growing conditions on 26 May were from a heated glasshouse
(minimum temperature 13C) and produced very few or no spears at
12.5C; plants transferred on 2 July had been chilled for 5 weeks;
plants transferred on 10 August had been chilled for 10 weeks.
4.4. DISCUSSION
4.4.1 A MODEL OF DORMANCY IN ASPARAGUS
Chilling markedly decreased the mean time to budbreak of the first spear to emerge after winter dormancy (Figs. 4.04 and 4.12), decreased the minimum temperature at which budbreak occurred (Figs. 4.04 and 4. 12), and increased the RSGR of that spear (Fig.4.06). The position of the fust spear to grow was also affected, becoming more basipetal in response to chilling (Fig.4.05). This indicates that chilling released winter dormancy w�ch implies that a form of internal dormancy i.e, paradonMncy or endodormancy, was operating. If ecodormancy had been operating during winter, chilling would not have changed the mean time to budbreak, or RSGR or position of the first spear to grow because temperature would have been the only factor limiting the initiation of growth. Also, the minimum temperature at which growth occurred would have been constant These observations are similar to the effects of chilling on other meristems which undergo dormancy, including the buds of deciduous fruit trees (Vegis 1963, 1964; Saure 1985; Fuchigami and Nee 1987). It seems reasonable to assume, on the basis of this data, that similar mechanisms are operating in both asparagus and other
i plants which undergo internal dormancy. Therefore dormancy models developed using
data from other plants should be able to be used for asparagus.
In Vegis' s mode�, the temperature range at which budbreak occurs changes during dormancy. A si
�
ilar process occurred in the three cultivars of asparagus in these experiments (Figs\ 4.04 and 4. 12). The minimum temperature for budbreak increased to above 12.5C in early winter (26 May) for 'Rutger's Beacon' and 'Jersey Giant'. The minimum decreased to near lOC in late winter (10 August) as a few plants of each cultivar evaluated had commenced growth while being stored at 10C (e.g. case 5, Table 4.09). However, genotypic differences seem to exist as the minimum for 'UC157' was below 12.5C throughout these experiments. Data from 20C did not clearly indicate whether the maximum temperature for budbreak changed during dormancy.Although plants grew at 20C throu.ghout the experiment, the ease of budbreak changed over time. The mean number of days to budbreak tended to be higher in early (May) and mid winter (July) than at the beginning (March) and end of dormancy (August) (Fig.4.04); and RSGR of fust -spear was lower in early winter than at other times
__
(Eig.4.fi6)� These-data- indicate -that initiation o.!
growth at 20C was most restricted in early�n
�er, and that 5 weeks of chilling had litde effect on the release of dormancy.In Vegis's model, maximum dormancy occurs when the temperature range at which budbreak occurs is narrowest. Data obtained at 12.5C indicated that the minimum temperature for initianon of growth in asparagus was highest in early winter, and higher in mid autumn than in mid winter. Asparagus growth at 20C was most restricted in early winter, and more restricted in mid winter than mid autumn. Thus the development of dormancy in asparagus seemed to follow Vegis's (1964) statement that as plants approach deepest internal dormancy, the decrease in maximum temperature for growth occurs after the increase in minimum temperature. This could have resulted in a change in the optimum temperature for budbreak as illustrated in Fig.4. 16, assuming that the optimum temperature is mid-way between the maximum and minimum temperatures. Thus, maximum restriction of growth at 12.5C preceded the maximum restriction of growth at 20C, and there was no clearly defined time of maximum internal dormancy when the temperature at which growth occurred was narrowest. Since asparagus
appeared to follow Vegis's model m progressing through internal dormancy, the factors
which induced dormancy were probably similar to those affecting deciduous fruit trees. Photoperiod often induces internal dormancy, but it's effects on asparagus could not be detemiined fr?m this study. The natural photoperiod had decreased from 15.7 hours in mid summer to 12.6 hours in late March (Francis 1972) before the plants were
transferred to either an ·g or 16 hour photoperiod. This decrease in natural photoperiod
may have induced internal dormancy. The experiment effec,tivelr .tested the effect of the 'current' photoperiod, and ignored the conditions which the plants had alrea�y
exp
�
rienced. 'Current' photoperiod had no effect on spear growth in a simulated autumn\ . .
harvest (Table 4.10). To determine the effect of photoperiod on internal dormancy, the experiment should either have commenced earlier in the year, or used supplementary lighting to extend photoperiod before 22 March.
The induction of internal dormancy by photoperiod can often be counteracted by warm temperatures. In this study, temperatures above 13C in late autumn-early winter prevented senescence of existing fern, but did not prevent the development of internal dormancy on plants with or without fern (Figs. 4.02 and 4.03).
\
In field grown asparagus, male plants tend to senesce later than females (Nichols pers. comm.) and tend to produce spears earlier in spring (Tiedjens 1 924; Robbins and Jones 1 925; Ellison and -Schermerhorn 1958; Ellison et al. 1960). Plant sex had very little effect on growth in these experim�nts. However, there was a slight interaction between plant sex and 'spring' temperature, and male plants tended to produce spears earlier than female plants at 12.5C (Table 4.1 1).
30 25 10 5 0 ,, ,, ,, ,, ,, ,, ,, ,,
March
,, ,, ,, ,, ' � .,,,, ,,,, .... ,,, ,,,, .. ··· ···· · ,,,,,, ... . ... , ... .·M
ay
July
Sept
Time
Key:
l l l l l l lMinimum temperature
• •
Maximum temperature
-