There is increasing demand for asparagus, particularly for fresh green spears especially in Japan and Europe. More and more countries including tropical and sub-tropical countries such as The Philippines establish asparagus cultivation to help meet world demand, creating a competitive market.
To gain economic benefit from a competitive market, high total yield of superior quality spears must be achieved. Compared to other countries, average production of asparagus in New Zealand is still low (2.5 Lha-J) and New Zealand production fulfils only 2% of world production.
The New Zealand Asparagus Council (NZAC) has set a goal of doubling yield of high quality spears by 2 0 1 0 and promoting the demand for asparagus. To achieve these goals, there are six research priorities: improved varieties, plant disease control, health and nutrition, food safety, innovative, smart products and crop physiology (VegFed, 2003).
Asparagus is in high demand particularly during Christmas time in New Zealand. Very little fresh asparagus is available because most asparagus harvesting in New Zealand ceases before Christmas. Growers decide when to end harvest in order to allow the plant to replenish carbohydrate for the following year's crop. But the basic information to support the decision making process has only just become available based on using the AspireNZ system (Wilson et al., 1 999).
Many asparagus growers in New Zealand have used the AspireNZ system to help in deciding the length of the harvest period. The system provides a root storage carbohydrate value based on brix readings from root samples to give an estimate of
available carbohydrate in storage roots in term of percent total soluble carbohydrate (TSC).
The extension of harvest length, nevertheless, must consider other internal factors of the plants. With the current system using AspireNZ, the decisions for harvest length is based only on carbohydrate percentage in the storage roots, rather than total carbohydrate available, which would require knowledge of total root mass.
Furthermore total yield will depend on spear weight and number of spears. Since spears are developed from buds, the number of suitable buds could limit the number of marketable spears produced. A limit number of suitable buds could reduce the harvest of high quality spears as well as leaving only small buds to develop into fern, which would then affect future fern vigour for carbohydrate replenishment. However, little work has been carried out on bud number associated with harvest and how the bud number could be modified to make it non-limiting. This is our main research that will verify the importance of bud number in lengthening harvest for achieving high total yield.
An asparagus life cycle is illustrated to give a better overview of the current study (Fig. 1 .5). Young shoots (spears) develop from buds in the spring from the underground rhizome. These shoots are initially harvested in spring and early summer to provide the saleable yield. Eventually however the shoots have to be allowed to develop, forming fern-like foliage. The ferns photosynthesise during the summer and early autumn producing carbohydrates (in this case fructans) that accumulate in the storage roots to provide supplies for next season's growth.
This study (printed in bold fonts in Fig. 1 .5) is about bud numbers and their relationship to root numbers, root mass, carbohydrate supply and final crop yield.
• How are bud numbers going to be affected by harvest or vice versa?
• Is there any way to modify bud number in relation to root mass when a large
root mass is present?
• Is bud number a limiting factor for spear yields?
Eight experiments were conducted to obtain sufficient information to answer these questions.
Summer fern
Bud number + Storage roots
Figure 1 .5 Growth cycle of asparagus with economic yield and research
interest reported in this thesis (in bold fonts).
To achieve the objective listed in general introduction, a linked senes of eight
experiments were done from 1 999 to 2002. Eight experiments and corresponding
CHAPTER 1
General introduction and literature
review Experiment 1 :
Effects of constant daylength of 15.5 and 1 3.5 h and different CHAPTER 2
pot size on seedlings were investigated for 18 weeks. Daylength and pot size influence on
Conducted in: March - September 1999 asparagus seedlings and six month
old plants
Experiment 2: Numbers and weight of shoot and
Effects of constant daylength and reducing daylength on root discussed seedlings and six month old plants were investigated.
The experiment was repeated and conducted in January - September 2000
Experiment 3:
Effects of cutting height and cultivar on bud spear
CHAPTER 3 production and quality
Spear production and quality using Conducted in: March - July 1 999 different cutting heights in relation
to bud numbers and carbohydrate
Experiment 4: supply
Effects of cutting height, cultivar, and plant sex on spear
production and quality
Investigation were carried out verify
Bud numbers and total soluble carbohydrate were assessed.
the effect of harvest length on bud
Conducted : August 2000 -May 2001
number. Bud dynamics during
harvest season was explored based
Experiment 5: on spear record and bud mapping.
Bud mapping for bud development on rhizomes in the field at different seasons from winter 2001 to summer 2002
Conducted between 2001 - 2002
Experiment 6: CHAPTER 4
First observation of spear elongation using aeroponics was set Correlative inhibition and spear
in March 1 999 and ceased after 2 months due to slow growth
growth of two years old plants
of the plants. The plants were allowed to grow until A�gust
grown in aeroponics
1 999. Then experiment was initiated using root prunmg treatments.
Conducted in: August - November 1999
Experiment 7: CHAPTER S
Assessment of seedling growth using aeroponics was done Asparagus seedling growth grown in using 2 cultivars. Observation on bud numbers without plant aeroponics
destruction.
Conducted in: November 1999 - July 2000
Experiment 8: CHAPTER 6
The use of hydroponics with different strength of EC was set Changes of morpho-physiology of for asparagus development in term of number asparagus plants under different
Conducted in: September 1999 -June 2000
strength of electrical conductivity
CHAPTER 7