Seedling emergence response to early sowings in unprimed and osmoprimed seeds of fiber sorghums for energy biomass under semi-arid climate

Fiber sorghum is a promising industrial crop for lignocellulosic biomass to be used for energy purposes. Limits to the adoption of early sowings (March-April) for this species are found in its thermal requirements for seed germination (>10°C). Seed osmopriming has been found to decrease the impact of low temperatures on seed germination of sorghum in laboratory. In this study, unprimed (UP) and primed (P) seed germination behavior in both laboratory at cold temperatures and open field conditions under early sowing in a typically Mediterranean environment, was examined in four cold-tolerant cultivars of fiber sorghum (Padana 4, PR 811F, Jumbo and PSE 24213). Seed germination in UP at 8°C was on average 82.4%, confirming the high cold-tolerance of all cultivars selected for the experiment. Germination was progressively delayed by the lowering of temperature from 25°C to 8°C. Seed osmopriming was effective in promoting germination speed at all temperatures in laboratory. It also resulted in an appreciable lowering of thermal time requirements for final germination. With the earliest planting date (March 10), minimum soil temperatures during the first ten days never dropped below 10.2°C, allowing a final seedling emergence of 79.2% in UP. However, low soil temperatures slowed down the rate of seed germination. At the 2nd sowing (end of March), daily maximum temperatures of soil exceeded 20°C but those minimum dropped below 10°C, slightly depressing seed germination in UP (76.3%). With the latest sowings (mid-April and early May), minimum soil temperatures always over exceeded minimum threshold for seed germination, allowing a regular seed germination in UP. Priming increased final emergence percentage in the two earliest sowings (those of March), whilst it enhanced the germination speed in all planting dates. Seed osmopriming also reduced thermal time requirements until the first seedling emerged from soil and promoted the seedling growth in all cultivars and sowing dates. These results suggest that it is possible to adopt early sowings (March) in cold-tolerant cultivars of sorghum under semi-arid climate, and that osmopriming allows the seed to partially overcome the limit imposed by cold soil to germination, contributing to a successful crop establishment when early plantings are adopted. Although the germination response to constant low temperatures in laboratory tests does not always provide a good index for predicting seedling emergence in soil under fluctuating uncontrolled temperatures, it may contribute to define desirable attributes for the identification of cultivars adapt to early sowings in the Mediterranean areas.