Adaptations to Environmental Change in Picea sitchensis Seedling-changes in Photosynthesis, Growth and Substrate Availability

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Abstract
Effects of a change in night temperature with or without reduction in the duration of photosyntheticallyuseful light were studied on assimilation and growth of Picea sitchensis seedlings. An increase in night temperature resulted in a rapid, but temporary, increase in photosynthesis while lowering the night temperature decreased photosynthetic rate. For the lowest night temperature treatments, reduction in light quantity resulted in increased photosynthetic efficiency while for the highest night temperature treatment, reduction in light quantity apparently checked the final decline in photosynthetic rate noted above. These changes tended to minimize treatment effects on absolute growth and net assimilation rates. The results are discussed in relation to the hypothesis or photosynthetic control by substrate level. Short-term redistribution of 14C labelled assimilate was examined in control plants and those in the most and least adverse environments. In the most unfavourable environment, total l4C transport and incorporation into turnover materials increased while labelling of reserves and new growth decreased. Paradoxically, labelling patterns for plants in the most favourable environment were similar although total 14C transport was much reduced. It is suggested that this indicates a substrate excess resulting from overloading of transport and utilization systems. Growth pattern was markedly affected in the case of developing branch initials and established branches only. The possibility that the observed changes represent adaptive responses is discussed.