Estructura poblacional y hábitat de un árbol tropical con frutos comestibles, Annona purpurea (Annonaceae), en el occidente de México / Population structure and habitat of a tropical tree with edible fruits, Annona purpurea (Annonaceae), in Western Mexico

Introducción: Annona purpurea es un árbol Mesoamericano, distribuido por la vertiente pacífica y atlántica de México hasta Sudamérica. Diferentes partes de la planta son de utilidad y los frutos son comestibles. Objetivo: Describir la estructura poblacional, el hábitat y los factores ambientales que influyen en la distribución y abundancia de A. purpurea en el occidente de México. Métodos: De abril a noviembre 2015, se establecieron 24 unidades de muestreo de 500 m2 cada una, con presencia de A. purpurea. En cada sitio se midieron e identificaron todas las especies leñosas con diámetro a la altura de pecho (dap) ≥ 2.5 cm y se registró información ambiental, geográfica y climática. Se obtuvo información sobre la repoblación de la especie. Se estimó la estructura de A. purpurea y se examinaron sus relaciones con las variables ambientales. El índice de asociación de Whittaker se determinó utilizando la matriz de índice de valor de importancia (IVI). Con la matriz de variables ambientales y la de IVI, se realizó un Análisis de Correspondencia Canónica para determinar la influencia de las variables ambientales sobre la especie y visualizar la distribución de las especies en el espacio multidimensional. Resultados: A. purpurea presentó 1 108 tallos en 1.2 ha, con el 85 % de ellos concentrados en las tres primeras categorías diamétricas. Su densidad presentó una relación positiva con la presencia de tocones, mientras que el área basal y el IVI se relacionaron de manera similar con la incidencia de incendios. Hubo poca o nula regeneración de A. purpurea debajo de su dosel. Los mayores IVI en la comunidad se registraron para A. purpurea, Tabebuia rosea, Quercus magnoliifolia y Enterolobium cyclocarpum y la prueba de perfiles de semejanza separó a A. purpurea y Guazuma ulmifolia como un grupo diferente. Las variables ambientales con mayor influencia en la distribución y abundancia de A. purpurea fueron la precipitación anual, incidencia de incendios, elevación, temperatura media anual, pedregosidad y pastoreo. Conclusión: A. purpurea presenta los mayores atributos estructurales en la comunidad, sus poblaciones se ven favorecidas en sitios con mayor precipitación y temperatura, con poca pedregosidad, donde hay incidencia de perturbaciones por fuego y ganado que generan claros grandes y su mayor asociación la presenta con G. ulmifolia.

Introduction: Annona purpurea is a Mesoamerican tree, distributed from the Pacific and Atlantic slopes of Mexico to South America. Different parts of the plant are of utility and the fruits are edible. Objective: To describe the population structure, habitat and environmental factors that influence the distribution and abundance of A. purpurea in Western Mexico. Methods: From April to November 2015, 24 sampling units were established, each of which were 500 m2 in area and had a presence of A. purpurea. At each site, all woody species with a diameter at breast height (dap) ≥ 2.5 cm were measured and identified, and environmental, geographic and climatic information recorded. Information regarding the regeneration of the species was obtained. The structure of A. purpurea was estimated and its relationships with environmental variables examined. The Whittaker association index was determined using the importance value index matrix (IVI). With the matrix of environmental variables and that of IVI, a Canonical Correspondence Analysis was performed to determine the influence of environmental variables on the species and to visualize the distribution of the species in multidimensional space. Results: A. purpurea presented 1 108 stems in 1.2 ha, with 85 % of these concentrated in the first three diametric categories. Its density presented a positive relationship with the presence of stumps, while the basal area and IVI were similarly related to the incidence of fire. There was little or no regeneration of A. purpurea under its canopy. The highest IVI in the community were found for A. purpurea, Tabebuia rosea, Quercus magnoliifolia and Enterolobium cyclocarpum, and the similarity profiles test separated A. purpurea and Guazuma ulmifolia as a distinct group. The environmental variables with the greatest influence on the distribution and abundance of A. purpurea were annual precipitation, fire incidence, elevation, mean annual temperature, stoniness and livestock. Conclusion: A. purpurea presents the greatest structural attributes in the community, its populations are favored in places with greater precipitation and temperature, with little stoniness and with incidence of disturbances by fire and livestock that generate large gaps and its greatest association is with G. ulmifolia.

