Date of Award

Spring 1996

Project Type


Program or Major

Plant Biology

Degree Name

Doctor of Philosophy

First Advisor

James Pollard


Critical photoperiod for flowering was determined for short-day (SD) and day-neutral (DN) genotypes of strawberry used in SD x DN crosses to investigate whether variation in critical photoperiod contributes to variation in frequency of DN progeny and, hence, may be an effective selection criterion for screening SD genotypes for use as parents in breeding regionally-adapted DN cultivars. Twelve SD and four DN genotypes were grown in controlled environments under eight photothermal regimes (four photoperiods (9, 11, 13, 15 hr) x two day/night temperatures (19/14 and 24/19$\sp\circ$C)). Among the SD genotypes, critical photoperiod varied more under cool than warm regimes. Under cool regimes, SD genotypes were ranked into three groups: rank 1 (four genotypes) having an apparent critical photoperiod of between 11 and 13 hrs, rank 2 (five genotypes) of between 13 and 15 hrs, and rank 3 (three genotypes) of greater than 15 hrs. Under warm regimes, apparent critical photoperiods were shorter, between 9 and 11 hrs for one SD genotype and between 11 and 13 hrs for eleven SD genotypes. Critical photoperiods were not detected among DN genotypes, as each flowered under every photothermal regime, nor was there variation in critical photoperiod within genotype, whether SD or DN.

Nine of the twelve SD genotypes, selected for variation in critical photoperiod, were crossed with the four DN genotypes. Fifty progeny per cross were classified as either SD or DN (distinguished by autumn fruiting) after two seasons in the field in Durham, NH. The proportion of DN progeny within crosses varied between 0.18 and 0.56 and, according to Spearman's rank correlation procedure, was not significantly related to critical photoperiod of the SD parent.

Chi-square tests for the probability that SD and DN phenotypes segregate 1:1 were conducted in response to recent reports that the DN trait is controlled by a single dominant gene. Forty-two percent of the SD x DN crosses failed to segregate 1:1 and, therefore, to fit the single gene model. A heterogeneity chi-square analysis revealed that the total population of 36 progenies was heterogeneous. All progenies which deviated from the expected 1:1 segregation ratio did so in favor of the SD phenotype.

The differences observed in proportion of DN progeny among the SD x DN crosses within this and between other studies suggest moderate to high levels of specific combining ability as well as considerable genotype x environment interactions for the DN trait. In the absence of phenotypic markers for the selection of superior SD parents for breeding regionally-adapted DN cultivars, progeny testing to take advantage of specific combining ability will remain important.