Relación C/N equilibrada

Germination experiments in controlled environments allow testing a wide range of temperatures, where no other factor can affect the response, and are therefore a basic requirement for cardinal temperature estimation (Bonhomme, 2000). However problems may still exist when germination experiments are used to determine cardinal temperatures. Angus et al. (1981) emphasized that not all seeds that germinate will result in emerged plants. Therefore care must be taken when the extrapolation of these results is used to other crop development events.

Thompson and Fox (1976) and Kondra et al. (1983) showed that germination responses of vegetables seeds to temperature were different among species but not among cultivars. Kondra et al. (1983) observed that cultivars of B. campestris had lower germination percentages than B. napus at temperatures below 9oC and at 25oC. Also B. napus cultivars had a faster germination rate at low temperatures (2, 3 and 5oC) while B. campestris cultivars were faster at higher temperatures. Wilson et al. (1992) showed that turnip cultivars were fastest to germinate followed by rape and kale cultivars in a study that tested the germination of these crops under a range of temperatures from 5 to 35oC. Vigil et al. (1997) obtained similar base temperatures of 0.9oC for the emergence of three spring and two winter canola cultivars in an experiment where temperatures tested were 0, 2, 4 and 16oC. However, this can change in species that have had an intense process of selection (e.g. maize and wheat) (Thompson and Fox, 1976).

Tokumasu et al. (1985) observed three groups of germination response to temperature for brassicas such as Chinese, purple and leaf mustards, Brussels sprouts, red and Chinese

34 cabbages, hakuran, kohl rabi, turnip and rape. The first group was composed of species that were sensitive to high (45oC) temperatures; the second group species were sensitive to low (5oC) temperatures and the third group was made of species that were sensitive to both extreme temperatures.

Another way to estimate cardinal temperatures is through the evaluation of development stages such as emergence (Moot et al., 2000; Black et al., 2006; Lonati, 2009; Wang et al., 2009), leaf appearance (Nanda et al., 1995; Adams et al., 2005) and anthesis or flowering in grain crops (Gilmore et al., 1958; Ellis et al., 1992; Slafer and Rawson, 1995; Wilson et al., 1995; Craufurd et al., 1998; Wang and Engel, 1998; Streck et al., 2008). Several studies have been done with maize and wheat but only a few were on brassicas (Hodgson, 1978a; Nanda et al., 1995; Nanda et al., 1996b; Vigil et al., 1997).

Hodgson (1978a) determined the base temperature and thermal time requirements to four development phases of ‘Torch’ (B. campestris) and ‘Midas’ (B. napus) annual rapeseed in a field experiment. ‘Torch’ had a base temperature of 7oC for the period of planting to bud appearance, 3oC from bud appearance to first flowering, 3oC from first flowering to pod formation and 4oC from pod formation to grain filling. ‘Midas’ had base temperatures of 1, 1, 6 and 1oC for each development period, respectively. The data presented by Hodgson (1978a) had a different base temperature for only one of the four stages observed for both cultivars. This contrasted with Wang (1960) and Monteith (1981) who showed that cardinal temperatures may differ with growth stages or cultivars.

Morrison et al. (1989) determined a base temperature for the growth of ‘Westar’ summer rape (B. napus) of 5oC in growth cabinets over a range of temperatures from 10 to 25oC and a 16 h photoperiod. The discrepancies between Morrison et al. (1989) and Hodgson (1978a) were attributed to the fact that winter sowing dates used by Hodgson (1978a) may have increased the vegetative phase duration, which decreased the rate and influenced the base temperature estimation (Morrison et al., 1989). Furthermore these

35 could have resulted from different ranges of temperatures tested between the two studies.

The results presented by Morrison et al. (1989) showed agreement between responses to temperature obtained in growth cabinets and in field experiments. This supported the use of a base temperature of 5oC for field data calculations. Nanda et al. (1996a) obtained a similar base temperature to Morrison et al. (1989) of 5-5.5oC between emergence and bud appearance stage of rapeseed. For leaf appearance Nanda et al. (1995) estimated a base temperature 5.7oC for B. campestris, 6.6oC for B. juncea, 4.6oC for B. napus and 5.3oC for B. carinata over a range of temperature from 15 to 24oC.

The studies undertaken by Morrison et al. (1989), Nanda et al. (1995) and Nanda et al. (1996b), among others, have focussed mainly on oilseed brassicas whereas few studies have been done with bulb brassicas. Adams et al. (2005) estimated the base temperature for leaf appearance of ‘Goliath’ rape, ‘Green Globe’ turnip and ‘Gruner’ and ‘Kestrel’ kales using two methods. Linear regressions between leaf appearance rates and mean air temperature resulted in a base temperature of 5.5oC while the least-variable method resulted in a base temperature of 4oC. However problems such as poor quality of fitting of the linear function, the narrow range of temperatures used, a high lowest temperature recorded during the exp=erimental period (10oC) and the least-variable method make these results questionable.

Collie and McKenzie (1998) used a base temperature of 0oC to calculate thermal time accumulation for turnips dry matter production. This same temperature was used by Wilson et al. (2004) and Zyskowski et al. (2004) when a crop model for yield prediction of kale and ‘Pasja’ turnip was presented. The discrepancy in the data for cardinal temperatures of brassicas is mainly related to methods of estimation, ranges of temperatures tested and species used. Therefore, determination of the base temperature is a focus of this research.

36 2.6.3.1 Thermal time requirements

Thermal time (Tt) requirements between two developmental stages are used to express the relationship between temperature and plant development (Arnold and Monteith, 1974). Several studies that have reported Tt requirements of different crops and Angus et al. (1981) observed that Tt required for emergence of tropical and temperate crops was species dependent.

Håkansson and von Polgár (1984) studied rapeseed emergence according to sowing depth and soil moisture and obtained requirements that ranged from 30 to 56oCd. Vigil et al. (1997) observed differences between spring and winter canola. They observed a Tt requirement to 50% emergence of 80oCd for spring canola and a range of Tt requirements that went from 68 to 117oCd for winter canola. Collie (1997) obtained Tt requirements for 50% emergence of turnip cultivars that went from 91oCd to 125oCd, according to sowing dates. Chakwizira (2008) studied methods of phosphorus application in kale and ‘Pasja’ turnip crops and observed that the emergence of both crops started at 87oCd but 50% emergence was reached after 300oCd.