The low intensity light reaction of stomata: effects of red light on rhythmic stomatal behaviour in Xanthium pennsyIvanicum

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Abstract
It has been shown (Mansfield 1964) that exposure to very low intensity light, during what would otherwise be a 16 h dark period, causes a phase shift in a rhythm of "opening ability" in Xanthium stomata so that they open less rapidly in bright light at the end of the period. This low intensity effect was found to be greatest at a wavelength of 703 nm, with old leaves, and this is now confirmed for young leaves, which are found to be even more sensitive. Interruptions of the 16 h dark period with short periods of relatively high intensity red light only produce a detectable phase shift if they are frequent. Phase shift is produced by 4 min of red light every half hour or 2 min every quarter hour and the latter treatment appears to be as effective as continuous irradiation with low intensity red light providing the same total energy. If the frequency or the intensity of the interruptions is reduced the resulting phase shift is much smaller. The large phase shift due to interruptions with red light every quarter hour is not reversed when each of these is immediately followed by a period of high energy far-red. The results suggest that this stomatal response to low intensity light operates through a pigment system different from that which drives the ordinary stomatal opening in light, for in the latter blue light is more effective than red. The possibility that phytochrome is involved in the low intensity reaction is considered.

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