Diurnal Investment of Floral Resources in Justicia aurea of southern Belize

Diurnal Investment of Floral Resources in Justicia aurea

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Diurnal Investment of Floral Resources in Justicia aurea of southern Belize

Ian Gerard Kinahan

University of Massachusetts Amherst,

Belize Foundation for Research and Environmental Education


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Abstract

Nectar in flowers of Justicia aurea was measured for volume and sugar content at

seven different times of day in order to determine if there was a diurnal rhythm of

nectar and sugar production, and whether or not this mirrored diurnal foraging

patterns of Justicia aurea’s pollinator species. Statistical analyses indicated no

correlation between time of day and nectar/sugar production. Even so, data

suggested that nectar volume and sugar of the plant were high in the morning and

evening, and low in the afternoon, which corresponds with known visitation

patterns of Justicia aurea-‐pollinating hummingbirds.

Keywords: Justicia, aurea, nectar, volume, sugar, diurnal, rhythm, pollinator, foraging, time

Introduction

Successful and sustained pollination of most plants depends largely on the volume

and content of their nectar (Pleasants 1468). The rate at which nectar is produced and

secreted into flowers is as important a factor in attracting pollinators as the flower color,

morphology, odor, etc. (Amorim et al. 326). Nectar production rate is affected by

microenvironmental factors like temperature, humidity and incident light, but is also

known to vary in accordance with the behavior of a species primary pollinators (Wyatt et

al. 636). For example, Mitchell showed that nectar volume changes in flowers of Ipomopsis

aggregata resulted in a detectable change in hummingbird behavior, with birds spending

significantly more time probing flowers of high nectar volume compared to low volume


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flowers (30). Furthermore, he observed that plants of this species with naturally high

nectar production rates dispersed significantly more pollen than did others of its species

that had low nectar production rates (Mitchell 28). Thus, generally high nectar volumes

obviously attract more pollinators, but sustaining high nectar production is costly to a

plant. Secreting nectar in accordance with specific times of day at which primary

pollinators forage may therefore allow a plant to make itself selectively more attractive to

pollinators, while saving as much energy as possible. Thus, a well-‐adapted hummingbird-‐

pollinated plant might exhibit a nectar secretion pattern consisting of high volumes in the

morning and evening, when hummingbirds are foraging, and low volumes in the afternoon,

when pollinators are not active. In a study on seven Neoptropical hummingbird-‐pollinated

plant species, McDade and Weeks discovered that six of the seven species exhibited nectar

secretion patterns that were highest in the early morning, and by mid-‐morning, flowers

held almost no nectar (224).

While nectar volume is essential in attracting pollinators, the amount of sugar

within the nectar is equally as important. Sugar contained in the nectar of flowers is the

main source of energy for hummingbirds’ high energetic requirements (Heyneman 198). It

therefore seems likely that hummingbirds would spend more time browsing flowers with

nectar that contains the most sugar. A plant species that is able to secrete significantly

more sugar than others will probably attract more hummingbirds for a greater amount of

time and greater number of visits and, again, increase dispersal of pollen. In a study on 47

species of angiosperms found in the tropics, a significant, positive correlation was found

between floral visitors and how much sugar was in the flowers’ nectar, and it was also

determined that hummingbird-‐pollinated species have generally higher sugar content in


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the nectar than plants pollinated by animals other than hummingbirds (Wolff 774).

Furthermore, McDade and Weeks observed a diurnal pattern of sugar secretion in 12

Neotropical hummingbird-‐pollinated plant species (196), which suggests that a number of

hummingbird-‐pollinated plants may have evolved to secrete nectar-‐sugar in a diurnal cycle

that matches the diurnal foraging pattern of hummingbirds.

In this paper I examine nectar properties of the Neotropical angiosperm Justicia

aurea (figure 1), in order to determine if there is an evolved rhythm of nectar and sugar

secretion that matches the diurnal pattern of foraging of its hummingbird-‐pollinator

species.

