EFFECTS OF PHOTOPERIOD ON REPRODUCTIVE DEVELOPMENT IN VELVETLEAF (ABUTILON-THEOPHRASTI)

When velvetleaf plants from Mississippi and Minnesota populations were maintained in growth chambers with day/night temperatures of 29/23 C and photoperiods of 11, 12, 13, 14, and 15 h, newer buds and open flow ers appeared first in the 12 h photoperiod. Buds and flowers appeared 2 to 4 d later at photoperiods of 11, 13, or 14 h, Increasing the phot operiod beyond 14 h to 15 h delayed bud appearance an additional 7 d i n the MN plants and 12 d in the MS plants, Open flowers appeared four to five nodes higher, 10 d later at 15 than at 14 h in the MN plants a nd 20 d later in the MS plants, Vegetative shoot weight and fruit weig ht 73 d after emergence were greater in 13 h or longer photoperiods th an at 11 or 12 h, In shorter photoperiods, MN plants produced more veg etative growth than MS plants, but the reverse occurred at longer phot operiods where MS plants were taller than MN plants, These growth diff erences occurred because earlier shifts in allocation to reproductive growth in MN plants limited their vegetative growth, particularly in t he longest photoperiods, Differences in rate of reproductive developme nt between populations were not evident until photoperiod exceeded 13 h, Reciprocal transfer of plants of the MS population between short an d long photoperiods revealed the durations of the juvenile (pre-induct ive), inductive, and post-inductive phases to be 3 to 5, 7 to 8 (short day) or 30 (long day), and 10 to 11 d, respectively, Differences in c ompetitive ability among latitudinal biotypes of photoperiodically-sen sitive weeds may depend on time of emergence in the field and conseque nt photoperiod exposure, Weed growth simulation models to be used in d evelopment of expert systems for weed management should take photoperi odic sensitivity into account.