Murray Grant, of Wellington, asks :-
What is the replenishment time for nectar in plants like flax, and pohutukawa that tuis feed on? i.e. how soon can a bird revisit the same flower?
Janice Lord, a botanist at the University of Otago, responded.
Your question is a bit like asking “how long is a piece of string”.
Flowers produce nectar to attract animals such as insects and birds, which then transfer pollen to other flowers to achieve cross-fertilisation. Although nectar is costly to produce, the benefit to the plant is that pollen is likely to be transferred more directly to another flower.
In general, nectar-producing flowers will replenish their offerings over the life time of the flower. However, the rate at which replenishment occurs depends on plant health, microclimate and flower age. Large, healthy, plants will not only produce more flowers, but each flower is likely to have more nectar and replace it more readily. Hot, dry conditions will cause nectar to evaporate or not be replenished as readily.
Some species will respond to pollinator visits by producing greater amounts of nectar, but for many species, nectar production is fairly constant until the flower reaches a certain age. For plants with hermaphroditic flowers, like NZ flax, or harakeke, each flower can function both as a male, exporting pollen to other flowers, and as a female, receiving pollen and producing seeds. When harakeke flowers first open they are functionally male, releasing pollen which is initially orange but which fades to yellow after 6 to 12 hours. Nectar production is greatest during this first day and is apparently replenished to some extent even if the flower has been visited. After 18-24 hours, the flower changes to the female phase and is ready to receive pollen from another plant. Nectar is still produced during this phase, but not in the same quantities as when the flower was releasing pollen.
This difference in nectar production between male and female phases is an efficient use of resources, as the amount of pollen exported to other flowers (success as a male parent) increases with the number of pollinator visits, hence it is worthwhile producing more nectar to attract more visitors. However, success as a female parent only requires one visit from a pollinator carrying enough pollen to fertilise all of the flower’s ovules, so there is little benefit in replenishing nectar.