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    Fruit composition, growth, and water relations of Braeburn apples under reduced plant water status : this thesis is presented in partial fulfilment of the requirements of the degree of Doctor of Philosophy in Horticultural Science at Massey University, Palmerston North, New Zealand
    (Massey University, 1996) Mills, Tessa Marie; Mills, Tessa Marie
    Water plays a major role in the physiological processes of plants. Effective irrigation relies on a comprehensive understanding of the impact of water on plant processes. As water becomes an increasingly scarce resource, the impact of reduced plant water status on crop performance (quality and yield) requires investigation. The effects of reduced plant water status on fruit composition, growth, and water relations were therefore studied using both field-grown and container-grown 'Braeburn' apple trees. Vegetative growth and carbon assimilation were also measured. Plant water deficit was imposed at various times during the growing season. The treatments were: control, which was fully irrigated during the experimental period, entire-season deficit, deficit irrigation from 55 days after full bloom (DAFB) until final fruit harvest (183 DAFB), early-season deficit (from 55 - 100 DAFB) followed by rewatering, and late-season deficit (from approximately 105 DAFB until final harvest). Reduced leaf water potential developed in all deficit irrigated trees during the stress period. Only the entire-season deficit irrigation treatment resulted in a significant reduction in vegetative growth as measured by total leaf area, shoot growth, and trunk growth. Return bloom was reduced under an early-, but not late-season deficit. Photosynthesis was generally reduced in water deficit treatments, as was stomatal conductance. Only an entire-season deficit irrigation reduced individual fruit weight. Fruit soluble solids and sugar concentration were generally increased under deficit irrigation treatments. However, upon rewatering of the early-season deficit trees, the values again became the same as controls. Fruit mineral concentration did not show consistent differences between treatments and the incidence of storage disorders was low in all treatments and unaffected by deficit irrigation. Early-season water deficit lowered both fruit water potential and osmotic potential. Despite turgor maintenance within the fruit during the stress period, growth was reduced at this time. A late-season water deficit did not modify fruit water relations. It appears that 'Braeburn' fruit are resilient to periodic water deficit during the season, and that water conservation is possible with limited impact on total crop yield. Additionally, a late-season deficit may even enhance some fruit quality attributes, such as increased total soluble solids. An early-season deficit reduced return bloom and must therefore be used with caution. An entire-season water deficit is not recommended due to the reduction in fruit size.
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    Regulation of ethylene biosynthesis in vegetative tissues of white clover (Trifolium repens L.) during water deficit : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Biology, Institute of Molecular Biosciences, Massey University, Palmerston North, New Zealand
    (Massey University, 2009) Nikmatullah, Aluh
    The investigation in this thesis is divided into two parts. In the first part, the expression and accumulation of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACO), the enzyme which catalyses the final step of ethylene biosynthesis in higher plants, is examined during exposure of white clover (Trifolium repens L.) to a water deficit. The second part of this thesis is focused on the identification and characterisation of a water-deficit-associated ACC synthase (ACS), the enzyme which catalyses the production of ACC. In the first part, two white clover varieties with differing sensitivity to water deficit, a drought-tolerant Tienshan ecotype and a drought-sensitive Grasslands Challenge cv. Kopu II cultivar were exposed to two water deficit treatments: one cycle of water deficit (designated non-prestressed; NPS) and a water deficit, a rehydration period and then a second water deficit treatment (designated pre-stressed; PS) in the New Zealand Climate Environment Laboratory (NZCEL). Treatments were terminated when the petiole elongation rate (PER) in the first fully-expanded leaf reached zero. Water relations, growth responses, the expression of the white clover ACO genes, TR-ACO1 TR-ACO2 and TR-ACO3 and the accumulation of two of the corresponding proteins, TR-ACO1 