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Studies on asparagus growth in relation to yield were undertaken in environmentally controlled growth cabinets and in greenhouses. Bud production during the annual growth cycle was also investigated in the field. Growth cabinet experiments showed that increasing the temperature had a significant effect on bud break and relative spear growth rate (RSGR), but although prior chilling had a significant effect on the length of time to bud break at 10°C and 15°C, the effect on RSGR was not so clear. The cytokinin-active compound, N-(2-chloro-4-pyridyl)-N'-phenylurea (CPPU), and the naturally occurring cytokinin, zeatin riboside (ZR) significantly stimulated spear elongation. However, spear leaf scale removal reduced spear elongation in the absence and presence of CPPU. CPPU only stimulated spear growth when spear leaf scales were present, indicating that other plant hormones may interact with cytokinins in promoting elongation. The importance of spear growth rate to yield was discussed. In greenhouse experiments, CPPU applied as a foliar spray at 10 or 20 mg L-1 was effective in producing longer and thicker cladodes that might be associated with increased photosynthetic rate. However, photosynthetic rate was unaffected by 10 mg L-1 CPPU treatment. Repeated CPPU applications to foliage reduced net assimilation rate (NAR) compared to untreated controls as determined by growth analysis studies. In asparagus plants, it was difficult to collect xylem sap and further experiments were undertaken with Capsicum annuum. The root exudate of CPPU-treated plants significantly decreased hypocotyl length in the lettuce gibberellin bioassay, suggesting that CPPU blocks gibberellin biosynthesis in roots. However, the application of GA3 to shoots did not reverse growth suppression caused by CPPU-treated roots. Bud production, both in growth cabinets and in open field plantings, started to occur during the spear harvest period in contrast to previously accepted views. During harvest three to four additional buds per cluster were produced in cabinet-grown plants and an average of 51 buds per m2 in field plantings. These results confirm that new bud initiation and development starts to occur during spear harvest, as well as during fern growth and establishment.

The results are presented of eight experiments designed to investigate the influence of interrelationships between bud population dynamics and carbohydrate supply from root stores on spear production in asparagus (Asparagus officinalis L). These investigations involved studies in the field and the greenhouse, and using aeroponics and hydroponics techniques to facilitate non-destructive studies of plant development. The evidence indicated that spear yield was limited by the number of buds of adequate size for developing into marketable spears, rather than total bud number. It was shown that bud development continues throughout the harvest period. About 14% of these buds contributed to fern production after harvest, but the majority were involved, following a period of dormancy, in development of the next season's spears. Approximately 16% of the new buds contributed to spear yield in the current harvest, 68% were dormant until the following summer and contributed to 18% of total buds at that time. Spear production was most efficient in plants with large crowns, since the effects of correlative inhibition on spear development were greater in small than large crowns. Nevertheless, increase in crown size in terms of root mass is not necessarily accompanied by an equivalent increase in bud number or cluster number, and bud availability is potentially an important yield limiting factor. However, large crowns reduced the period of correlative inhibition within a bud cluster. Crown size and bud population were sensitive to nutrient supply, and it is suggested that control of nutrient supply over the harvest period may be best achieved by use of slow-release fertilizer or split application of nitrogen. Carbohydrate partitioning and possibly photosynthetic rate were also sensitive to daylength, and there was some evidence of genotypic variation in the response to daylength changes and contrasts. Principal component analysis indicated that numbers of buds and bud clusters, plant size and chlorophyll content were the main determinants of spear yield, and cluster analysis demonstrated potentially important genetic variation for these variables in potentially high yielding cultivars. Spear yield is the product of harvest intensity and harvest duration, and harvest duration itself was shown to be sensitive to genotype and management effects on bud initiation and development. A conceptual model is used to illustrate the influence of bud population and bud cluster characteristics on harvest intensity and duration, and on spear yield, and the relative importance of management manipulation of bud dynamics and carbohydrate supply to spear yield.