Since stalk elongation in bulbous plants has also been shown to be due to the parallel activity of auxin and gibberellins, we suggest that auxin signals not only play a role in the induction of shoot growth, but also probably affect the onset of GA biosynthesis. The accumulation of the latter in growing shoots induces the remobilization of all reserves in bulbs in order to supply nutrients for growth. For instance, accumulation of GA leads to an enhanced expression of invertase genes, which establishes a large sink of sucrose transported from the bulb and provides hexoses necessary for shoot elongation. A normal growing shoot also induces an increase in respiration and water flux. Therefore, according to our hypothesis, the differences observed between pre-cooled and non-cooled plants during storage and after planting (in carbohydrate content, enzyme activities, water content and respiration rate, etc.) independently do not explain growth inhibition. These parameters are only secondary effects of the absence of growth, which is regulated by temperature via auxin signals and GA biosynthesis.
6. Temperature