Development in wheat as affected by timing and length of exposure to long photoperiod

Seeds of a spring wheat (Triticum aestivum L. cv. 'Condor') were vernalized and then grown at 19 degrees C in two naturally-lit environments, one with a moderate (12 h) and the other with long (18 h) photoperiod. Treatments consisted of transfers of plants from the moderate to the long photoperiod chamber on different occasions, or for periods of different durations, The main objectives were to determine whether wheat development responds to current and previous photoperiodic environments and whether there is a juvenile phase when the plants are insensitive to photoperiod. Plants under constant 18 h photoperiod had fewer leaves which appeared faster than those under constant 12 h photoperiod (i.e. phyllochron was increased from 4.4 to 5.1 d leaf(-1)). Plants transferred from 12 h to 18 h photoperiod at terminal spikelet appearance (ISA) reached anthesis 4 d earlier than plants retained at 12 h, while plants under continuous long photoperiod (18 h) completed this phase most rapidly, Thus, there was some evidence for a historic effect of photoperiod on development. Exposure to long photoperiod during the first 5 d after plant emergence accelerated the rate of development towards anthesis, suggesting that there was no juvenile period of photoperiodic insensitivity. There were, however, changes during ontogeny in the degree of sensitivity to long photoperiod, increasing from seedling emergence to a maximum c. 15 d later, and then decreasing again. Although all treatments were imposed before TSA, the response was not limited to the pre-TSA phase, suggesting that well before the terminal spikelet appeared, the plant was already committed to the initiation of this spikelet. Spikelet number decreased with delayed transfer to long photoperiod with a minimum for plants transferred to long days from 16-20 d after seedling emergence. Additionally, there was a trend for an increase in the rate of leaf appearance (decrease in phyllochron) when the plants were exposed to long days between 10 and 35 d after seedling emergence. Although the differences were small, when considered in conjunction with the effects on final leaf number they become important in explaining differences in time to anthesis.