Agave — Characteristics
Since Agave is a whole genus with many different species, each one has specific characteristics, however, in general, they are known for having similar characteristics that will be described below.
Agave is one of the most interesting and complex plant groups, due not only to its great diversity in species, but also to its characteristics and adaptations, some really remarkable in the plant kingdom.
Among its adaptations we can mention the suicidal reproduction, where after growing for many years, they produce largest inflorescences and then die; its remarkable coevolution with its main pollinators, nectar-eating bats; its remarkable photosynthetic metabolism that allows them to use water very efficiently; and its great succulence, which helps store water and resources for your suicide bloom.
The Agave adaptations have evolved thanks to an intricate interplay between natural selection, local adaptation, and chance. This complexity has generated not only the different species and adaptations, but a whole mosaic of intermediate populations.
Source: Eguiarte Fruns Luis E. (2019). Agave, a master piece of evolution. IV International Symposium On Agave Integral and Sustainable Use of the Agave, IV, 25, Oaxaca, México.
Aspect
Agaves are perennials, with strong, long, thick, and fleshy leaves arranged in a rosette at the apex of a stem, which is usually extremely short.
Agaves are formed by a central fibrous stem, aka piña, around which leaves grow forming a spiral rosette following a Fibonacci number pattern that allows some leaves to shade others, thereby reducing the temperature on their surface. New leaves emerge from a central meristem, so that the youngest leaves are always at the center of the stem.
Source: Monja-Mio, K. M., Herrera-Alamillo, M. A., Sánchez-Teyer, L. F., & Robert, M. L. (2019). Breeding Strategies to Improve Production of Agave (Agave spp.). Advances in Plant Breeding Strategies: Industrial and Food Crops, 319–362. https://doi.org/10.1007/978-3-030-23265-8_10
Leaves
The number of leaves goes from 20 to 200 and they live for a long time, 12–15 years, often for the full cycle of the plant.
Their color varies from bright green to deep bluish-gray. Variations in the thickness of the cuticle can affect the color of the leaf, with horizontal banding appearing. In some species the cuticle is fine. The leaves are generally glabrous and have elongated fibers throughout their length.
The organization of the leaves allows capturing more radiation for the photosynthesis process and channel the water to the superficial roots, which in times of drought can decrease in size and thus avoid water loss.
The leaves can be flexible or very rigid. The abundance of fibers in the leaf tissues maintains rigidity during the dry periods preventing the leaves from deformation and maintaining their ability to capture enough light for photosynthesis.
Source: Monja-Mio, K. M., Herrera-Alamillo, M. A., Sánchez-Teyer, L. F., & Robert, M. L. (2019). Breeding Strategies to Improve Production of Agave (Agave spp.). Advances in Plant Breeding Strategies: Industrial and Food Crops, 319–362. https://doi.org/10.1007/978-3-030-23265-8_10
The leaves have an impermeable cuticle covered by waxes that help preserve water from evaporation. When the cuticle of the terminal spine continues along the leaf margin, the spine is said to be decurrent.
The leaf margin can be smooth, interrupted by teeth or with elongated fibers. It can also be straight between the teeth or along its entire length, wavy, with strong prominences or curved. Most Agave species have sharp marginal teeth, an extremely sharp terminal spine that can be long or short, straight, curved or cylindrical, with or without basal excavation, but in most cases pointed.
The teeth have great morphological diversity: teeth corneas can be originated from projections of tissue known as mamilas, or located on a continuous horny band, while in some of them it is filiferous and breaks off into fibers (aka filaments). The leaf generally ends in a thorn that can measure from a few millimeters to several centimeters.
The development of succulence in the leaves helps to store water during the rainy season, and the leaves contain parenchymas capable of storing water for use during the dry season.
All agaves are succulent plants with CAM photosynthesis, among other adaptations that lead to water conservation. These are considered productive plants under harsh conditions for exploitation of biomass and carbohydrates. While the water-soluble carbohydrates (WSC) are the main photosynthates, water in leaves is the resource to be saved. Both resources, succulence and carbohydrate productivity, are unique characteristics to be exploited in the agave plants that should be better understood among different species.
While PCA results revealed that some traits can address the ability of saving water of agave plants in a novel way. Independent carbohydrate and water distribution in leaves seem to delimit each other. This is specific to the species and doesn’t correspond to conserved anatomical characters. Even though WSC represent the major proportion in agave leaves, independent distribution of carbohydrates and water throughout agave leaves was visualized. Succulence was manifested fundamentally at the cellular level and is not translated to morphological succulence.
Source: Pérez Areli et. al. (2019). Insights into anatomy and physiology of water and carbohydrate storage in leaves of 3 agave species. IV International Symposium On Agave Integral and Sustainable Use of the Agave, IV, 31, Oaxaca, México.
One of the most remarkable adaptations of the agaves, is the structure of the stomata linked to the crasulacean acid metabolism (CAM) that allows them to close during the day and open at night, preventing the loss of water by evapotranspiration.
Agave plants also avoid physiological damage caused by drought through nocturnal assimilation of CO2, thick cuticles, low frequency of stomata and succulence in the leaves. These last two adaptations allow the continuous movement of stored water from the medullary parenchyma to the chlorénchyme during periods of drought conferring the ability to withstand prolonged periods in drought.
In many species of Agave, the opening of their stomata at night, favoring the capture of CO2 and minimizing the loss of water through perspiration. It is important to mention, that drought stress can also affect some quantitative characteristics in agaves.
Regarding temperature, in agaves, has shown that low temperatures (20–25 ° C) favor the assimilation of CO2 while temperatures above 30°C stop this assimilation, being this a characteristic physiological response of CAM type plants.
Source: Gschaedler, A. C., Gutiérrez Mora, A., Contreras Ramos, S. M., Davila Vazquez, G., & Gallardo Valdez, J. (2017). CIATEJ. Panorama del aprovechamiento de los Agaves en México. México. Retrieved May 11, 2020, from http://ciatej.repositorioinstitucional.mx/jspui/handle/1023/646
The gas exchanges take place during the night when carbon dioxide is fixed as malic acid in the cytoplasm of mesophyll cells by enzyme reactions, like the one in the C4 pathway, and stored in vacuoles. During the daylight period, CO2 is released again and enters the stroma of the chloroplasts, and is transferred to the Calvin cycle to be incorporated into sugars.
It is very important to consider that CAM plants have the ability to function in a similar way to C3 plants when environmental conditions allow it, for example, rainy season or under irrigation where there is water available.
The energy cost of performing photosynthesis is higher for CAM plants than for the other two groups of photosynthetic metabolisms. CAM has 740–790 kilojoules per net molecule of CO2 fixed, while C3 has 500 and C4 has 640–690. For this reason, agave plants have a slow natural development and have high-cost plant structures that accumulate reserves to reach to complete its biological cycle.
