(with Prof. Judith Bronstein, Dr.
bacctus – An important fruit-bearing plant in the Israeli desert
Occhradenus bacctus’s fruit (Photo:
The yellow-vented bulbul (Pycnonotus xanthopygos) (left) and Tristram's grackle (Onycognathus tristramii) (Photo: Haim Shochat) (right) are of the major fruit consumer and seed dispersal-agent of Occradenus baccatus.
1. The impact of environmental conditions on the morphology, nutritional value and secondary metabolites in Ochradenus baccatus fruits.
Many plants disperse their seeds by fleshy fruit that is consumed by animals. The chemical, morphological and phenological traits of these fruits may control plant fitness, because they determine how many fruits will be consumed, by whom, how far the seeds will be transferred, where the seeds will be deposited and in what shape. Likewise, these traits simultaneously affect seed predators and pathogens and hence determine the level of seed damage. These fruit characteristics are related to species phylogeny but also to some extent to the characteristics of the potential seed dispersers and predators within the distribution region of the plant. Although intraspecific variation is the raw material of natural selection, little is known about the intraspecific variation of fruit traits and how it is affected by the abiotic environment.
In this study we have examined the effect of environmental conditions
on different fruit traits (morphology, nutritional value and secondary
metabolites) in Ochradenus baccatus,
a desert shrub that produces fleshy fruits, a rare phenomenon in the arid
environment. First, we assessed the intraspecific variation of several fruit
characters by comparing four O. baccatus
populations along a north-south geographical gradient in its natural habitat in
We found that although O. baccatus is a desert plant exposed to limited and unpredictable water conditions, its fruits have similar chemical content and morphological attributes to fleshy fruits from other habitats. O. baccatus pulp has high water content, which tends to be an attractive feature to dispersers, especially in arid habitats where it grows. However, we also found high intraspecific variation in several fruit morphological and chemical traits. This spatial and temporal variation was detected among populations, within populations and between seasons. The results indicate that a large proportion of this variation is explained by differences in the environmental conditions in the microhabitat of each plant. As water conditions improve, along with some enhancement (but to a lesser extent) of nitrogen and other soil minerals, O. baccatus fruits are larger but have lower dry weight, protein, fat and secondary metabolites. This trend was significant when all sampled plants (n=40) were examined, but not significant when each population was examined separately. Because the arid environment is water-limited, water is the main factor in the environment axis, affecting fruit morphology and chemistry. Moreover, plant water potential by itself was positively correlated with fruit size and water content, and negatively correlated with several chemical variables. These relationships were found among as well as within populations.
This is the first time that glucosinolates have been found in fleshy fruits. Their concentration in the pulp is higher than in the seeds. The two main glucosinolates [2-(α-l-Rhamno pyranosyloxy) Benzylglucosinolate; and O-(α-Arabinosyloxy) enzylglucosinolate[ have chemical structure which is typical to the Resedaceae family, one of those is new to science. The considerable intraspecific variation of glucosinolate content in the pulp was also partially explained by the variation in the environmental conditions. The greenhouse experiment indicated that glucosinolate concentration in fruit pulp was mainly affected by the sulfur content in soil. To a lesser extent, the pulp glucosinolate concentration was negatively related to water availability. Glucosinolate level varied among sites and between seasons, and was depended upon extrinsic rather than intrinsic factors of the plant. These findings support the theory that the level of secondary metabolites in fruit pulps is a non-adaptive trait. Nevertheless, many questions about the role of glucosinolates in plants and about the factors that govern their production remained unsolved.
In sum, we detected considerable intraspesific variation in fruit attributes. We also showed that this variation is partially explained by abiotic environmental conditions. Nevertheless, part of this variation is probably due to factors that were not examined here, such as differential selective pressures by different fruit consumers (dispersers and predators) or genetic differences among individuals and among populations.
2. Review of fleshy-fruited plants and frugivores in desert ecosystems
A traditional emphasis on the rarity of fleshy fruit in deserts has
contributed to a paucity of information about the importance of frugivory in
desert systems. We compared the Ochrodenus
baccatus frugivory system to that of the
This project was supported by the International Arid Land Consortium (IALC) ($100,000; 2003-2006).