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Abstract
Various histological criteria were employed in studying the chronological and morphological development of the apex of field-grown plants of Brassica oleracea var. italica, cv. ‘Coastal’. Time of differentiation was based on changes in the size and configuration of the apex. The earliest evidence of reproductive differentiation was 5 weeks after sowing or at the time of macroscopic appearance and unfolding of the eighth true leaf. First order floral stalks began to appear at 7 weeks or at the time of macroscopic appearance of the 14th true leaf. Thus, the time interval for the transition from a vegetative to a reproductive apex appears to be approximately 2 weeks, under field conditions. By 9 weeks after sowing, or after the macroscopic appearance of the 22nd true leaf, second order floral stalk initiation and hence inflorescent head formation predominated.
Abstract
Environmental growth chamber studies were employed to determine the influence of temperature, daylength, and seed vernalization on reproductive differentiation of the shoot apex and subsequent formation of the inflorescent head or “button”. Under the conditions employed, low temperature alone did not directly influence differentiation or button formation. However, low temperature markedly retarded vegetative growth so that the plant underwent differentiation and formed buttons at a younger morphological age. Low temperature had no influence on the chronological time of differentiation or button formation except under continuous light. Therefore, in studying the influence of temperature on flowering, a chronological index should be used alone or conjointly with a morphological index.
Under continuous light, seed vernalization or low ambient plant growing temperatures resulted in both chronological and morphological earliness of differentiation and subsequent button formation. It is suggested from these results that a low temperature-mediated phytochrome system may play a part in the control mechanism for flowering of green sprouting broccoli.