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).

种子萌发后的脱水耐受性（DT）与季节性干旱热带森林（SDTF）物种幼苗存活的成功直接相关。本研究旨在评估 Anadenanthera colubrina 和 Cenostigma pyramidale 种子的脱水耐受性是否存在，以及耐受性维持至幼苗发育的哪个阶段。种子从果实残留物、枝条和其他物种的种子中分离出来，并在冷干室（±10 °C/45% RH）的布袋中储存6个月，直至实验开始。新鲜的 A. colubrina 和 C. pyramidale 种子表现出85%和90%的发芽率，水分含量（WC）分别为8%和8.1%，在脱水耐受性试验之前。将种子播种在用蒸馏水湿润至干纸重量2.5倍的发芽纸上，并根据 BRASIL（2013年）提供的最佳种子发芽条件，在25 °C下对 A. colubrina 进行培养，在30 °C下对 C. pyramidale 进行培养，光照周期为12/12小时（光通密度为30 W/m2的白光）。由于这些物种发芽速度非常快，24小时后即可观察到并使用数字卡尺（精度为0.001 mm）测量根尖突出。将幼苗分为四个国际长度等级（IRL）：1.00–2.99 mm、3.00–4.99 mm、5.00–6.99 mm 和 7.00–10.99 mm。来自每个 IRL 类别的幼苗（每组10粒种子的5个重复）在24小时和72小时内进行脱水处理。脱水是通过将幼苗转移到放置在发芽箱（11 × 3.5 cm）中的铝筛上，并在100 g蓝色硅胶（4–8 mm）中进行，对 A. colubrina 在25 °C，对 C. pyramidale 在30 °C下进行培养，光照周期为12/12小时光/暗（BRASIL，2013年）。脱水24小时和72小时后，将幼苗重新水化，并允许其在湿润的发芽纸上生长，并在与之前描述相同的温度和光照周期条件下，培养7天和14天。种子水分含量（WC）通过在105 ± 3 °C下烘干每种物种25粒静息种子（约3 g）的两种样品进行重量法测定（BRASIL，2009年），因为这是一种破坏性方法。结果以平均百分比（以鲜重为基础）表示，并被认为是每个单独干燥种子的初始WC。在发芽前，每个静息种子都单独称重。每个类别的幼苗在根尖突出后、脱水24小时或72小时后以及随后的7天或14天重新水化后都单独称重。为了评估WC的变化，我们使用了来自 Hong 和 Ellis（1996年）的以下方程式进行适配。脱水后重新水化并恢复到原始水分含量后，幼苗恢复生长时被认为是脱水耐受性。实验结束时，在重新水化14天后，分别使用数字卡尺（精度为0.001 mm）测量脱水并重新水化幼苗的最终根和茎长。