Nitrogen in the defense system of Annona emarginata (Schltdl.) H. Rainer.

The concentration of nitrogen can generate different strategies in plants in response to stress. In this study, we investigated how nitrogen concentration interferes with the defense system of Annona emarginata. Low concentrations of nitrogen increased the allocation of photosynthetic resources to carbon metabolism, resulting in an increase in the synthesis of volatile substances involved in signaling and defense that contributed to antioxidant enzymes in overcoming stress. The availability of nitrogen at 5.62 mM concentration might have helped to induce increased resistance in the plants because at this concentration, signaling substances and defense substances (monoterpenes and sesquiterpenes) were observed. Plants cultivated with the highest nitrate concentration displaced energy for the reduction of this ion, likely forming nitric oxide, a signaling molecule. This condition, together with the decrease in carbon skeletons, may have contributed to the lower synthesis of volatile substances of the specialized metabolism that are also involved with signaling. Varying the nitrogen in Annona emarginata cultivation revealed that depending on the concentration, volatile substances show higher or lower synthesis and participation in the system of signaling and defense in the plant. These results may suggest that volatile substances participate in resistance to pests and diseases, which is a necessary condition for Annona emarginata to be preferentially used as rootstock for Annona x atemoya.

Ontogenic synchronization of Bephratelloides cubensis, Annona macroprophyllata seeds and acetogenins from Annonaceae.

The seeds of Annona macroprophyllata Donn. Sm. contain idioblasts with toxic acetogenins, including laherradurin and rolliniastatin-2, in relatively high proportions. Both metabolites are cataloged as potent insecticides for several species, even so, the wasp Bephratelloides cubensis Ashmead fulfills almost its entire life cycle inside the seeds of this and other annonaceous species, to such a degree, that they constitute a strong selection pressure. In order to document the chemical relationship between the two species, it is reported for the first time in this paper the presence of idioblasts and acetogenins during the ontogenic development of the seeds of A. macroprophyllata, and contrasted with the development of B. cubensis. The results indicate that idioblasts with laherradurin and rolliniastatin-2 acetogenins are formed in the middle stages of the endospermic development, also that both acetogenins are biosynthesized simultaneously, and that their proportion is dependent on the degree of development. The acetogenins are present in high amounts that suppose a sufficient toxic barrier and, in this case, laherradurin is the most abundant (> 1000 µg g dry weight-1). The wasp B. cubensis only emerges from the seeds to copulate and returns for oviposition; its larval phase coincides with the appearance of acetogenins, so it feeds on the acetogenic endosperm. The absence of acetogenins in the tissues and excreta of the insect supposes a metabolization of the molecules, which would explain the tolerance to its toxicity.

Identification and characterization of a new Bacillus atrophaeus strain B5 as biocontrol agent of postharvest anthracnose disease in soursop (Annona muricata) and avocado (Persea americana).

