Contrasting adaptive strategies to terminal drought-stress gradients in Mediterranean legumes: phenology, productivity, and water relations in wild and domesticated Lupinus luteus L.

C Ludwig,J D Berger

doi:10.1093/jxb/eru006

Abstract

Our understanding of within-species annual plant adaptation to rainfall gradients is fragmented. Broad-scale ecological applications of Grime's C-S-R triangle are often superficial, while detailed drought physiology tends to be narrow, focusing on elite cultivars. The former lack the detail to explain how plants respond, while the latter provide little context to investigate trade-offs among traits, to explain where/why these might be adaptive. Ecophysiology, combining the breadth of the former with the detail of the latter, can resolve this disconnect and is applied here to describe adaptive strategies in the Mediterranean legume Lupinus luteus. Wild and domesticated material from low- and high-rainfall environments was evaluated under contrasting terminal drought. These opposing environments have selected for contrasting, integrated, adaptive strategies. Long-season, high-rainfall habitats select for competitive (C) traits: delayed phenology, high above- and below-ground biomass, productivity, and fecundity, leading to high water-use and early stress onset. Terminal drought-prone environments select for the opposite: ruderal (R) traits that facilitate drought escape/avoidance but limit reproductive potential. Surprisingly, high-rainfall ecotypes generate lower critical leaf water potentials under water deficit, maintaining higher relative water content than the latter. Given that L. luteus evolved in sandy, low-water-holding capacity soils, this represents a bet-hedging response to intermittent self-imposed water-deficits associated with a strongly C-selected adaptive strategy that is therefore redundant in R-selected low-rainfall ecotypes. Domesticated L. luteus is even more R-selected, reflecting ongoing selection for early maturity. Introgression of appropriate C-selected adaptive traits from wild germplasm may widen the crop production range.

Highlights

Ecological C-S-R frameworks such as Grime’s triangle (1977) enhance our understanding of plant adaptation by evaluating traits in the context of environmental selection pressure
The use of wild populations that evolved under contrasting terminal drought stress facilitates C-S-R type comparisons, while the inclusion of domesticated material highlights adaptive strategies favoured by breeders, making it possible to speculate how these may have influenced crop development
The results show that Grime’s (1977) C- and R-selected adaptive strategies do lead to contrasting water-use and stress onset, leading to rather surprising tradeoffs in tolerance to water deficit in L. luteus

Summary

Introduction

Ecological C-S-R frameworks such as Grime’s triangle (1977) enhance our understanding of plant adaptation by evaluating traits in the context of environmental selection pressure. Used to describe species composition and adaptive traits between contrasting environments, they can provide insight into intra-specific variation, and have been applied in Mediterranean annuals along aridity gradients (Table 1). According to Grime (1977), as rainfall decreases, or becomes more variable (i.e. habitats become more stressful, or likely to be disturbed by terminal drought), reproductive strategies become increasingly conservative (ruderal, R), advancing reproduction and senescence at the expense of biomass production capacity. It is suggested that this limits aboveand below-ground resource acquisition Root–shoot ratio Leaf area Hard seededness Seed size Growth rates Gas exchangec WUEd Water relationse.

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Conclusion