Optimum Mean Temperature for a Plant Growth Calculated by a New Method of Summation

The relationship between temperature and stem elongation has been investigated for representative herbaceous plants sampled from populations of seven species which were growing under semi—natural conditions in the Botanical Garden of the University of Padua (Italy). The growth of the floral stem was measured for the following species: Galanthus nivalis L., Corydalis cava Schw. et Krt., Anemone nemorosa l., Symphytum tuberosum L., Symphytum tuberosum L., Allium ursinum L., Aegopodium podagraria L., and Campanula rapunculoides L. Official records of daily maximum and minimum air temperatures have been elaborated by means of a new method of temperature summation. This method takes into consideration the concept that plants possess a range of sensitivity to temperature between maximum and minimum thresholds and that there is an optimal temperature for each stage of development. Data for stem elongation have been optimized (using the technique of best fit) as a function of a logistic curve which may represent growth under ideal conditions. Differences between the logistic curve and the real curve of growth may be related to the amount of deviation of values for environmental factors form optimal values. The temperature ranges between 0° C and 30° C which gave the best correlations with stem elongation have been calculated for the entire period of this process of growth. The average between the two thresholds on each day has been considered as the optimum mean temperature for that day. The optimum mean temperature increases from the start of the process to ripening, with the exception of a period just before flowering when all plants seem to require lower daily temperatures. The results of this analysis have also indicated that unknown factors other than temperature are affecting stem elongation.