Thermal time requirements for germination of four subterranean clover cultivars

Figures & data

Table 1. Subterranean clover cultivars, subspecies, seed lot and reported mean seed weight (mg) and commercialisation year of seeds used. Letters B, S and Y denote subspecies: B = brachycalycinum, S = subterraneum and Y = for yanninicum. (Nichols* et al. 2013).

Cultivar	Subspecies	Seed lot Mean Seed weight (mg)	Reported Mean Seed weight (mg)*	Year commercialised*
Antas	B	9.2	10	1999
Denmark	S	6.4	7.1	1992
Monti	Y	9.8	9.9	2013
Narrikup	S	8.8	5.4	2009

Figure 1. Cumulative germination percentage for ‘Narrikup’ subterranean clover seeds incubated at 10°C (▴) and 2.5°C (•).

Figure 2. Number of days to T₅₀ for ‘Antas’, ‘Denmark’, ‘Monti’ and ‘Narrikup’ subterranean clover seeds estimated by Gompertz and Approx functions (r = 0.99) for individual experimental units at 2.5 (□), 5 (○), 7.5(△), 10 (+) 12.5 (X), 17.5 (▽), 20 (

), 25 (

), 30 (

) and 35°C (

) incubation temperatures. Dashed line represents the 1:1 line; 95% Ci = 0.95–0.99.

Figure 3. Mean maximum germination percentage for ‘Antas’ (○), ‘Denmark’ (△), ‘Monti’(□) and ‘Narrikup’ (

) sub clover seeds incubated at different temperatures. Error bars are the maximum standard error of differences of means. LSD _{(cultivar × temperature)} = 33.0, P = 0.02.

Figure 4. Mean cumulative germination percentage (%) for subterranean clover cultivars ‘Antas’, ‘Denmark’, ‘Monti’ and ‘Narrikup’ at different incubation temperatures 2.5 (□), 5 (○), 7.5(△), 10 (+) 12.5 (X), 17.5 (▽), 20 (

), 25 (

), 30 (

) and 35°C (⊕). Error bars are the maximum standard error of the mean for final germination percentage.

Figure 5. Mean number of days to T₅₀ for ‘Antas’ (○), ‘Denmark’ (△), ‘Monti’ (□) and ‘Narrikup’(

) sub clover seeds incubated at different temperatures. LSD (_{cultivar × temperature}) = 1.1, P < 0.001. Bars are the maximum standard error of means.

Figure 6. Germination rate (1/ days to T₅₀) for ‘Antas’ (○), ‘Denmark’ (△), ‘Monti’ (□) and ‘Narrikup’(

) subterranean clover seeds incubated at different temperatures. LSD _{(cultivar × temperature)} = 0.10, P < 0.001. Error bars are the maximum standard error of the means (SE).

Figure 7. Mean number of days to T₅₀, maximum germination percentage (%) and germination rate (1/days) for subterranean clover cultivars ‘Monti’ (yanninicum) ‘Narrikup’ (subterraneum) originated in Australia (○) and Antas’ (brachycalycinum) and ‘Denmark’ (subterraneum) originated in Italy (•) tested at Field Research Centre, Lincoln University, New Zealand. Time to T₅₀: P = 0.0002, LSD = 0.9; maximum germination (%):P = 0.08, LSD = 9.0; Rate (1/days to T₅₀): P < 0.001, LSD = 0.06. Error bars represent the standard error.

Table 2. Mean base (T_b), optimum (T_opt) and maximum (T_max) temperatures (°C) for 50% of germination estimated for sub clover seeds of ‘Antas’, ‘Denmark’, ‘Monti’ and ‘Narrikup’. SE = standard error. Ci = confidence interval.