Figure 1. An inflorescence of Justicia aurea, showing ornithophilous morphology.

Methods

Measurements of Nectar Volume-‐ Five Justicia aurea plants were examined in the

rainforest of the Belize Foundation for Research and Environmental Education. One

inflorescence on each individual plant, which was identified to be composed entirely of

unopened flowers that showed no signs of nectar robbery, was bagged at approximately

18:00. The following day, at 06:00, nectar was drained completely from one flower of each


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of the five inflorescences, using separate 10μl microcapillary tubes, for a total of five

samples. Flowers from which nectar was drawn were removed from the inflorescence after

draining, so as to prevent repeated measurements from the same flower later in the day. A

centimeter stick was used to measure the volume of nectar in each tube. This process was

repeated at times 07:30, 09:30, 11:00, 12:30, 14:00, 15:30, 17:00.

Measurements of Sugar-‐ At 06:00, all five acquired nectar samples were placed in a

refractometer in order to determine sugar concentration (all samples were used because

the volume of nectar in individual flowers was too small for the refractometer to analyze).

The total amount of sugar in the nectar was determined by multiplying the sugar

concentration, given by the refractometer, by the combined volume of all five flowers at

06:00. This was repeated at times 07:30, 09:30, 11:00, 12:30, 14:00, 15:30, 17:00.

Data analysis-‐ All analyses were conducted using JMP 11. Each data set was tested

for significance using t-‐tests. Linear and nonlinear fits were applied to change in nectar

volume and sugar over time. In the case of the nonlinear fits, outliers were removed from

the data set (i.e. flowers that secreted zero nectar at all times of day).

Results

Measurements of Nectar Volume-‐ Nectar volume did not appear steady when

measured in the field; it was observed to be considerably variable throughout the day. R2

value of 0.006 indicates no linear correlation between time of day and nectar production,

p-‐value of 0.6549 indicates that the data is not representative of a linear time-‐volume

relationship (figure 2).


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However, when outliers were removed from the data set (flower measurements that

were zero at all times of day and therefore deviated significantly from the rest of the data),

and a 2nd-‐order, polynomial fit was applied, a weak nonlinear relationship could be seen

between nectar volume and time of day. Figure 3 shows nectar volume to appear relatively

high in the morning and evening, and low in the afternoon; however, R2 and p-‐values are

still insignificant.

Measurements of Sugar-‐ Another 2nd order polynomial fit applied to a plot of sugar

against time showed a nonlinear pattern similar to that of nectar volume vs. time: relatively

high amounts in the morning and evening with low sugar present in the nectar in the

afternoon. Carbon investment by Justicia aurea thus appeared to have a diurnal pattern; yet

statistically speaking, the results were not significant (figure 4).

Figure 3. There appeared instead to be a nonlinear correlation between nectar volume and time of day; one in which the volume was high between the hours of 06:00 and 9:30, and from approximately 15:00 on, with low volumes in the afternoon. (R2 = 0.033, p = 0.6518)

Figure 2. Beginning at 06:00, as time of day increased, nectar volume did not increase accordingly, indicating that for Justicia aurea there is no linear correlation between time of day and nectar volume. (R2 = 0.006; p = 0.6549)


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Discussion

Measurements of Nectar Volume-‐ Figure 2 illustrates that there was no linear

correlation between nectar volume and time of day. This aligns with the hypothesis that

Justicia aurea may have evolved to secrete nectar strategically. For example, it is known

that hummingbirds visit Justicia aurea in the morning and evening, and not in the afternoon

(McDade and Kinsman 950). It is also known that the afternoon is the hottest time of day in

the Neotropical environment. Thus, it would be maladaptive for the plant to not only waste

energy increasing nectar volume in the afternoon, when hummingbirds are not likely to

visit the plant, but also to not use the fluid it is depositing in the nectar in order to increase

transpiration, or some other means by which to cope with heightened temperatures. Thus,

this lack of a linear correlation between nectar volume and time in Justicia aurea may

indicate that the plant has evolved a more adaptive strategy of nectar secretion.

