BROCCOLI (Brassica oleracea var. italica) HEAD INITIATION UNDER FIELD CONDITIONS

A two–year study on the influence of temperature on broccoli head initiation was carried out at the “Marcelin” experimental station of the Poznań University of Life Sciences. In each year of the study, plants were planted in the field at four dates. The evaluation of the developmental phase of the broccoli shoot apex was based on the analysis of microscope slides. The date of head initiation was assumed as the day on which the first of the examined apices were found to be at the early generative phase. The plant characteristics (number of leaves, leaf area and stem diameter) on the date of initiation were also determined. Variation in length of the period from planting to head initiation was found both between dates of planting and between experimental years. The shortest period from planting to initiation was when the plants were planted in April and June (17-18 days) in the first year and the longest one for planting in April in the first year of the study (29 days). The length of the period from planting to head initiation depended on mean daily air temperature. The higher the temperature was, the shorter was the period.


INTRODUCTION
The transition from the vegetative to the generative phase in plants is a complex morphogenetic process. It occurs as a result of induction of flowering, which may be caused by light, temperature, water availability or chemical agents such as hormones and growth regulators (K o p c e w i c z and C e n t k o ws k a , 1980). The environmental factors are perceived by different parts of the plant. Photoperiod and irra-diance are perceived mainly by mature leaves in intact plants. Temperature is perceived by all plant parts, although low temperature (vernalisation) is often perceived mainly by the shoot apex. Water availability is perceived by the root system (B e r n i e r et al. 1993). Flower induction follows immediately after the juvenile phase in which the plant shows no sensitivity to stimuli of flowering (K o z u m p l i k and C h r i s t e , 1972). The length of the juvenile phase is specific to species and variety (W i e b e , 1994).
According to M i l l e r et. al (1985) and W u r r et al. (1995), the main factor for the transition from the vegetative to the generative phase of broccoli plants is the temperature. According M o u r ã o and B r i t o (2000), at 21.5 o C initiation occurred in broccoli after 32 days, and at 9 o C after 88 days after planting. F e ll o w s (1997) reported that at temperatures of 5, 10, 15, 20 o C initiation occurred successively after 96, 51, 36 and 64 days. F u j i m e et al. (1988) found that the vernalisation effect of temperature also depends on the length of the day. According to these authors, effectiveness of lower temperatures is higher at 16 hours of the day than at 8 hours. The aim of this study was to determine the effect of temperature on broccoli head initiation when grown at four planting dates in two consecutive years in the field. This paper is an extension of a previously published study of the first author (K a ł u ż e w i c z et al. 2002).

MATERIALS AND METHODS
The experiment was carried out in two successive years at the "Marcelin" Experimental Station of the Poznań University of Life Sciences. The study comprised one cultivar of broccoli -'Fiesta'. Transplants were planted out in the field at four dates (21 April, 10 May, 12 June, and 9 July in 1997 as well as 8 April, 12 May, 19 June, and 21 July in 1998). The trial was established in a randomised block design with four replications. In each plot, one hundred transplants were planted at the 4-leaf stage at a spacing of 0.5×0.5 m.
The experiment was established on podzolic soil; the plough layer was loamy sand underlying sandy loam. The soil was tested before planting and 100 kg×ha -1 of P 2 O 5 , 200-250 kg×ha -1 of K 2 O and 60 kg·ha -1 of nitrogen were applied before soil preparation. Additionally, nitrogen (150 kg×ha -1 ) was applied in three doses during the growing season. Plants were watered when the soil water potential had exceeded -0.04 MPa.
From the first week after planting until the time when a head of about 1 mm in diameter was observed, every 3-4 days, 4-8 apices were collected in order to estimate their stage of development. The obtained microscopic slides were subjected to histological assessment. The date of head initiation was assumed as the day on which the first of the examined apices were found to be at the early generative phase. The date of initiation was determined for each date of planting. A description of the developmental phases of broccoli apices was presented in the previous publication (K ał u ż e w i c z et al. 2002).
Meteorological data were obtained from the Meteorological Station located 2 kilometers from the Experimental Station. The sum of day-degrees was calculated from the base temperature of 0 o C.

RESULTS AND DISCUSSION
In both years, of the study found large variations in length of the period from planting to broccoli head initiation between the dates of planting were found (Table 1). These results are contrary to the results of our previous study (K a ł u ż e w i c z et al. 2002) in which the duration from planting to head initiation was the same at each date of planting.
The biggest differences between years in the duration of the period from planting to head initiation were in the case of the April date of planting, while this duration was the shortest for the May date of planting.
The time to head initiation depended on mean daily air temperature -the higher it was, the shorter the time to head initiation was (Fig. 1). A remarkable variety in the time from planting to head initiation was also confirmed by G r e v s e n and O l e s e n (1999). These authors conducted a field experiment in which they used three cultivars with different growing periods. The plants were planted in three consecutive years and at four dates every year. Initiation time was considered to be the day when the head diameter was 0.6 mm. For the earliest cultivar, the time from planting to initiation was from 14 to 43 days, for the latest one -from 19 to 49 days. For all the cultivars, the longest time from planting to initiation was observed at the earliest planting date.
In our study, the plants planted at the earliest date had the lowest number of leaves as well as the smallest leaf area and stem diameter on the date of head initiation ( Table 2). The later the date of planting was, the higher the values of the above features were. Significant correlation coefficients were observed between the sum of day-degrees, mean daily temperature, number of leaves and leaf area on the day of head initiation (Table 3). The correlation coefficient between the sum of day-degrees and stem diameter was also highly significant. However, the correlation coefficient between mean daily temperature and stem diameter was insignificant.
Moreover, it was observed in our study that temperature sum during the time from planting to initiation was the lowest when the plants were planted in April in the first year of the study and the highest when they were planted in July in the first year of the study (Table 4). Such a variation in temperature sum in the time from planting to initiation may be related to the reaction of broccoli to different temperature ranges. Ac Effectiveness of initiation temperature depends on the phase in which it is applied. According to F o n t e s et al. (1967), broccoli head initiation did not occur when low temperature (4.4 o C) was used for three-week-old plants. When four-week-old plants were cooled, initiation was observed in 22-25 % of plants, depending on the cultivar. Initiation occurred in all plants when low temperature was applied to five-week-old plants. The above-mentioned authors reported that devernalisation could be observed in broccoli. F o n t e s et al. (1967) proved that moving plants to temperature of 22-26 o C after exposure to low temperatures eliminated the effects of their application. This can explain the differences in the time from planting to initiation. Under field conditions, there is considerable variation in temperature, especially between day and night. Therefore, it is difficult to determine how long the plants must be subjected to low temperatures for vernalisation to occur, as well as how long high temperature must operate to eliminate the effect.
Broccoli is believed to have a juvenile phase (W a r n e , 1961; W i e b e , 1989). During this period, it is not possible to induce head initiation even under optimum temperature conditions. It is very difficult to determine the end of the juvenile phase, because so far researchers have not found the growth features making it possible to say that broccoli plants have finished the juvenile phase. Some researchers have tried to determine the end of the juvenile phase for cauliflower but their results differed. According to A d i t y a and F o r d h a m (1995), the juvenile phase in cauliflower may finish at the 7-9 leaf stage, while W u r r and F e l l o w s (2000) claim that it ends when plants have 10-22 leaves, depending on the cultivar.