|dc.description.abstract||This thesis investigated the storage and remobilisation of carbohydrate reserves in perennial woody stem tissue, using one-year-old apple stems. In this study, the theories developed in herbaceous annual species were applied to woody perennial plants. Perennial plants are more complicated to study as they need reserves over a longer temporal scale and the woody tissue makes experimental manipulation difficult. In addition, apple is a multiple carbohydrate transporter and sorbitol (a sugar alcohol) is a major translocation product. These complications were overcome in this thesis, allowing hypotheses, developed from knowledge of carbohydrate movement in herbaceous plants, to be tested. The hypotheses were tested by manipulating sink conditions, 14C and 11C tracer experiments, use of inhibitors and enzyme extractions.
The findings allowed models of carbohydrate storage and remobilisation in stem tissue of apple to be made. The sieve elements of apple are leaky pipes and assimilates are lost through passive leakage or active unloading during their journey from source to terminal sink. During remobilisation of carbohydrate in spring, retrieval of this leaked assimilate back into the phloem stream is up-regulated. This retrieval is likely to be directly into the sieve elements and involves carriers similar to those found in herbaceous annuals. Buffering also occurs along the stem length to prevent interruption to flow from short-term changes in photoassimilate supply. Buffering, leakage and retrieval probably occur into the apoplast. Starch is initially remobilised from storage cells closest to the phloem region in the spring. Sorbitol in the stem probably cannot be metabolised, which allows sorbitol to exist in high concentrations in the stem where it could act as a temporary (days to weeks) storage pool and could play a buffering role, maintaining a constant flow of sorbitol to sink tissue.
During storage in autumn, when terminal sink tissue is either removed or saturated, assimilates continue to be lost from the leaky sieve tubes. However, retrieval is down-regulated so high concentrations of sugars build up in the stem apoplast. This results in storage of carbohydrate as starch, initially into the cells surrounding the phloem. Storage in the stem is given the lowest priority in terms of accumulating available carbohydrate and only occurs when there is an excess of photoassimilate, such as when higher priority sinks like fruit are removed or saturated. Eventually starch storage will saturate (at a concentration of 50mg g-1 in trees growing in the Hawkes Bay). Saturation of storage is likely to reduce concentration gradients between sieve elements and the apoplast, leading to a build up of sugars in the sieve elements and reduced assimilate flow. This is likely to result in a reduction or stoppage of photosynthesis, followed by leaf senescence.
The results of this thesis show that most of the processes of carbohydrate storage and remobilisation in herbaceous plants also occur in woody perennial plants and this has increased our knowledge of basic plant processes. In order to manipulate source-sink relationships in plants to increase crop yields, we need to understand the physiological processes involving the movement of carbohydrates. The retrieval and release of sugars along the transport phloem is important, as there is a balance between supplying terminal sinks with photosynthate and retention of photosynthate along the pathway. In addition, physiological studies into apple trees are sparse. It was important to go to the next stage of carbohydrate studies in apple trees, beyond orchard management, and understand the physiological processes involved in carbohydrate movement and remobilisation. Apple is a high yielding crop and knowledge of the carbohydrate processes in this plant could help to increase yields in other commercially important tree species.||