Morphological criteria and temperature for evaluation of cupuassu seed germination

Abstract

The objectives of this study were to determine the temperature range of Theobroma grandiflorum seed germination, to indicate the most suitable temperature for the germination test and propose morphological evaluation criteria to provide data for the inclusion of this species in the Brazilian Rules for Seed Testing. The experiments were carried out in germination chambers with a 12 hour daily photoperiod, using vermiculite as substrate. Seed of T. grandiflorum were collected from the same population however of different trees at two occasions: February (lot 1) and April 2010 (lot 2). Seed germination was assessed at constant temperatures between 10 and 40°C, with an interval of 5°C. Three morphological criteria were evaluated: the protrusion (≥ 5 mm) of the first visible structure (physiological criterion), epicotyl length (≥ 3 cm; corresponding field emergence) and normal seedling development (technological criterion). The experimental design was completely randomized, with seven constant temperatures and four replicates with 25 seeds. For each criterion, germination percentage and variables of germination speed were assessed. In T. grandiflorum the hypocotyl is the first visible structure in the germination process. Its protrusion was observed between 10 and 40°C in seed lot 1 and, in seed lot 2, between 10 and 35°C. The lack of epicotyl growth and the development of normal seedlings at 10, 15 and 40°C, in both seed collections, indicate that the shoot is more sensitive to extreme temperatures than the hypocotyl and root. The lower temperature limit for shoot growth was between 15 and 20°C and the upper limit between 35 and 40°C. The temperature of 30°C is recommended for the germination test of T. grandiflorum. Observing normal seedling development, the first count could be done at day 21 and final count at day 35. Epicotyl length (≥ 3 cm) is suggested as an alternative for earlier assessment of seed germination, with the first count at day 14 and final count at day 20.