RESPONSE-SURFACE MODELS TO DESCRIBE THE EFFECTS AND PHYTOTOXIC THRESHOLDS OF MICROWAVE TREATMENTS ON BARLEY SEED-GERMINATION AND VIGOR

The effects of various combinations of microwave treatment factors: ab sorbed microwave power (AMP), treatment duration (DUR), pulsing (PUL, the time in seconds the microwave is on/off per min of duty cycle) and initial seed moisture content (SMC) on barley (Hordeum vulgare L.) se ed germination and seedling vigour were studied using a factorial appr oach and surface response methodology, Among the variables studied, th e AMP and PUL were the most important factors affecting seed germinati on and seedling vigour. High seed germination (> 90.0%) can be sustain ed at combinations of AMP < 0.38 W/g and PUL < 38/22 s, when the DUR a nd SMC were held constant al: 75 min and 21%, respectively. Seedling v igour, expressed as the mean plumule length (MPL), was much more affec ted than seed germination, and the nature of interactions among microw ave treatment factors were quite complex. Only PUL had significant mai n effects on vigour; however, the magnitude and direction of these eff ects were dependent on the levels of other factors. At fixed DUR and S MC, the MPL dropped rapidly below 70.0 mm when PUL > 47/13 s and AMP < 0.55 Wig. At fixed DUR and SMC, any combinations of AMP from 0.2 to 0 .6 W/g and PUL < 38/22 s or AMP < 0.38 W/g and PUL < 47/13 s yielded g ermination values > 90.0% and MPL > 70.0 mm. These two values were est ablished as thresholds considering the commercial seed certification l imit of 85.0% seed germination and because the response surface slopes were steeper beyond these values. These two seed quality thresholds w ere quite close to values observed for the non-microwave treated check (germination = 92.5% and MPL = 77.0 mm). Therefore the combinations o f microwave treatment factors, that do not reduce seed quality below t he thresholds established here; could be considered for potential inac tivation of seed-borne pathogens of barley.