Pineapple stem-derived bromelain based priming improves pepper seed protein reserve mobilization, germination, emergence and plant growth.

Abstract

Pepper seeds are slow to germinate and emergence is often non-uniform and incomplete, reducing gains from this cash crop. This study investigated the effects of pineapple stem- derived protease (stem bromelain) based priming on pepper seed germination in relation to reserve mobilization (specifically, proteins and aminoacids), germination, emergence and plant growth. These parameters were compared across two controls, (1) unsoaked seeds and (2) seeds soaked in deionized water, and seeds soaked in pineapple stem bromelain crude extract (treatment). Seeds were soaked in bromelain crude extract possessing a proteolytic activity of 6.25 tU or deionized water (first control) for 3h at 35°C. Light microscopy revealed an abundance of protein bodies in the endosperm of the seeds prior to imbibition. When observed for a period of 96h, these bodies were progressively degraded, with the rate of this degradation being fastest in bromelain-treated seeds. Quantitative analysis of protein levels confirmed this observation: 17.2mg proteins/g FW at 120h after priming in bromelain-treated seeds compared with 22.1mg/g FW in controls (average). The bromelain treatment also increased levels of free amino acids from 3.9mg/g FW in the controls to 4.6mg/g FW after 120h of imbibition. Germination and emergence percentages were initially higher in bromelain-treated seeds: 92.0% germination in bromelain-treated seeds vs. ~ 52.2% in the controls at 18 d; 100% emergence in protease-treated seeds vs. ~ 72.2% in the controls at 18 d. However, these parameters were comparable across the treatment and the controls at 28 d. Importantly, plant fresh and dry weights were significantly higher when seeds were primed with bromelain. The results suggest the use of bromelain extracts for priming pepper seeds based on their proteolytic activity, since germination is dependent on the availability of crude protein and essential aminoacids. The benefits of bromelain seed priming appear to translate into improved seedling growth as well.