Seedlings of dry forest adapted species resume growth after desiccation
Desiccation tolerance (DT) in germinated seeds is directly linked to the success of seedling survival of seasonally dry tropical forests (SDTF) species. The objective of this study was to evaluate whether the seeds of Anadenanthera colubrina and Cenostigma pyramidale present post-germinative DT and until what stage of seedling development the tolerance persists. Seeds were separated from fruit remains, branches, and seeds of other species and stored for 6 months in cloth bags in a cold and dry chamber (±10 °C/45% RH) until the beginning of the experiment. Fresh A. colubrina and C. pyramidale seeds showed 85% and 90% germination and WC was 8% and 8.1%, prior to DT trials. Seeds were sowed on germination paper moistened with distilled water 2.5 times the dry paper weight and incubated at 25 °C for A. colubrina and 30 °C for C. pyramidale and photoperiod of 12/12 h (white light with photon flux density of 30 W/m2) light/dark, according to specific instructions for best conditions for seed germination (BRASIL, 2013). Since these are very fast germinating species, after 24 hours, radicle protrusion was observed and measured using a digital caliper (0.001 mm accuracy). The seedlings were separated into four IRL categories: 1.00–2.99 mm, 3.00–4.99 mm, 5.00–6.99 mm, and 7.00–10.99 mm. Seedlings (5 replicates of 10 seeds) from each IRL category were desiccated for 24 and 72 hours. Desiccation was performed by transferring seedlings to aluminium screens placed in germination boxes (11 × 3.5 cm) with 100 g of silica gel blue (4–8 mm) and incubated at 25 °C for A. colubrina and 30 °C for C. pyramidale, photoperiod of 12/12 h light/dark (BRASIL, 2013). After 24 and 72 hours desiccation, seedlings were rehydrated and allowed to grow by transferring to germination paper moistened with distilled water and incubated at same temperature and photoperiod conditions as previously described, for 7 and 14 days. WC of seed lots was determined gravimetrically by oven-drying two samples of 25 quiescent seeds (approximately 3 g) of each species at 105 ± 3 °C for 24 h (BRASIL, 2009), since this is a destructive method. The results were expressed as a mean percentage (fresh weight basis) and this result was considered the initial WC of each individual dry seed. Each quiescent seed was individually weighed prior to germination. Seedlings of each category were individually weighed after radicle protrusion, after 24 or 72h of desiccation, and after 7 or 14 days of subsequent rehydration. To evaluate changes in WC, we used the following equation adapted from Hong and Ellis (1996). DT was considered when seedlings resume growth after desiccation and rehydration to original water content. At the end of the experiment, at 14 days after rehydration, final root and shoot length of desiccated and rehydrated seedlings were individually measured using a digital caliper (0.001 mm accuracy).