The influence of seed size and seed pre-treatment on germination, early seedling growth, field survival and growth of Pinus elliottii

Abstract

Successful tree species establishment depends on the quality of nursery-raised seedlings and good silvicultural practices such as appropriate site x species matching, site preparation, tending and tree protection. Commercial nurseries depend on quality seed, which has a high germination and will germinate rapidly, uniformly, and consistently. Pinus elliottii seed germination has been described as unpredictable, where one seed lot may result in 90% germination while another lot may only germinate at 30%. This phenomenon forces nursery managers to buy more seed and double sow to reach production targets. However, this has a negative influence on nursery profitability — hence the main objective of this research study, which is to investigate the impact of seed size, pre-treatment, and their interaction on germination, early seedling growth, field survival, and growth of Pinus elliottii in South Africa. Seed sizes included: small (3.1–4 mm), large (5.1–6 mm), and mixed (a mixture of 88% large and 12% small seed) seeds. The pre-treatments included hydrogen peroxide, hydro-priming, kelp-p-max, stratification, hydrogen peroxide + hydro-priming, hydrogen peroxide + kelp-p-max and a control. Germination trials were done under laboratory (controlled environmental conditions in a growth chamber) and nursery (uncontrolled) conditions. Other germination parameters used to measure seed quality included: Time to 50% Germination (T50), Mean Germination Time (MGT), Germination Value (GV), Germination Rate Index (GRI), and Coefficient of Velocity of Germination (CVG). Seedlings were kept in the nursery to measure, after 5 months, early seedling growth (seedling height, root collar diameter and sturdiness ratios) and then planted infield, where growth (seedling height, ground line diameter and biomass index) and survival assessments were done 12 months after planting. Large seed had the highest germination (79.1%) followed by mixed seed (76.9%), while the small seed had the worst performance (60.7%) under laboratory conditions. The composition of the mixed seed (a mixture of 88% large and 12% small seed) caused it to have a similar germination to that of large seed. The hydrogen peroxide + kelp-p-max_large interaction treatment had the highest germination (92.3%), while the control_small treatment (60.8%) was the worst under nursery conditions. Large seed had the highest seed quality by having the best T50 (6.2 days), MGT (8.4 days), GV (21.7), GRI (11.5%), GI (1507.3), and CVG (12.2) values, with the small seed having the worst T50 (8.6 days), MGT (10 days), GV (10.1), GRI (7.6), GI (1041.8) and CVG (10.2) values, in the laboratory. T50 (15.4 days), MGT (19.0 days) and GRI (5.4%) for large seed and T50 (11.9 days), MGT (15.7 days) and GRI (6.6%) for the stratification pre-treatment demonstrated the highest germination speed with the best values in the nursery. The combination of final germination and germination speed was best for the stratification_large interaction treatment by showing the best GV (11.3), GI (3106), and CVG (7.3) values under nursery conditions. This indicates that large seed (in laboratory and nursery trials) was significantly more vigorous than small seed. Seedling height and sturdiness ratio was highest for large seed (33.2 cm and 8.7, respectively), while small seed had the lowest (22.4 cm and 6.7, respectively). RCD was highest for mixed seed (3.9 mm), while small seed had the lowest (3.4 mm). Seedling height and RCD (29.3 cm and 3.9 mm, respectively) were the highest for the stratification pre-treatment, while the hydrogen peroxide + kelp-p-max (8.1) pre-treatment had the highest sturdiness ratio. The hydrogen peroxide + hydro-prime pre-treatment had the shortest seedlings and smallest RCDs (26.1 cm and 3.5 mm, respectively), while the control had the lowest sturdiness ratio (7.0). Seedling height, ground line diameter, and biomass index were the highest for seedlings from large seeds (800.6 mm, 26.7 mm, and 634 454, respectively). Seedlings from the kelp-p-max pre-treatment were the tallest (749.6 mm) and were significantly taller than the seedlings from the hydrogen peroxide + hydro-prime pre-treatment (699.9 mm). Survival in the trial was high and survival for seedlings from the control_large, control_small, hydrogen peroxide_large, hydrogen peroxide_mixed, hydrogen peroxide + hydro-prime_small, hydrogen peroxide + kelp-p-max_large, hydro-prime_large, kelp-p-max_small treatments (100%) was significantly higher than for the kelp-p-max_mixed treatment (83.3%). Thus, the interaction between seed size and seed pre-treatment influenced germination under nursery conditions, while under laboratory conditions germination was dependent on seed size. Seed size was important for early seedling growth in the nursery, with large seed producing the tallest seedlings. Infield growth 12 months after planting was influenced by seed size and pre-treatment, while survival depended on the interaction between seed size and pre-treatment.