PHOTOPERIODISM IN SEEDS AND BUDS 



P. F. WAREING 



Department of Botany, University College of Wales, 

 Aberystwyth, Wales 



Although only a small fraction of the total work on photoperiodism 

 has been devoted to the effects of day length on dormancy, neverthe- 

 less it is clear that such effects are very widespread in higher plants, 

 particularly in woody species. The seedlings of many woody species 

 respond to short days by ceasing active extension growth and forming 

 resting buds, whereas under long-day conditions the onset of dormancy 

 is delayed or entirely suppressed (for the relevant literature, see 

 Wareing, 1956). Moreover, in certain species, such as Fagiis sylvatica, 

 Betida pubescens, Larix europaea, and Pinus spp., seedlings that have 

 been rendered dormant by previous short-day treatment can be induced 

 to resume growth by transferring them to long-day conditions or con- 

 tinuous illumination. Even leafless dormant seedlings of these last 

 species expand their buds under long days, whereas under short days 

 they remain dormant. There is every indication that this is a real 

 photoperiodic effect (Wareing, 1953). Thus, in these species we have 

 photoperiodic sensitivity even in the very young leaf primordia present 

 in the resting buds. When this fact was first established it seemed very 

 unusual to find photoperiodic sensitivity in embryonic tissue, since it 

 was generally held that, in the photoperiodic control of flowering in 

 herbaceous species, it is only the fully expanded leaves which are sensi- 

 tive to daylength conditions. More recently, however, it has been 

 shown that, in Xcmthium, it is the half-expanded leaves which are most 

 sensitive (Khudairi and Hamner, 1954). The difference between 

 woody and herbaceous species with respect to the sensitivity of the 

 immature leaves would thus appear to be one of degree only. 



SEED GERMINATION IN VARIOUS SPECTRAL REGIONS 



The observation that the buds of birch seedlings show photoperiodic 

 responses led us to investigate whether the seeds of this species show 



73 



