Abstract
Heterodichogamous reproduction in plants involves two flowering morphs, reciprocal in their timing of male and female sexual functions. The degree of synchrony in floral sex phase, within and between individuals of each morph, determines the flowers’ potential fertilization partners. Complete within-morph synchrony enables across-morph mating alone, whereas unsynchronized floral sex phases may allow fertilization within a plant individual (geitonogamy) or within a morph. We documented the disruption of flowering synchrony in the heterodichogamous Ziziphus spina-christi towards the end of its seven-month flowering season. This desert tree has self-incompatible, protandrous, short-lived (2-day) flowers that open before dawn (‘Early’ morph) or around noon (‘Late’ morph). We counted flowers in the male and female phase on flowering branches that were sampled monthly during the 2016–2018 flowering seasons. In 2018, we also tagged flowers and followed their sex-phase distributions over two days at the start, middle, and end of the season. The switch to the female phase was delayed at the end-season (November-December), and 74% of the flowers did not develop beyond their male phase. Differences in male-phase duration resulted in asynchrony among flowers within each tree and among trees of both flowering morphs. Consequently, fertilization between trees of the same morph becomes potentially possible during the end-season. In controlled hand-pollination assays, some within-morph fertilizations set fruit. The end-season breakdown of synchronous flowering generates variability within morphs and populations. We suggest that this variability may potentially enable new mating combinations in a population and enhance its genetic diversity.
Highlights
Dichogamy entails the separation in time between the male and female functions of hermaphrodite flowers [1,2]
Sex expression became asynchronized between trees under some conditions. Another example of environmentally induced asynchronized flowering was reported for M. thunbergii, in which the female phase of the flowers was delayed on rainy days, potentially leading to overlap with male-phase flowers on the same trees [12]
Such anecdotal observations suggest that some ambient conditions interfere with synchronized flower development. They raise a number of open questions relating to the breakdown of synchrony in heterodichogamous plants: Are both flowering morphs and sex phases disrupted? Is flowering synchrony disrupted to the same extent within and among plant individuals? In addition, can the disruption of synchrony allow fertilization within each flowering morph?. We addressed these questions in the heterodichogamous desert tree Z. spina-christi (Rhamnaceae), one of the Ziziphus species exhibiting heterodichogamy [19,21,22]
Summary
Dichogamy entails the separation in time between the male and female functions of hermaphrodite flowers [1,2]. In heterodichogamous species, perfect synchrony in the flower development across a population is manifested as the exact concurrency of the male phase of one morph with the female phase of the other. Another example of environmentally induced asynchronized flowering was reported for M. thunbergii, in which the female phase of the flowers was delayed on rainy days, potentially leading to overlap with male-phase flowers on the same trees [12] Such anecdotal observations suggest that some ambient conditions (such as low temperatures) interfere with synchronized flower development. They raise a number of open questions relating to the breakdown of synchrony in heterodichogamous plants: Are both flowering morphs and sex phases disrupted? Disruption of between-plant synchrony would generate possibilities for within-morph reproduction
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