Low temperature exposure of garlic cloves: effects on growth and yield of non-bolting and bolting genotypes

In garlic, the main phenological stages such as clove sprouting, emergence, bulb initiation and maturation are mainly controlled by temperature. This research was conducted to evaluate the effect of low temperature treatments for different durations (0, 2, 4, 6 weeks) on the growth, bulb formation and yield of two local garlic cultivars (‘Gravina’ as non-bolting and ‘Rosso di Sulmona’ as bolting one) over two growing seasons (2016–2017 and 2017–2018). The results indicated that cultivar, chilling duration and experimental year significantly influenced garlic bulbing and yield. The highest bulb diameter (55.6 mm), bulb weight (50.2 g), bulblets weight (4.0 g), and bulb yield (11.5Mg ha−1) were reached in the second year by the ‘Gravina’ cultivar treated for 4 weeks at low temperature before planting. In general, in the first year, production was higher with the use of bulbs refrigerated for 4 and 6 weeks, while in the second year the pre-treatments for 2 and 4 weeks were more effective. Principal component analysis (PCA) highlighted a clear separation between the two cultivars. ‘Gravina’ showed a stronger response in terms of morphological and productive parameters to pre-planting treatments, especially exposing cloves to longer chilling time than ‘Rosso di Sulmona’. The results indicated the importance of low temperature to promote the growth of garlic and increase the bulb size and yield.