Water availability shapes temporal patterns of extrafloral nectar secretion and ant visitation to a Neotropical legume

Mutualistic plants use non-structural sugar (NSC) to produce carbon-based resources to reward partners. Then, any factor compromising NSC plant supplies should increase the relative costs of these rewards for plants. Drought, for instance, can initially reduce plant growth but not necessarily photosynthesis, which could boost NSC supplies. However, if drought persists and compromises plant water status, photosynthetic rates decline, reducing NSC reserves and potentially impacting the plant's investment in mutualistic rewards. We hypothesized that plants would initially increase extrafloral nectar investment - a sugary solution attractive to ants - at the onset of water shortage, increasing their attractiveness to ants. However, such investment and attractiveness gradually decrease over time as drought lasts and the plant water status is compromised. Here, we experimentally manipulated soil water availability and water potential of Chamaecrista nictitans (Fabaceae) and evaluated the effects of soil water decline over time on patterns of extrafloral nectar secretion and plant visitation by ants. We observed that C. nictitans had more active extrafloral nectaries producing more concentrated nectar at drought onset, while plant water potential was not affected. However, it gradually declined as the water shortage progressed, and plant water potential declined. Ant visitation followed a similar temporal pattern, peaking at the experiment onset and diminishing over time as the drought lasted. These findings emphasize the pivotal role of drought severity in shaping the temporal dynamics of extrafloral nectar secretion and likely other carbohydrate-based mutualistic rewards, shedding light on the physiological mechanisms regulating mutualisms.