Base temperature as a function of genotype: a foundation for modeling phenology of temperate fruit species

Abstract

To complete each phase of the growing season, plants must accumulate thermal time at lower base temperature (Tb). Little information exists on Tb variation between either fruit species or cultivars of the same species. We therefore aimed to determine the lower base temperature for contrasting genotypes in precocity of peach, plum, grape, pear, and kiwi. Twigs 25-35 cm long for the following cultivars: peach, Tropic Beauty (TB) and Eragil (ER); plum, Gulf Blaze (GB) and Letícia (LE); grape, Chardonnay (CH) and Cabernet Sauvignon (CS); pear, Smith (SM) and Packham’s (PA); and kiwi, Golden King (GK) and Hayward (HA) were collected in orchards in Veranópolis, RS Estate, on 06/13/2014, with 0 h at temperatures ? 7.2°C (chilling hours; HC) in the field. Intact twigs packed in black plastic film were subjected to 1,008 HC at 0°C in incubators to overcome dormancy and then transferred to temperatures of 2, 4, 6, 8, 10, and 12°C on single-node cuttings planted in phenolic foam to define effective heat temperature for the genotypes. Over 110 d, budburst of the buds was evaluated in 2-3-d intervals in the green-tip stage. The resulting inverse data of number of days to budburst (1/days to budburst) was inserted into regression curves to estimate Tb for each genotype. Historical phonological series comprised of 10 years for the analyzed cultivars and meteorological data of the cultivation sites were used to determine thermal time (degree-days) for the fruit trees during the growing season, considering different phenological phases. Temperate fruit species exhibited different Tb behaviors: Tb was lower for early cultivars (TB and GB = 2.2°C; CH = 2.1°C; SM = 4.4°C; GK = 4.3°C) and higher for late cultivars (ER = 6.3°C; LE = 6.2°C; CS = 4.3°C; HA and PA = 8.2°C) for all cultures. The Tb f fruit cultivars related directly with genotype chilling requirements: the higher the chilling requirement, the higher the Tb of the cultivar. Cultivars of the same fruit species yielded a sum of degree-days almost equal to finalize the growing season, regardless of the degree of precocity (TB = 1720; ER = 1801; GB = 1680; LE = 1718; CH = 2310; CS = 2369; SM = 2096; PA = 2003 GD; GK = 2775; HA = 2691). Regarding phenological phases, 82% of the assessed cases responded more to thermal time (degree-days) than to chronological time (d) to complete phenological steps. Differences in Tb between genotypes are a relevant factor for improving the accuracy and applicability of phenology models in agriculture.