Physiological characters underpinning cultivar differences in spear yield of field-grown asparagus (Asparagus officinalis L.)
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Although it has long been recognized that genetic variation in spear yield in asparagus (Asparagus officinalis L.) is related to the amount of storage carbohydrate reserve in the storage roots, which in turn is linked to photoassimilate production in the previous season, the physiological basis for this variation is not known. In this study, diurnal and seasonal changes in photosynthetic parameters, carbon partitioning parameters and carbon utilization in developing spears were investigated in two asparagus cultivars with contrasting yield. The purpose of the investigations described in this thesis was to characterize the physiological characters underlying cultivar differences in spear yield of asparagus, with the emphasis on carbon assimilation, partitioning, utilization and sucrose metabolism during an annual cycle. Seasonal patterns in photosynthetic parameters were strongly dependent on cladophyll developmental stage in both cultivars. The greatest photosynthetic rates (A) of 8.94 ± 0.54 /µmol m-2 s-1 for the high-yielding cultivar (ASP-69) and 6.50 ± 0.38 /µmol m-2 s-1 for the low-yielding cultivar (ASP-03) were observed in fully expanded cladophyll tissue measured in mid-summer (February) when both photon flux density (PFD) and temperature were at a maximum. A significant decline in A was measured in April. This was accompanied by a significant decrease in both stomatal conductance (gs) and ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) activity. A close correlation between A and gs (r = 0.84) was observed. Although a correlation between A and total rubisco activity did not exist, both in vivo and fully activated rubisco activities in ASP-69 were significantly greater than in ASP-03, indicating the important role of this enzyme to cultivar differences in photosynthetic capacity. The difference in photosynthetic capacity between the two cultivars was related to significant differences in cladophyll thickness and specific leaf weight (SLW). Maximum photosynthetic rate (Amax) was positively correlated with sucrose phosphate synthase (SPS) activity (r = 0.86). ASP-69 exhibited greater SPS activity and sucrose content than ASP-03 in fully expanded and mature cladophyll tissue. ASP-69 also displayed a higher percentage of soluble sugar in stem cell sap than did ASP-03. These results suggest that carbon translocation rate in ASP-69 is higher than in ASP- 03. Sucrose synthase (SS) activity in storage roots in ASP-69 was significantly greater than in ASP-03 during fern growth season. Total non-structural carbohydrate (TNC) concentration in storage roots did not differ in the two cultivars. Biomass analysis revealed that ASP-69 had a greater root/shoot ratio than in ASP-03, suggesting that the total carbohydrate storage pool rather than carbohydrate concentration is an important determinant of asparagus yield. Rates of assimilate export estimated from A and dry mass changes were highest at midday and coincided with maximum assimilation rate in both cultivars. A positive correlation was found between A and assimilate export rate (r = 0.87) and this relationship did not differ between the two cultivars studied. The greater carbon export rate per unit cladophyll area measured in ASP-69 was associated with significantly higher A and sucrose concentration in the cladophyll tissue in comparison to ASP-03. Biochemical evidence indicated that the greater A and sucrose accumulation in ASP-69 were associated with a significantly higher SPS activity (P < 0.05). Phloem 14C exudate analysis confirmed the results estimated by dry mass changes and revealed that 14C flux out of cladophyll tissue in ASP-69 was significantly greater than in ASP-03. The greater spear elongation rate measured in ASP-69 was associated with a significantly higher hexose accumulation (P < 0.05) in spear tissue in comparison to ASP-03. However, sucrose content was similar in the two cultivars, suggesting more efficient machinery for transport and catalysis of carbohydrate in spears of ASP-69. Biochemical evidence indicated that the greater elongation rate in ASP-69 was associated with a significantly higher acid invertase (AI) activity (P < 0.05) in the elongation zone, whereas SS activity was not significantly different between the two cultivars. There was little neutral invertase (NI) activity detected in either cultivar. These results strongly suggest that it is AI and not SS or NI that is an important determinant of the difference in sucrose metabolism between the two asparagus cultivars in metabolising imported sucrose in the elongation region, which in turn plays a part in regulating the import of sucrose into spear tissue. The profile of sucrose cleaving enzyme activities along spear sections indicated that SS was the dominant enzyme in both tip and base of spears, whereas AI was the dominant enzyme in the elongation zone. Overall the data substantiate the conclusion that changes in activities of sucrose cleaving enzymes are correlated with sink functions in developing spears. The results obtained from this study are consistent with a feed-forward relationship among photosynthesis, sucrose synthesis and assimilate export in the cladophyll tissue. Both metabolic and anatomical factors appear to play significant roles in determining differences in photosynthetic capacity between the two asparagus cultivars studied. For the role of carbohydrate storage roots, it is the pool of total carbohydrate storage rather than carbohydrate concentration that is an important determinant of asparagus yield. This was indicated by the fact that high-yielding cultivar (ASP-69) exhibited a high percentage of young roots to the total biomass than the low-yielding cultivar (ASP-03). This difference was related to a great SS activity in ASP-69. In developing spears, ASP-69 displayed great sucrose cleaving enzyme activities than in ASP-03, indicating that carbohydrate demand in the sink tissue is an important determinant of spear development. The overall results substantiate the conclusion that spear yield is influenced by both source and sink properties, in which spear elongation is closely related to spear ability to import carbon and the overall yield is determined by the available carbohydrate reserve accumulated in the carbohydrate storage pool, which in turn is linked to assimilate production.