Effect of flower color and sampling time on volatile emanation in alfalfa flowers

Flower volatile compounds of alfalfa (Medicago sativa L.) are involved in a ttractiveness to pollinating insects, and thus influence seed set. This stu dy investigated differences in volatile emanation in this species by varyin g flower color of the sampled material or time of flow er collection. Chemi cal determinations were made by gas chromatography and gas chromatography-m ass spectrometry analyses. The first investigation examined volatile concen tration in clones characterized by seven different color ratings. Significa nt variation among colors was found with a trend for greater emanation From dark- than from light-colored flowers. This trend could bear an evolutiona ry significance to compensate for the possibly lower visibility of dark flo wers in a stand. The second investigation analyzed volatiles from intact in florescences and from florets and rachises taken separately. A substantial similarity of volatile amount and composition among the three kinds of samp les was observed, The final investigation assessed the volatiles produced a t different times of the day in each of the first three flowerings of the g rowing season, during which alfalfa pollination was expected to take place. Confirming previous evidence, temporal patterns of emanation were observed , with a sharp increase from the flowering in May to those in July and Augu st and a strong time-of-day effect during the two summer flowerings (burst of volatiles around midday). The marked seasonal effect appeared somewhat r elated to temperature increase while the daily variation of emanation in su mmer paralleled that of solar radiation, There seems to be a coincidence be tween conditions for maximum volatile emanation and reported optima for ins ect foraging activity.