For example, figure 3 shows a better fit of the data; it shows that nectar volume is

relatively high in the morning and evening and low in the afternoon. This reinforces the

Figure 4. Carbon investment by the plant appears high in the morning and evening, and low in the afternoon. This pattern appears diurnal, yet R2 and p-‐values are not significantly indicative of such a correlation. (R2 = 0.400, p = 0.3639)


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hypothesis that Justicia aurea may partition its floral resources in an adaptive way: by

corresponding high volumes with times of hummingbird foraging, and low volumes with

times of physiological need (transpiration). Because hummingbirds exhibit a diurnal

foraging pattern, and Justicia aurea (as seen in figure 3) exhibits a seemingly diurnal nectar

secretion pattern, the hypothesis that the plant has evolved to secrete more nectar when

hummingbirds are active is reinforced. For example, in a study on hawkmoth behavior in

relation to nectar properties of Mirabilis multiflora, it was determined that the more nectar

present in the plant’s flowers, the more frequently the plant would be visited by its

pollinator species, the hawkmoth. This was an important discovery because it was

subsequently determined that the greater the number of hawkmoth visits to this plant, the

greater the amount of pollen was distributed to other individuals of the species (Hodges

199). Thus, Justicia aurea’s apparent diurnal nectar secretion pattern may be an adaptive

strategy for enhanced pollen dispersal.

Measurements of Sugar-‐ Figure 4 illustrates carbon investment by Justicia aurea.

Statistical analysis showed this data, sugar amount vs. time, to have the strongest

correlation of all data collected. Thus, it is more likely that Justicia aurea exhibits a diurnal

pattern of sugar secretion than nectar secretion. Considering that the plant’s primary

pollinators forage diurnally (McDade and Kinsman 950), the most adaptively valuable

pattern of sugar secretion for Justicia aurea would be a diurnal cycle of high nectar-‐sugar in

the morning and evening, and low in the afternoon. Figure 5 shows McDade and Kinsman’s

findings on hummingbird visitation rates to Justicia aurea. The graph confirms that the

hummingbird species responsible for pollinating Justicia aurea visit the plant in a diurnal

pattern. When a nonlinear fit is applied to this visitation rate of hummingbirds to Justicia


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aurea, and is laid over the trendline from figure 4 (change in nectar-‐sugar amount over

time), a striking similarity can be seen between visitation rate and sugar secretion pattern

(figure 6). Justicia aurea thus appears to secrete sugar in a rhythmic manner that mirrors

the diurnal foraging pattern of its hummingbird pollinator species. High nectar-‐sugar is

found in the morning and evening, which corresponds with high visitation rates of

hummingbirds at these same times. Thus, even though the data is statistically insignificant,

this matching pattern is indicative of and reinforces the hypothesis that Justicia aurea has

evolved to invest its floral resources according to the foraging patterns of its primary

pollinator species for a competitive, efficient means of sustained and successful pollination.

Figure 5. McDade and Kinsman’s findings on hummingbird visitation patterns. Hummingbirds visited Justicia aurea in the morning, between the hours of 06:00 and 11:00; this was then followed by a period of zero visits to the plant during the afternoon. Visits picked up again at 15:00 and increased as the evening progressed. The hummingbirds thus exhibited a diurnal pattern of visits to Justicia aurea when foraging.


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Figure 6. Combined graphs of nectar-‐sugar amount over time and times at which hummingbirds visited Justicia aurea; the purple and orange line representing sugar investment pattern and hummingbird visitation pattern, respectively. Blue diamonds are data points for McDade and Kinsman’s hummingbird visitation rate data. There appears an appreciable similarity between sugar investment by the plant and visitation pattern of pollinating hummingbirds. Both appear to be diurnal.


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References

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Diurnal Investment of Floral Resources in Justicia aurea of southern Belize

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