
Control	FS+SD	99.78 ab
	FS	108.2 ab
	SD	93.53 b
Continuum V2	FS+SD	80.81 b
	FS	117.1 ab
	SD	104.4 ab
Dune	FS+SD	90.82 b
	FS	139.3 a
	SD	107.2 ab
Kelpak Maxx	FS+SD	103.3 ab
	FS	88.7 b
	SD	78.49 b
p value		0.031
^z Different lowercase letters suggest significant difference among means within a column according to Tukey's HSD test at p < 0.05. ^y Each biostimulant treatment was applied through three methods, including foliar spray until runoff (FS), substrate drench (SD) at a rate of 120 mL per plant, and a combination of both (FS+SD).

Table 7.

Iron concentration as affected by the interaction between biostimulant type and application method.