Specific Daylength Effects on Leaf Growth and Dry-matter Production in High-latitude Grasses

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Abstract
Plants of Phleum pratense L. cv. Engmo (origin 69° N) and Bromus inermis Leyss. cv. Löfar (origin 64° N) raised in short days gave large and significant increases in d. wt, height and leaf area upon exposure to continuous light, compared with 8 h short days, at essentially identical daily inputs of radiant energy (8 h summer daylight ± low intensity extension). The increases in both plant height and leaf area were caused mainly by increases in the dimensions of the leaf sheaths and blades, which, in turn, were the result of increased cell size and number. However, there was a pronounced interaction between temperature and daylength such that the greatest photoperiodic stimulation occurred at lower growing temperatures ( ≤ 15 °C), suggesting that these grasses are well adapted to the cool, long days of the high-latitude summer. Application of growth analysis to the results for P. pratense indicated that photoperiodic stimulation of relative growth rate was caused principally by increases in the leafiness of the plants (leaf area ratio) rather than any effect on net photosynthesis, although there was a short-lived rise in net assimilation rate at 21 °C. The observed increases in leaf area ratio were not caused by changes in the partitioning of assimilate (leaf weight ratio) but by increases in specific leaf area associated with the increased succulence of plant tissues developing under continuous illumination. The results are discussed in relation to previous work on high-latitude cultivars of Poa pratensis and Dactylis glomerata.