Thermal requirements and estimates of number of leaves of Annona squamosa L. grown under different shading conditions

Abstract

The objective of this study was to determine the thermal requirements and compare methods for estimating
leaf emergence in Annona squamosa L. under different shading conditions during the dry and rainy seasons
in a transition region between the Cerrado Amazon biomes, Mato Grosso, Brazil. Seedlings were grown under
full-sun conditions (reference) and black polyolefin screens with 35%, 50%, 65%, and 80% attenuation of global
radiation. Basal temperatures were determined using the method of least standard deviation; accumulated
thermal sum was estimated using the method described in Ometto (1981); the relationship between number of
leaves and thermal sum were evaluated using the phyllochron and Wang & Engel (1998) models. The minimum
and maximum basal temperatures for the growth of A. squamosa were 11.0 and 38.0 °C, respectively. The
accumulated thermal sum presented small variation between shading conditions and seasons, resulting in
means of 1744±44 and 4434±118 degree-days at 113 and 260 days after transplanting (DAT), corresponding to
the end of the dry and rainy seasons, respectively. The thermal requirement for leaf emergence varied among
the different shading conditions, with higher phyllochrons under full sun conditions and 80% shading (109.8 °C
day leaf-1) and lower phyllochrons under 50% shading (91.74 °C day leaf-1). The phyllochron model presented
higher efficiency in estimating leaf emergence compared to the Wang & Engel model. Growing A. squamosa
seedlings under 50% shading is recommended as it resulted in higher degree-day accumulation and lower
phyllochron for leaf emergence.