16.4  Adaptation to low availability of nutrients

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Worldwide, plants evolve on depauperate sites with adaptive devices that enhance both acquisition and internal utilisation of soil nutrients. For example, impoverished soils of coastal areas in eastern, southern and southwestern Australia support natural communities of ‘heath’ vegetation that comprise dense stands of seasonal herbs plus perennial sclerophyllous shrubs and small trees. These communities had attained an equilib-rium with meagre soil nutrient resources during their evo-lutionary history, remaining sustainable for millennia. By contrast, crop plants introduced into those areas require extensive supplements, and especially additional supplies of N, P, K, Ca, Cu, S, Mo and Zn. What nutritional features distinguish these two plant categories?

Grundon (1972) addressed this issue in southeast Queensland by comparing the nutritional physiology of three crop plants with representative ‘Wallum’ species (an Aboriginal word for Banksia aemula, a small shrubby tree endemic to a coastal region of extremely infertile sandy soils north of Brisbane). Six Wallum species were grown simultaneously in nutrient culture (coarse sand medium) with sorghum, tomato and white clover. Growth responses to N, P, K and Ca were compared.

Wallum plants grew optimally at P and Ca levels that resulted in severe deficiency symptoms on crop plants. Growth response to N and K was masked by P toxicity in Wallum plants, and especially those cultured on low N or K. Crop plants were not so affected by P toxicity, but appeared to use plant P with less efficiency than Wallum plants. Grundon’s ‘P utilisation quotient’ for this experiment (g dry mass formed per mg of plant P) was 0.83 units for tomato on low P compared with 3.18 in Banksia robur, but these values were both decreased and reversed on high P, 0.31 for tomato compared with 0.10 for B. robur.

P toxicity in native plants on nutrient-rich media described by Grundon (1972) was also noted by Groves and Keraitis (1976) for three sclerophyllous species, Banksia serrata, Acacia suaveolens and Eucalyptus pilularis, findings with implications for revegetation of disturbed sites where supplementary nutrients are commonly included in management strategies. These same three species varied in their sensitivity to N and P combinations. B. serrata died on high P plus high N; A. suaveolens died when supplied with high P irrespective of accompanying N, while E. pilularis failed to survive either high P or high N. By contrast, failure to exclude luxury P by Wallum species was linked to imbalance in either the N:P or K:P ratio of solution cultures (Grundon 1972). By inference from both sets of experiments, a predisposition to luxury uptake and consequent P toxicity in these native species may well be a corollary to their low-nutrient adaptation.

A distinctive nutritional physiology in plants so adapted is complemented in nature by an enhanced acquisition of sparse nutrients due to a broad range of features. These include root morphologies (cluster roots in Section 3.1), biotic associations with soil microorganisms (mycorrhizas) and even highly specialised lifestyles such as parasitism and carnivory. These adaptations have been reviewed comprehensively by Lamont (1982) with particular reference to South Africa and those parts of southwest Western Australia subject to highly seasonal winter and spring rain (Mediterranean-type climates). In both cases, soils are highly infertile, and plant communities are subject to low winter temperature at times of abundant soil moisture, followed by hot and dry summers. Such edaphic and seasonal restrictions on root physiology are offset by highly developed adaptations that enable plants to tap novel sources of nutrients as outlined below.