Yield and quality of asparagus (Asparagus officinalis L.) as affected by carbohydrate distribution in relation to daylength, fructan levels, and bud number : a thesis presented in partial fulfillment of the requirements for the degree of Master of Applied Science in Plant Science at Massey University, New Zealand
Three different aspects of carbohydrate reserves in asparagus were evaluated in this thesis. The first aspect was the effect of daylength on dry matter partitioning between fern and storage roots (Chapter Two), the second aspect was remobilization of soluble carbohydrates (fructans) from different roots during spear harvest (Chapter Three), and the last aspect was the effect of cutting height at harvest on spear yield and quality and stimulation of additional spears using hormone treatment (Chapter Four). The experiments using controlled climate growth chambers were conducted to evaluate the effect of daylength on dry matter partitioning in asparagus plants. The treatments were long constant daylength (15.5 h) and reducing daylength. The results showed that partitioning carbohydrates between fern and crown was influenced by daylength. Plants exposed to reducing daylength showed reductions in plant height, shoot number, number of lateral per shoot, length of lateral, and fern dry weight. This reduction in fern growth was followed by decreasing NAR (net assimilation rate) except at daylengths around 14 hours. However under reducing daylength relatively more carbohydrate partitioned to crown than to fern, as indicated by root:shoot dry weight ratio and allometric ratio between crown and fern. The results also suggested that daylengths around 13.5 to 14 hours seem to be particularly favorable for storing carbohydrates in the roots. In addition, cultivar differences exist in the response to daylength. 'Jersey Giant' was more responsive to daylength than 'UC157' and 'Italian Hybrid' showed a little or no response to daylength. Radioactive labeling using ,¹⁴CO₂ was used to study fructan remobilization from different roots during spear harvest and fructans separation was done using HPLC System. The results showed that spear growth utilized carbohydrate, not only from the nearest roots, but also from more distant new roots. Spears also utilized carbohydrates from distant old roots during harvest but not to the some extent as from new roots. The HPLC system used in this work was able to separate fructans up to a degree of polymerization of 10 (DP10) and produced a single large peak of long chain fructans. The source of carbohydrates used to support spear growth was mainly from long chain fructans (DP more than 10) as long chain frutans decreased sharply during spear growth while short chain fructans (DP3 to DP 10) only decreased slightly. Changes in individual fructans suggested that hydrolysis rates of DP4 and DP3 seem to be a limiting process during fructan hydrolysis. Finally, utilization of carbohydrate reserves and buds to produce marketable spears was studied by the application of cutting height at harvest. The results showed that marketable yield (both first class and total marketable yields) showed an optimum cutting height. The marketable yield increased with increasing cutting height and reached a maximum marketable yield at certain height then decreased with increasing cutting height. The optimum cutting heights for first class marketable yield (quality 1 spears) was lower than those for total marketable yield and varied with cultivars so that variation in cutting height during spear harvest should be used for cultivar evaluation. In addition the application of hormone mixture (BA and GA₃), when spear production had nearly ceased, induced additional spear production indicating that spear production was not limited by bud number but probably by carbohydrate level in storage roots, under the conditions of these experiments. However, considerable variation existed between individual plants.