Cultivar	Tb	Tb	Topt	Topt	Tmax	Tmax
	(°C)	95% Ci	(°C)	95% Ci	(°C)	95% Ci
‘Antas’	−4.5 (±3.1)^b	−15; 5.2	22.0 (±0.8)^ab	18.0; 27.0	36.0 (±0.3)^a	35.0; 37.0
‘Denmark’	1.6 (±0.4)^a	0.37;2.9	24.0 (±0.4)^a	23.0;26.0	35.0 (±0.3)^a	34.3; 36.1
‘Monti’	−0.8 (±0.9)^ab	−3.7; 2.2	19.0 (±1.7)^bc	13.4; 24.5	39.0 (±0.6)^b	36.9; 41.0
‘Narrikup’	0.7 (±0.5)^a	−0.9;2.3	16.0 (±0.8)^c	13.3; 18.5	39.0 (±0.9)^b	36.1;42.1
P	0.05		0.003		0.003
LSD	4.6		3.8		2.0
Mean	−0.8 (±1.9)		20.0 (±2)		37.0 (±1.0)

Table 3. Coefficients and their standard errors and coefficient of determination (R², %₎ for the linear regressions between germination rate (Y- axis, 1/ days to T₅₀) and temperatures (X- axis, °C) for subterranean clover cultivars.

Cultivar	Regression	Temperature (°C)	Intercept	Slope	R^2 (%)
‘Antas’	ascending	2.5–20	0.08 (±0.04)	0.03 (±0.002)	93
	descending	25–35	1.8 (±0.6)	−0.05 (±0.02)	84
‘Denmark’	ascending	2.5–25	−0.07 (±0.04)	0.04 (±0.004)	94
	descending	25–35	3.0 (±1.4)	−0.09 (±0.05)	77
‘Monti’	ascending	2.5–17.5	0.02 (±0.04)	0.03 (±0.002)	94
	descending	25–35	1.1 (±0.34)	−0.03 (±0.006)	84
‘Narrikup’	ascending	2.5–17.5	0.02 (±0.06)	0.03 (±0.006)	85
	descending	25–35	0.8 (±0.12)	−0.02 (±0.004)	91

Table 4. Mean sub (Tt_sub, from T_b to T_opt) and supra (Tt_sup, T_opt to T_max) optimal thermal time requirements (°Cd) for T₅₀ estimated for sub clover seeds of ‘Antas’, ‘Denmark’, ‘Monti’ and ‘Narrikup’. Values in brackets (± SE) are the mean standard error.

Cultivar	Ttsub (°Cd)	Ttsup (°Cd)	Tt (°Cd)	Tt (Tb = 0,°Cd)
‘Antas’	44.0 (±11)	21.0 (±2.5)^b	50.0 (±1.8)^a	33.0 (±8.5)
‘Denmark’	25.0 (±2.0)	12.0 (±1.4)^b	42.0 (±0.6)^b	32.0 (±3.0)
‘Monti’	38.0 (±4.0)	39.0 (±6.0)^a	49.0 (±1.9)^a	40.0 (±7.0)
‘Narrikup’	31.0 (±1.5)	48.0 (±5.0)^a	48.0 (±0.6)^a	38.0 (±4.7)
P	0.115	0.001	0.002	0.06
LSD	16.0	15.0	3.8	7.0
Mean	35.0	30.0	47.0	36.0

Table 5. Mean base (T_b), optimum (T_opt) and maximum (T_max) temperatures (^oC) estimated for germination for sub clover cultivars in New Zealand.

Cultivar	Tb	Topt	Tmax	Author
		(°C)
‘Mount Barker’	0	–	–	(Moot et al. 2000)
‘Tallarook’	0	–	–
‘Woogenellup’	0	–	–
‘Mount Barker’	0.1	13	27	(Lonati et al. 2009)
‘Dalkeith’	0.7	19	31.9	(Monks et al. 2009)
‘Leura’	0.1	20	31
‘Woogenellup’	1.3	26	35
‘Mount Barker’	0.4	20	41.6
‘Napier’	–	30	–	(Sharifiamina et al. 2017)
Mean	0.3	21.3	33.3

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