Seed priming has been suggested to improve germination performance under temperature or water stress. The objective of this study was to develop an optimal osmopriming protocol for spinach (Spinacia oleracea L. cv. Bloomsdale) seeds and examine its effect on the germination performance at suboptimal and supraoptimal temperatures, and under water stress. Standard germination tests at optimal germination temperature (10°C) was conducted to evaluate performance of 'Bloomsdale' spinach seeds primed in sixty different priming treatments arising from various combinations of osmoticum (PEG 8000 concentration), priming temperature and priming duration. Germination performance was evaluated by final germination percentage, germination rate, and uniformity. An optimal priming protocol was selected and used to test seed germination performance at suboptimal (5°C) and supraoptimal (20°C) temperatures, and three levels of water stress (-0.8,-1.2, and-1.6 MPa). Priming at-0.6 MPa at 15°C for 8 d was the optimal protocol for osmopriming 'Bloomsdale' spinach seeds. This treatment also improved seed stress tolerance by improving germination performance at 5 and 20°C, and water stress of-0.8 and-1.2 MPa.
Aquaporins (AQPs) are proteinaceous channels known to regulate transmembrane water transport, and therefore may be important component of imbibition during osmopriming and germination. To explore the association between AQPs and osmopriming-led enhanced germination performance, we studied the expression patterns of four spinach (Spinacia oleracea) AQP coding genes (SoPIP1;1, SoPIP1;2, SoPIP2;1, and SoδTIP) during osmopriming and subsequent germination under optimal conditions, chilling, and drought. All these genes were up-regulated within 2 -4 d of priming (phase II-imbibition). We hypothesize such up-regulation to facilitate the pressure potential-driven cell expansion and increase germination potential of primed seeds. Our data during post-priming germination suggest that SoPIP1;1 and SoδTIP were more closely associated with enhanced germination performance. In general, all AQPs were downregulated under chilling and drought. However, under chilling, SoPIP2;1 was expressed at relatively higher level in primed seeds that also exhibited greater chilling tolerance, whereas SoPIP1;2 and SoδTIP exhibited opposite pattern. Similarly, SoPIP1;1, SoPIP2;1, and SoδTIP exhibited higher expression in primed seeds that also had greater drought tolerance.
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