Photosynthesis during progressive water stress in interior spruce (Picea glauca (Moench Voss) : physiology and protein composition

Although the stress responses of conifers have been extensively investigated in terms of growth and survival, the ramifications for the perennial photosynthetic apparatus are not well characterized. Coping with excess radiation is particularly important for conifers because the structurally expensive needles usually support multiple seasons of photosynthesis, surviving a range of conditions limiting carbon dioxide fixation. This thesis represents the first detailed characterization of a coordinated set of photosynthetic responses to progressive water stress by young interior spruce (Picea glauca (Moench) Voss XP. engelmanni Parry hybrid complex). Analysis of photosynthesis at the tissue and thylakoid membrane levels revealed a pattern of down-regulation that corresponded generally to that described for other species; however, it differed in that protective mechanisms were initiated during a moderate level of stress. As water potentials declined, an integrated repertoire of mechanisms to cope with excess light energy unfolded. As mild water stress developed at water potentials below -1 MPa, gas exchange measurements indicated stomatal closure and minimal carboxylation. As moderate water stress developed below -1.6 MPa, chlorophyll fluorescence revealed decreases in photochemical efficiency. Nonphotochemical quenching was readily light-saturated in spruce seedlings at any water status; however, quenching increased during moderate water stress at low irradiances, revealing an effect on energy dissipation. Most significantly, electron flow through Photosystem II (PSII) decreased during moderate stress, in contrast to reports for angiosperm species. Details of the thylakoid response to water stress were obtained by immunoquantification of proteins using antibodies to two of the oxygen-evolving enhancer proteins (OEE1 and OEE2), the D l reaction centre protein, cytochrome b₅₅₉, chlorophyll a/b binding proteins, and ATP synthetase at different stress phases defined by physiological criteria. This revealed that only D l levels diminished early in the imposition of water stress; other protein levels were unaffected until the stress became severe. Synthesis of D l , calculated from ³⁵S-methionine labelling of needle proteins, decreased by 80 % in stressed trees whereas it was considerably enhanced in recovering trees, suggesting that D l synthesis was limiting during water stress. A novel observation was that the membrane association of OEE2 with PSII, assessed by susceptibility to removal by detergent, decreased significantly during moderate stress. This occurred before photoinhibition suggesting that this nuclear-encoded protein is intimately involved in PSII down-regulation in spruce, possibly as a component of reversible inactivation.Science, Faculty ofBotany, Department ofGraduat