Alteration of Sex Expression in Summer Squash
by Gibberellic Acid, West Tennessee Experiment Station, 2000
‘Multipik’ produced more first flowers which were pistillate than ‘Gold Slice’. The pistillate/staminate ratio of first flowers and the average number of days from first pistillate to first staminate flower were higher in ‘Multipik’ compared with ‘Gold Slice’. GA at 50 or 100 ppm caused development of more early staminate flowers and reduced early fruit number and early yields.
Many summer squash hybrids begin flowering by producing one to several pistillate flowers before any staminate flowers are produced. The period of time when no staminate flowers, and consequently no pollen, is available can be five to eight days, depending on the hybrid and the season of the year. This usually causes fruit which develop during this time period to either abort immediately or to grow for a few days and then wither and become inedible. If this is a spring crop, it will be at a time when squash prices are highest and loss of the early crop will be costly.
The objective of this research was to determine if a growth regulator, gibberellic acid (GA), will change the sequence of flower production in summer squash in order to have staminate flowers present when pistillate flowers start to develop.
Materials and Methods
Yellow summer squash hybrids ‘Gold Slice’ and ‘Multipik’ were planted at the West Tennessee Experiment Station at Jackson on 16 May. The test was replicated four times and arranged in a split plot design with squash hybrids as main plots and GA rates as subplots. Fertilization and pest control measures were according to recommended practices for summer squash production in West Tennessee. GA treatments were 50 and 100 ppm GA applied when the first true leaf was one to two cm in diameter and repeated on the third day following the first treatment. GA was sprayed on the seedlings to runoff at each treatment. A control treatment was sprayed to runoff with water. First and second GA treatments were applied on 26 and 29 May, respectively.
Pistillate and staminate flowers were counted on each plant in the test from 14 June when flowering began and continued daily for 13 days. Fruit were harvested every two or three days from 23 June through 5 July. Data are reported on number and yield of marketable fruit less than 2 ˝ inches in diameter.
Two calculations of GA effectiveness were made. First, the ratio of the number of plants in a plot which first produced pistillate flowers to those which first produced staminate flowers was calculated. Second, a "flowering index" was calculated to estimate the average days from first pistillate to first staminate flower. If a plant produced a staminate flower before a pistillate, the number of days between the two events was assigned a positive number; if a pistillate flower was produced first, the number of days until a staminate flower was produced was assigned a negative number. The negative and positive numbers were totaled and the sum was divided by the number of plants in the plot. Analysis of flowering and yield data was by appropriate ANOVA procedures and differences were separated by LSD.
Results and Discussion
‘Multipik’ produced significantly more pistillate first flowers than staminate first flowers than ‘Gold Slice’ (Table 1). This indicated by the pistillate/staminate ratio of first flowers as well as the average number of days from first pistillate to first staminate flowers on the two hybrids. This characteristic may have also contributed to the significantly higher number of early fruits produced and the higher early yield of ‘Multipik’ compared with ‘Gold Slice’.
Application of GA at 50 or 100 ppm caused significant reductions in the ratio of pistillate to staminate flowers and the average number of days from first pistillate to first staminate flower (Table 1). No significant differences were found between GA rates.
The data on fruit numbers and marketable yield indicate that early in the development of squash flower buds, the GA applied at both rates altered the final sex expression of the flowers. Those which would have normally developed into pistillate flowers were converted into buds which developed into staminate flowers. This conversion from female to male flowers resulted in less yield from the treated plots for the first three harvests, although only the June 23 harvest was reduced significantly.
Application of GA to a few rows in a planting of commercial summer squash should increase early yields, even though early yields on the treated rows will probably be reduced.
zAverage days from first pistillate flower to first staminate flower on a squash plant
yBased on 17 plants/plot
xFirst three harvests; GA treatments were not significant for remaining three harvest days
wMeans for treatments followed by the same letter are not significant, LSD 5% level of P.