and TR-ACO2, were then examined. The soil water content (SWC) and leaf water potential (LWP) measured in both varieties and in both water deficit treatments declined progressively. The rate of decline in SWC and LWP was slower in the Tienshan ecotype with no difference between the NPS and PS treatments. However, the LWP in the Tienshan ecotype at the point at which the PER ceased was less negative (ca. -1.4 MPa) compared to Kopu (ca. -1.7 MPa). In addition, the decline in the PER differed between NPS- and PS-treated Kopu. In the NPS-treated Kopu, the PER was maintained at a high rate when plants were exposed to SWC above 18%, but declined sharply as the SWC declined further. However, in the PS-treated Kopu, the PER declined more progressively in a similar pattern to that determined for NPS- and PS-treated Tienshan. Expression of TR-ACO1 and accumulation of TR-ACO1 was observed in the apical structure of the stolon. As the water deficit progressed, no significant alteration in TR-ACO1 expression and TR-ACO1 protein accumulation was observed in the apical structures of both the NPS- and PS-treated Tienshan ecotype suggesting some degree of protection of the meristem tissues in this more drought-tolerant variety. However, a discernable decline in expression of TR-ACO1 and accumulation of TR-ACO1 protein was observed in the NPS-treated Kopu suggesting some degree of tissue injury in this more drought-susceptible variety. However, after the pre-stress (PS) treatment, no real changes in TR-ACO1 expression and TR-ACO1 protein accumulation were observed, in common with the observations for the NPS- and PS-treated Tienshan ecotype suggesting that meristem protection may now be occurring. The results suggest further that the pre-stress treatment of the more drought-susceptible Kopu may result in a degree of acclimation to the water deficit. For the first-fully expanded leaves, expression of two transcripts, TR-ACO2 and TR-ACO3 and accumulation of TR-ACO2 protein was monitored as the SWC decreased. The expression of TR-ACO2 and accumulation of TR-ACO2 decreased as the water deficit progressed in both the NPS- and PS-treated Tienshan ecotype and correlated with the decrease in PER. By contrast, in the NPS-treated Kopu, TR-ACO2 expression and TR-ACO2 protein accumulation increased, but again, after a period of pre-stress, TR-ACO2 expression and TR-ACO2 accumulation decreased, in common with the Tienshan ecotype. Again, the pre-stress treatment of the drought-susceptible Kopu may result in a degree of acclimation to the water deficit such that the responses become similar to those observed in the more drought-tolerant Tienshan ecotype. However, in both NPS- and PS-treated Tienshan and Kopu there was no significant alteration in the expression of TR-ACO3 in the first fully-expanded leaf. The expression of TR-ACO2 and TR-ACO3 and accumulation of TR-ACO2 protein were also observed in the second fully-expanded leaves (an older tissue). Again similar patterns in the expression of TR-ACO2 and TR-ACO3 and accumulation of TR-ACO2 protein were observed in both NPS- and PS-treated Tienshan and Kopu. In these leaves, expression of TR-ACO2 and accumulation of TR-ACO2 protein decreased as the water deficit progressed, but expression of TR-ACO3 increased as the water deficit decreased to less than 10%. These results suggest that responses of younger tissues (apical structure; first-fully expanded leaf) maybe the critical determinant for the tolerant (or otherwise) of white clover plants to water deficit. In the second part of this thesis, four ACS genes were identified from the Tienshan ecotype exposed to water deficit and designated TR-ACS-T. Three of these were similar to previously identified TR-ACS genes from Grasslands Challenge genotype 10F while the fourth was a novel gene designated TR-ACS4-T. TR-ACS4-T is 64%, 64% and 63% homologous to TR-ACS1-T, TR-ACS2-T and TR-ACS3-T, respectively in terms of nucleotide sequence. In the GeneBank database, TR-ACS4-T shares highly homology to ACC synthase sequences from a wide range of tissues including seedlings and fruit tissues, in addition to a high homology to ACS genes induced in auxin-, wounding- and ethylene-treated tissues. The pattern of TR-ACS4-T expression observed during leaf development suggests that the gene is expressed initially in the apical structures and in the newly initiated leaves, and then again in the later mature leaves and those at the onset of senescence. Expression decreases again during senescence. TR-ACS4-T expression is not altered by water deficit, but is induced by both ethylene and NAA treatment, but the auxin-induced TR-ACS4-T is mediated by ethylene treatment.