Anthracnose is a fungal disease caused by Colletotrichum species that is detrimental to numerous fruit, including soursop and avocado. The use of fungicides to maintain the high quality of fruit creates a potential health risk. One alternative to this problem is the biological control, which has been applied successfully during postharvest. The Bacillus species are one of the most studied biological agents against postharvest pathogens because accomplish their biocontrol performance by producing a variety of metabolites. In this study, we evaluated the activity of metabolites contained in the cell free supernatant, obtained from Bacillus strain B5 culture, against micelial growth and spore germination of two virulent strains of C. gloeosporioides isolated from soursop and avocado. On the basis of 16S rDNA gene sequence analysis, this strain was identified as Bacillus atrophaeus. A preventive treatment using cell free supernatant, reduced severity and incidence of anthracnose disease on harvested soursop and avocado fruit. B. atrophaeus strain B5 harbors genes involved in the production of antibiotics such as surfactin, bacillomycin and iturin, which could be contributing to the efficiency of the preventive treatment during postharvest. The antagonistic role of metabolites contained in the cell free supernatant against anthracnose disease, provide a new approach by which to attack this problem and can help reduce the use of chemical pesticides, environmental pollution, leading to the safer fruit preservation.

Comparative transcriptomic analyses of normal and malformed flowers in sugar apple (Annona squamosa L.) to identify the differential expressed genes between normal and malformed flowers.

BACKGROUND: Sugar apple (Annona squamosa L.), a popular fruit with high medicinal and nutritional properties, is widely cultivated in tropical South Asia and America. The malformed flower is a major cause for a reduction in production of sugar apple. However, little information is available on the differences between normal and malformed flowers of sugar apple. RESULTS: To gain a comprehensive perspective on the differences between normal and malformed flowers of sugar apple, cDNA libraries from normal and malformation flowers were prepared independently for Illumina sequencing. The data generated a total of 70,189,896 reads that were integrated and assembled into 55,097 unigenes with a mean length of 783 bp. A large number of differentially expressed genes (DEGs) were identified. Among these DEGs, 701 flower development-associated transcript factor encoding genes were included. Furthermore, a large number of flowering- and hormone-related DEGs were also identified, and most of these genes were down-regulated expressed in the malformation flowers. The expression levels of 15 selected genes were validated using quantitative-PCR. The contents of several endogenous hormones were measured. The malformed flowers displayed lower endogenous hormone levels compared to the normal flowers. CONCLUSIONS: The expression data as well as hormone levels in our study will serve as a comprehensive resource for investigating the regulation mechanism involved in floral organ development in sugar apple.

Acetogenins and alkaloids during the initial development of Annona muricata L. (Annonaceae).

In plants, the presence and distribution of specialized metabolites during the early stages of development are not documented enough, even though their biosynthesis is one of the most important strategies for survival. In this study, five alkaloids and four acetogenins were detected in Annona muricata L. during early development seedling, including three phases of root emergence and three of seedling formation. Hexane and alkaloid extracts were obtained from each organ, which were analyzed in a gas-mass chromatograph and in a high-performance liquid chromatograph coupled with a photodiode array UV detector (HPLC-DAD). This research shows the presence of the acetogenins cis-uvarimicin IV, mosinone, muricina B, and cis-annonacin-10-one, as well as of the alkaloids reticuline, coreximine, anonaine, asimilobine, and nornuciferine, both groups with a variable organ-specific distribution, related with the formation of organs and tissues.

Phenological characterization and temperature requirements of Annona squamosa L. in the Brazilian semiarid region.

The phenological growth stages of various species of monocotyledonous and dicotyledonous plants can be uniformly coded using the Biologische Bundesanstalt, Bundessortenamt, und Chemische Industrie (BBCH) scale. The aim of the present study was to determine the duration of different phenological stages and the temperature requirements of the sugar-apple, Annona squamosa, during two crop cycles in the semiarid region of Brazil. Phenological stages were divided into eight of ten possible principal stages: (0) bud development, (1) leaf development, (3) shoot/branch development, (5) inflorescence emergence, (6) flowering, (7) fruit development, (8) fruit maturity and (9) senescence and the beginning of dormancy. The phenological cycle of the sugar-apple from having closed leaf buds to the fruit ripening stage lasted 149 and 164 days with temperature requirements of 1684.5 and 1786.7 degree days (DD) for the first and second crop cycles, respectively. The results provided important information that will inform the correct timing for crop management practices.

