Temporal changes in floral nectar production, reabsorption, and composition associated with dichogamy in annual caraway ( Carum carvi ; Apiaceae)

The dynamics of nectar production were studied in perfect florets of two varieties (Karzo, Moran) of annual caraway ( Carum carvi L., Apiaceae). Florets were protandrous and strongly dichogamous, lasting 7–15 d but producing nectar from the stylopodia for 4–12 d, in an interrupted fashion. Nectar secretion began during a floret's phase of stamen elongation and anther dehiscence. After reabsorption of uncollected nectar, at which point nectary surfaces were completely dry, the two styles elongated and a second bout of secretion commenced during the female phase, up to 5 d later, when a floret became receptive to pollination. During the male and female phases, respectively, 0.392 ± 0.064 μL and 1.083 ± 0.261 μL of nectar of similar solute concentration (844 mg/mL) was produced per ten florets. On a daily basis, florets yielded 1.5‐fold more nectar in the female than during the male phase. First‐time nectar removal throughout the female phase did not match the sum of nectar quantities from male and female phases combined, suggesting that under natural conditions, any uncollected male‐phase nectar, once reabsorbed, is not made available to visitors of the same florets when in the female phase. Nectar‐sugar composition differed between bouts of secretion; it was hexose‐rich (59.6% fructose, 26.9% glucose, 13.6% sucrose) initially, but hexose‐dominant (70.2, 26.8, 3.1) during the female phase. A 5.7‐fold difference in mean nectar production per floret occurred among plants.