Photoperiod and the Determination of Potential Seed Number in Chenopodium rubrum L.

Individuals from two latitudinal populations of Chenopodiumrubrum, a short-day annual, were induced in two inductive photoperiods,15 h and 12 h, to examine the dynamics of reproductive developmentthat determine the potential number of seeds produced. The northernpopulation (50° N) is induced in both photoperiods, whilethe southern population (34° N) is induced only in the 12h photoperiod. Individuals were given either 2, 6, 10 or continuousinductive cycles and dissected at intervals after the startof inductive treatments to determine the rates of initiationand differentiation of primordia on the main axis and selectedaxillary buds. Initial reproductive data indicated that the duration of reproductivedevelopment among individuals of the northern population, whengrown in the longer photoperiod, was 25 per cent greater, butthe number of seeds was increased by a factor of 46. Likewisethe duration of reproductive development among individuals ofthe southern population, when grown in the same photoperiodas the northern population, was 50 per cent longer and the numberof seeds was increased by a factor of 66. Dissection of reproductively-developing individuals revealedthat induction leads to a stimulation in the rate of initiationof primordia followed by a complete inhibition coincident withthe differentiation of terminal floral structures. The timingof this stimulation-inhibition process on each axillary buddepends upon its age relative to the timing of induction. Thuscertain primordia on individuals prematurely removed from inductivetreatments escape floral differentiation and remain vegetative.The total number of floral primordia (potential number of seeds)is determined early in reproductive development by (1) the numberof axillary buds at the start of induction, (2) the stimulatedrate of initiation of primordia after induction, and (3) therate of differentiation of induced primordia. Among individuals of the northern population, the longer inductivephotoperiod leads to a greater stimulation in the rate of primordialinitiation and a decrease in the rate of floral differentiation,which together lead to the production of more primordia. Likewiseamong individuals of the southern population in the shorterphotoperiod, the rate of organ initiation is similar to thatof the northern population in the same photoperiod, but therate of floral differentiation is lower, leading to the initiationof many more primordia. The effects of photoperiod on seed number are discussed in termsof physiological and ecological criteria of optimality. Sincenatural induction occurs in the longest, physiologically-'sub-optimarphotoperiods possible, and this leads to the greatest productionof seeds, it is suggested that the critical photoperiod is amore meaningful focus of interpretation than the physiologically‘optimal’ photoperiod. Of the factors influencing potential seed number, the most significantdifference between the two latitudinal populations is the responseof the rate of floral differentiation to the photoperiod ofinduction. Thus potential seed number in natural populationsis intimately related to the prevailing photoperiods throughthe rates of developmental events. Selection for changes inor maintenance of a particular reproductive ecology must bemediated through developmental responses. Limitations on the potential growth rate of plants are discussedin terms of the ratio of meristematic cells to the total cellpopulation in the plant. Thus the number of growing axillarybuds greatly contributes to the potential growth rate, and thedegree of correlative inhibition is interpreted as a cost ofselection for vertical growth among terrestrial plants competingfor light. By means of a simple model the cost of correlativeinhibition is also discussed in terms of potential seed number.Reproduction is seen as a release from the developmental constraintsrequired by plant form and a stimulation of growth that leadsto a very high production of potential seeds.