Beetle pollination and flowering rhythm of Annona coriacea Mart. (Annonaceae) in Brazilian cerrado: Behavioral features of its principal pollinators.

The conservation and sustainable management of Annona coriacea requires knowledge of its floral and reproductive biology, and of its main pollinators and their life cycles. In this work, we analyzed these aspects in detail. Floral biology was assessed by observing flowers from the beginning of anthesis to senescence. The visiting hours and behavior of floral visitors in the floral chamber were recorded, as were the sites of oviposition. Excavations were undertaken around specimens of A. coriacea to determine the location of immature pollinators. Anthesis was nocturnal, starting at sunset, and lasted for 52-56 h. The flowers were bisexual, protogynous and emitted a strong scent similar to the plant´s own ripe fruit. There was pronounced synchrony among all floral events (the period and duration of stigmatic receptivity, release of odor, pollen release and drooping flowers) in different individuals, but no synchrony in the same individuals. All of the flowers monitored were visited by beetle species of the genera Cyclocephala and Arriguttia. Beetles arrived at the flowers with their bodies covered in pollen and these pollen grains were transferred to the stigmata while foraging on nutritious tissues at the base of the petals. With dehiscence of the stamens and retention within the floral chamber, the bodies of the floral visitors were again covered with pollen which they carried to newly opened flowers, thus promoting the cycle of pollination. After leaving the flowers, female beetles often excavated holes in the soil to lay eggs. Larvae were found between the leaf litter and the first layer of soil under specimens of A. coriacea. Cyclocephala beetles were the main pollinators of A. coriacea, but Arriguttia brevissima was also considered a pollinator and is the first species of this genus to be observed in Annonaceae flowers. Annona coriacea was found to be self-compatible with a low reproductive efficiency in the area studied. The results of this investigation provide ecological data that should contribute to the conservation and economic exploitation of A. coriacea.

Phenological characterization and temperature requirements of Annona squamosa L. in the Brazilian semiarid region

ABSTRACT The phenological growth stages of various species of monocotyledonous and dicotyledonous plants can be uniformly coded using the Biologische Bundesanstalt, Bundessortenamt, und Chemische Industrie (BBCH) scale. The aim of the present study was to determine the duration of different phenological stages and the temperature requirements of the sugar-apple, Annona squamosa, during two crop cycles in the semiarid region of Brazil. Phenological stages were divided into eight of ten possible principal stages: (0) bud development, (1) leaf development, (3) shoot/branch development, (5) inflorescence emergence, (6) flowering, (7) fruit development, (8) fruit maturity and (9) senescence and the beginning of dormancy. The phenological cycle of the sugar-apple from having closed leaf buds to the fruit ripening stage lasted 149 and 164 days with temperature requirements of 1684.5 and 1786.7 degree days (DD) for the first and second crop cycles, respectively. The results provided important information that will inform the correct timing for crop management practices.

Diverse patterns of stored water use among saplings in seasonally dry tropical forests.

Tree species in seasonally dry tropical forests likely vary in their drought-survival mechanisms. Drought-deciduousness, which reduces water loss, and low wood density, which may permit dependence on stored water, are considered key traits. For saplings of six species at two distinct sites, we studied these and two associated traits: the seasonal amount of water released per stem volume ("water released") and the hydraulic capacitance of the stem (C). Two deciduous species with low stem density, Cavanillesia platanifolia and Bursera simaruba, had high C and high dry-season stem water potential (Ψ(stem)), but differed in dry-season water released. C. platanifolia did not use stored water during the dry season whereas B. simaruba, in a drier forest, released stored water. In both, water released was highest while flushing leaves, suggesting that stored water supports leaf flushing. In contrast, two deciduous species with intermediate stem density, Annona hayesii and Genipa americana, had intermediate C, low dry-season Ψ(stem), and high seasonal change in water released. Meanwhile, two evergreen species with intermediate stem density, Cojoba rufescens and Astronium graveolens, had relatively low C, low dry-season Ψ(stem), and intermediate seasonal change in water released. Thus, at least three, distinct stored-water-use strategies were observed. Additionally, bark relative water content (RWC) decreased along with Ψ(stem) during the dry season while xylem RWC did not change, suggesting that bark-stored water buffers Ψ(stem) seasonally. Together these results suggest that seasonal use of stored water and change in Ψ(stem) are associated with functional groups that are characterized by combinations of deciduousness and stem density.

