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Delayed chilling appears to counteract flowering advances of apricot in southern UK
Temperatures are rising across the globe, and the UK is no
exception. Spring phenology of perennial fruit crops is to a large extent
determined by temperature during effective chilling (endo-dormancy) and
heat accumulation (eco-dormancy) periods. We used the apricot flowering
records of the UK National Fruit Collections (NFC) to determine the
influence of temperature trends over recent decades (1960 to 2014) on
apricot (Prunus armeniaca L.) flowering time. Using Partial Least Squares
(PLS) regression, we determined the respective periods for calculating
chill and heat accumulation. Results suggested intervals between
September 27th and February 26th and between December 31st and April 12th
as the effective chilling and warming periods, respectively. Flowering
time was correlated with temperature during both periods, with warming
during chilling corresponding to flowering delays by 4.82 d°C-1, while
warming during heat accumulation was associated with bloom advances by
9.85 d°C-1. Heat accumulation started after accumulating 62.7 ± 5.6 Chill
Portions, and flowering occurred after a further 3744 ± 1538 Growing
Degree Hours (above a base temperature of 4°C, with optimal growth at
26°C). When examining the time series, the increase in temperature during
the chilling period did not appear to decrease overall chill accumulation
during the chilling period but to delay the onset of chill accumulation
and the completion of the the average chill accumulation necessary to
start heat accumulation. The resulting delay in heat responsiveness
appeared to weaken the phenology-advancing effect of spring warming.
These processes may explain why apricot flowering time remained
relatively unchanged despite significant temperature increases. A
consequence of this may be a reduction of frost risk for early flowering
crops such as apricot in the UK
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