Dynamic Nature of the Photoperiod Response in Maize1

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Abstract
A general quantitative description of maize (Zea mays L.) response to photoperiod for use with widely different genotypes and environments is important in order to accurately evaluate stages in maize ontogeny. This paper describes a method for combining photoperiod and maturity information into a relatively simple model to estimate the interval from seedling emergence to tassel initiation and tassel emergence in maize. Twenty contrasting cultivars of maize were grown to tassel emergence in the Duke Univ. Phytotron at 25°C under photoperiods ranging from 10 to 17.5 h. Dates of tassel initiation were determined destructively and tassel emergence nondesrructively. An apparently general linear relationship was found for estimating the thermal time (degree days) between seedling emergence and tassel initiation from the thermal time between seedling emergence and tassel emergence. This implies that the timing of tassel initiation can be estimated with reasonable accuracy when the temperature and date of tassel emergence are known. The time to tassel initiation was not influenced by photoperiod for all cultivars studied. Those that were photoperiod sensitive experienced a delay in rime to tassel initiation only when the photoperiod was greater than a critical threshold value ranging from 10 to 13.5 h. Below the threshold photoperiod, plants appeared to be insensitive to photoperiod. For photoperiods above the threshold value, the delay in thermal time to tassel initiation was described as a linear function of photoperiod above the threshold value. For the cultivars studied, the photoperiod sensitivity ranged from zero for six insensitive early maturity cultivars to 36 degree days per hour increase in photoperiod for one late maturity cultivar.

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