Microspore development in Annona (Annonaceae): differences between monad and tetrad pollen.

UNLABELLED: • PREMISE OF THE STUDY: Permanent tetrads are the most common form of pollen aggregation in flowering plants. The production of pollen in monads is plesiomorphic in angiosperms, but the aggregation into tetrads has arisen independently different times during the evolution of flowering plants. The causes behind the recurrent evolution of pollen aggregation from monads remain elusive. Permanent tetrad pollen is quite common in the Annonaceae, the largest family in the early-divergent order Magnoliales. In some genera, such as Annona, both tetrad- and monad-producing species can be found.• METHODS: In this comparative study of pollen development, we use immunolocalization, cytological characterization, and enzymatic assays of four species in the genus Annona and one species in its closely related genus Asimina that release pollen in tetrads and two species in the genus Annona that release pollen in monads.• KEY RESULTS: The main difference between species with tetrad and monad pollen is a delayed digestion of callose and cellulose at the pollen aperture sites that resulted in nonlayering of the exine in these areas, followed by a rotation and binding of the young microspores at the aperture sites.• CONCLUSIONS: Small changes in development resulted in clear morphological changes on pollen dispersal time and open a window on the possible selective advantage of the production of aggregated pollen.

Stenospermy and seed development in the "Brazilian seedless" variety of sugar apple (Annona squamosa).

Stenospermy was identified in naturally occurring sugar-apple (Annona squamosa) mutants with great potential for use in genetic improvement programs. However, to date, there have been no detailed studies of the development of aspermic fruit in this species. The aim of the present study was to characterize the anatomy of developing fruit in the 'Brazilian Seedless' mutant. Flower buds in pre-anthesis and developing fruits were subjected to common plant anatomy techniques. The abnormal ovules are unitegmic and orthotropic and have a long funiculus. There is evidence of fertilization, including the presence of embryos in early development and the proliferation of starch grains in the embryo sac. However, the embryos and embryo sac degenerate, although this does not affect pericarp development. Ovule abortion does not occur. The perisperm, which is formed from the peripheral layers of the nucellus, fills the cavity left by the embryo sac. The mature fruit contains numerous small sterile seeds with abundant perisperm and unlignified integument that is restricted to the micropylar region. The majority of perisperm cells are living and appear to be metabolically active in the periphery. Therefore, stenospermy leads to the formation of sterile seeds in A. squamosa, and the perisperm possibly play an important role in fruit development.

Maintenance of a living understory enhances soil carbon sequestration in subtropical orchards.

Orchard understory represents an important component of the orchards, performing numerous functions related to soil quality, water relations and microclimate, but little attention has been paid on its effect on soil C sequestration. In the face of global climate change, fruit producers also require techniques that increase carbon (C) sequestration in a cost-effective manner. Here we present a case study to compare the effects of understory management (sod culture vs. clean tillage) on soil C sequestration in four subtropical orchards. The results of a 10-year study indicated that the maintenance of sod significantly enhanced the soil C stock in the top 1 m of orchard soils. Relative to clean tillage, sod culture increased annual soil C sequestration by 2.85 t C ha(-1), suggesting that understory management based on sod culture offers promising potential for soil carbon sequestration. Considering that China has the largest area of orchards in the world and that few of these orchards currently have sod understories, the establishment and maintenance of sod in orchards can help China increase C sequestration and greatly contribute to achieving CO2 reduction targets at a regional scale and potentially at a national scale.

