Branching the risks: architectural plasticity and bet‐hedging in Mediterranean annuals

Abstract

It has been suggested that architectural plasticity in shoot size and number allows plants to manage environmental risks. Simpler structures require shorter development times and fewer resources, which secure minimal fitness even under risky and unfavourable conditions. Here we tested the hypothesis that the magnitude of such architectural plasticity depends on the species' developmental strategy. Specifically, species with late reproduction were expected to express the highest levels of architectural plasticity in response to environmental cues predicting high probability of abrupt deterioration in growth conditions. This hypothesis was tested by comparing Mediterranean and semi-arid populations of three species, which differed in growth strategy: Trifolium purpureum, a determinate and late flowerer, and Emex spinosa and Hippocrepis unisiliquosa that flower indeterminately throughout the season. All plants were exposed to varying levels of water availability and competition, but only T. purpureum displayed plastic architectural responsiveness to the experimental manipulations. In contrast, the early and extended step-by-step flowering of both E. spinosa and H. unisiliquosa reflected a relatively deterministic bet-hedging reproductive schedule, whereby minimum fitness is secured even under adverse conditions. These two opposing strategies gave contrasting results, with E. spinosa and H. unisiliquosa displaying reduced efficiency under favourable conditions under which T. purpureum had the highest reproductive efficiency. The evolutionary interplay between deterministic risk-averse and plastic risk-prone growth strategies might reflect contrasts in the probability and severity of environmental risks, and the costs of missed opportunities.

Fixed allocation patterns, rather than plasticity, benefit recruitment and recovery from drought in seedlings of a desert shrub

“…In some sense, fixed allometric allocation patterns can decrease the costs for plasticity while increasing fitness (e.g. survival rate) in stressful environments ( DeWitt et al 1998 ; Poorter et al 2012 ; Shemesh and Novoplansky 2013 ). In the Gurbantonggut Desert, the precipitation is infrequent, discrete and largely unpredictable ( Xu and Li 2006 ; Li et al 2011 ; Salguero-Gómez et al .…”

Explaining pre-emptive acclimation by linking information to plant phenotype

“…Evolution of anticipatory responses depends on the availability of suitable cues and signals. Bet-hedging is an alternative strategy that moderates risk of catastrophic failure even when the time course of environmental variation cannot be reliably anticipated ( Childs et al , 2010 ; Shemesh and Novoplansky, 2013 ). Bet-hedging can, for example, be implemented as a constitutive tolerance response to a stressor that appears as a ‘waste’ of resources in the absence of the stressor.…”

Decision-Making: Are Plants More Rational than Animals?

“…This has not been tested yet and deserves further attention. However, differences between Mediterranean annuals regarding architectural plasticity in response to environmental cues versus more fixed bet-hedging strategies [10] suggest that it should at least be possible. A problem that may affect animals and humans more than plants is that a seeming risk-averse behavior may be evolutionarily more stable than risk-prone behavior.…”

“…general-purpose associative learning mechanisms in the medial temporal lobes. Numerous studies have shown that the hippocampus can rapidly bind arbitrarily-related features that are present in awareness [9], even if the features pertain to very low-level stimulus features like orientation [10] and even if the to-be-associated features are only present in memory, not in perception [11][12][13]. By limiting the spread of color information to early visual cortex, the neurofeedback procedure used by Amano et al [1] eliminates the possibility of using higher-level association-formation mechanisms.…”