Physiological activity in persistent bulbils of Agave vilmoriniana (Agavaceae)

Abstract

Plants of Agave vilmoriniana are unique members of a reproductively diverse genus since they produce prolific numbers of both bulbils and viable seeds. Bulbils were observed to remain attached to the flowering stalk into a second summer, instead of abscising in the year they developed. Hence, a number of hypotheses were tested to establish the physiological basis for their longevity and evaluate their influence upon the plant's reproductive biology. Bulbils produced following anthesis were abundant (>2000 bulbils), large (0.44 g dry mass per individual), and their combined leaf surface area (one side) could exceed 3 m2. Thus, the current‐year inflorescence may act as a photosynthate source. Second‐year, attached bulbils are healthy‐looking, even after leaves of the parent plant have senesced, since maternal roots continue to absorb rain water. Rehydrated 2nd‐yr bulbils resume Crassulacean acid metabolism, sometimes demonstrating net daytime tissue acidification. Water conservation by bulbils is very efficient, exhibiting very low cuticular conductances (0.016‐0.0035 mm/s). Bulbils detach over all climatic seasons after having formed >3 and >18 root primordia in their 1st and 2nd year, respectively. About one‐third of bulbils are capable of forming roots after a 1‐yr storage at 35°C, when their water contents averaged 37%. Irrigated bulbils produce water‐absorbing roots within 2 d, with de novo chlorophyll synthesis and nocturnal acidification commencing within 4 d. Two bulbils were observed to root and become established in shaded microhabitats during this 2‐yr study of 29 plants.