THE EFFECTIVENESS OF VERNALIZATION OF IMMATURE EMBRYOS OF WINTER-WHEAT VAR GRANA AS RELATED TO AGE AND EXOGENOUS PHYTOHORMONES

Starting from the 10th day after pollination, immature embryos of wint er wheat var. Grana were isolated and then vernalized for 4 to 8 weeks on Murashige and Skoog nutrients containing BAP (0.5 or 2 mg/dm3), NA A (0.5 or 2 mg/dm3), or GA3 (5 or 20 mg/dm3). Vernalized seedlings wer e cultivated in a greenhouse and the number of days to the heading of individual plants as well as the percentage of plants capable of gener ative development were estimated. The lower limit of size for 50 % sur vival of embryos strongly depended on the phytohormone used: from 0.9 mm in control, 1.1 mm in nutrient containing BAP, 1.2 mm for NAA, up t o 1.7 mm in nutrient with GA3. Exogenous GA3 was lethal for embryos yo unger than 18 days but induced elongation of older embryos. Embryos is olated 2.5 to 4 weeks after pollination showed minimal requirements fo r the length of vernalization. BAP increased the percentage of heading plants originated from the youngest embryos. GA3 improved partial ver nalization, strongly increasing the percentage of heading plants, but did not change the time from the end of vernalization to heading. It h as been postulated that GA3 increases number of plants capable of over coming the threshold of induction of generative development but does n ot accelerate the flowering process. Hormonized plants showed no defor mation of generative development. As the effectiveness of vernalizatio n increased, the heading of plants was faster but they were shorter, f orming spikes with a smaller number of spikelets and producing fever l ateral shoots. The very young embryos probably have in reserve suffici ent amounts of auxins and gibberellins and therefore exogenous GA3 dec reases their viability or even exerts a deleterious effect. However, a s the embryos' ageing, gibberellin starvation develops. This being esp ecially pronounced during vernalization. The de novo synthesis or acti vation of gibberellins takes place during the second stage of vernaliz ation. This is why exogenous hormone improves the effectiveness of par tial vernalization, though it is not possible to substitute by gibbere llins the vernalization requirements of immature embryos.