Pollen performance, cell number, and physiological state in the early-divergent angiosperm Annona cherimola Mill. (Annonaceae) are related to environmental conditions during the final stages of pollen development.

Pollen performance is an important determinant for fertilization success, but high variability in pollen behavior both between and within species occurs in different years and under varying environmental conditions. Annona cherimola, an early-divergent angiosperm, is a species that releases a variable ratio of bicellular and tricellular hydrated pollen at anther dehiscence depending on temperature. The presence of both bi- and tricellular types of pollen is an uncommon characteristic in angiosperms and makes Annona cherimola an interesting model to study the effect of varying environmental conditions on subsequent pollen performance during the final stages of pollen development. In this work, we study the influence of changes in temperature and humidity during the final stages of pollen development on subsequent pollen performance, evaluating pollen germination, presence of carbohydrates, number of nuclei, and water content. At 25 °C, which is the average field temperature during the flowering period of this species, pollen had a viability of 60-70 %, starch hydrolyzed just prior to shedding, and pollen mitosis II was taking place, resulting in a mixture of bi- and tricellular pollen. This activity may be related to the pollen retaining 70 % water content at shedding. Temperatures above 30 °C resulted in a decrease in pollen germination, whereas lower temperatures did not have a clear influence on pollen germination, although they did have a clear effect on starch hydrolysis. On the other hand, slightly higher dehydration accelerated mitosis II, whereas strong dehydration arrested starch hydrolysis and reduced pollen germination. These results show a significant influence of environmental conditions on myriad pollen characteristics during the final stages of pollen development modifying subsequent pollen behavior and contributing to our understanding of the variability observed in pollen tube performance.

Liriodenine alkaloid in Annona diversifolia during early development.

Plants of the Annonaceae family produce a series of alkaloids, including liriodenine oxoaporphine. Its distribution in these primitive angiosperms suggests that it plays an important role, but very little is known about which plant organs it accumulates in, or in which developmental stages it is synthesised. Accordingly, liriodenine production was studied during the early stages of germination and seedling development in Annona diversifolia Saff. Liriodenine samples were obtained from the roots and were characterised on the basis of spectroscopic data. Quantification was done by HPLC in the organs and tissues of newly collected seeds, seeds following 1-, 2-, 5- and 10-day imbibitions, upon emergence of the radicle and at the seedling stage. According to our results, liriodenine could not have originated from the parent plant, nor during embryogenesis because it appears for the first time in the endosperm approximately 5 days after the start of imbibition. Therefore, its synthesis does not depend directly on photosynthesis. During the seedling stage it is found in the root and the stem but it is absent from the cotyledonary leaves and the first true leaves. Liriodenine biosynthesis begins during the early stages of development in the endosperm and seed radicles.

The progamic phase of an early-divergent angiosperm, Annona cherimola (Annonaceae).

BACKGROUND AND AIMS: Recent studies of reproductive biology in ancient angiosperm lineages are beginning to shed light on the early evolution of flowering plants, but comparative studies are restricted by fragmented and meagre species representation in these angiosperm clades. In the present study, the progamic phase, from pollination to fertilization, is characterized in Annona cherimola, which is a member of the Annonaceae, the largest extant family among early-divergent angiosperms. Beside interest due to its phylogenetic position, this species is also an ancient crop with a clear niche for expansion in subtropical climates. METHODS: The kinetics of the reproductive process was established following controlled pollinations and sequential fixation. Gynoecium anatomy, pollen tube pathway, embryo sac and early post-fertilization events were characterized histochemically. KEY RESULTS: A plesiomorphic gynoecium with a semi-open carpel shows a continuous secretory papillar surface along the carpel margins, which run from the stigma down to the obturator in the ovary. The pollen grains germinate in the stigma and compete in the stigma-style interface to reach the narrow secretory area that lines the margins of the semi-open stylar canal and is able to host just one to three pollen tubes. The embryo sac has eight nuclei and is well provisioned with large starch grains that are used during early cellular endosperm development. CONCLUSIONS: A plesiomorphic simple gynoecium hosts a simple pollen-pistil interaction, based on a support-control system of pollen tube growth. Support is provided through basipetal secretory activity in the cells that line the pollen tube pathway. Spatial constraints, favouring pollen tube competition, are mediated by a dramatic reduction in the secretory surface available for pollen tube growth at the stigma-style interface. This extramural pollen tube competition contrasts with the intrastylar competition predominant in more recently derived lineages of angiosperms.

