The kinetics of spear growth and asparagus productivity : control by environmental and internal factors : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, Palmerston North, New Zealand
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.