Pollen development in Annona cherimola Mill. (Annonaceae). Implications for the evolution of aggregated pollen.

BACKGROUND: In most flowering plants, pollen is dispersed as monads. However, aggregated pollen shedding in groups of four or more pollen grains has arisen independently several times during angiosperm evolution. The reasons behind this phenomenon are largely unknown. In this study, we followed pollen development in Annona cherimola, a basal angiosperm species that releases pollen in groups of four, to investigate how pollen ontogeny may explain the rise and establishment of this character. We followed pollen development using immunolocalization and cytochemical characterization of changes occurring from anther differentiation to pollen dehiscence. RESULTS: Our results show that, following tetrad formation, a delay in the dissolution of the pollen mother cell wall and tapetal chamber is a key event that holds the four microspores together in a confined tapetal chamber, allowing them to rotate and then bind through the aperture sites through small pectin bridges, followed by joint sporopollenin deposition. CONCLUSION: Pollen grouping could be the result of relatively minor ontogenetic changes beneficial for pollen transfer or/and protection from desiccation. Comparison of these events with those recorded in the recent pollen developmental mutants in Arabidopsis indicates that several failures during tetrad dissolution may convert to a common recurring phenotype that has evolved independently several times, whenever this grouping conferred advantages for pollen transfer.

Vegetation responses to extreme hydrological events: sequence matters.

Extreme hydrological events such as flood and drought drive vegetation dynamics and are projected to increase in frequency in association with climate change, which could result in sequences of extreme events. However, experimental studies of vegetation responses to climate have largely focused on responses to a trend in climate or to a single extreme event but have largely overlooked the potential for complex responses to specific sequences of extreme events. Here we document, on the basis of an experiment with seedlings of three types of subtropical wetland tree species, that mortality can be amplified and growth can even be stimulated, depending on event sequence. Our findings indicate that the impacts of multiple extreme events cannot be modeled by simply summing the projected effects of individual extreme events but, rather, that models should take into account event sequences.

Flooding, root temperature, physiology and growth of two Annona species.

The effects of root zone temperature (RZT) and flooding on physiology and growth of Annona glabra L. (pond apple) and A. muricata L. (soursop) were investigated. Trees planted in containers were exposed to RZTs of 5, 10, 20, 25 or 35 degrees C in controlled root temperature chambers. Trees at each RZT were either non-flooded (control) or continuously flooded. There were four replications over time for each treatment combination. Pond apple was more flood-tolerant than soursop. A combination of flooding and RZTs of 5 and 10 degrees C resulted in tree mortality of both species by Week 4. Only trees that appeared to develop morphological adaptations survived continuous flooding. In both species, net CO2 assimilation (A) decreased to nearly zero within 1 week following exposure to RZTs of 5 or 10 degrees C and became consistently negative over the remaining experimental period. Flooding reduced leaf chlorophyll index (measured with a SPAD meter), A and plant growth, and increased root electrolyte leakage from soursop. Optimum growth occurred at RZTs of 25 to 35 degrees C for non-flooded pond apple trees and at 20 to 25 degrees C for flooded trees. Soursop exhibited maximum growth at RZTs of 35 degrees C under non-flooded conditions and at 25 degrees C under flooded conditions.