Nutrient ratios in the foliage of plants in southwestern Western Australia

(writing in progress)

Source: Foulds (1993, https://nph.onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.1993.tb03901.x and https://pubmed.ncbi.nlm.nih.gov/33874598/and https://www.jstor.org/stable/2558261).

FOLIAR CONCENTRATIONS OF NITROGEN

Categories of plants in which the foliar concentrations of nitrogen (on a dry matter basis), in decreasing order:
legumes 1.5%
zamia 1.45%
daisies 1.25%
drosera 1.1%
grasses 0.98%
casuarinas 0.81%
restioids 0.8%
types 0.8%

My commentary:

The foliage of Drosera, which supplement nitrogen by consuming insects, is not particularly rich in nitrogen. However, the nitrogen-fixation of legumes does result in nitrogen-richness. Rather than merely compensating for a shortage of nitrogen in the soil, they recharge the ecosystem with this nutrient.

POTASSIUM/NITROGEN RATIOS

Plants with succulent foliage but non-succulent fruits (excluding parasitic plants)

Plants with succulent foliage and succulent fruits

Plants with succulent fruits but not succulent foliage

Parasitic plants with succulent foliage

Casuarinas

The expectation is upheld that foliar concentrations of nitrogen in nitrogen-fixing plants exceed those in 'normal' plants (e.g. grasses and daisies) - if one considers that no grass or daisy is as scleromorphic as some of the legumes, casuarinas, and zamia. In other words, the inability of most grasses and daisies to adapt to extremely poor soils makes a simple comparison difficult. So, I think that nitrogen-fixing plants do indeed have nitrogen-rich foliage, and the small values for casuarinas reflect their sclerophylly and their relatively inefficient systems of nitrogen-fixation. Grasses and daisies have greater foliar concentrations of potassium (1.3-1.6%) than do Western Australian legumes, zamia, drosera, and particularly casuarinas. I interpret this as reflecting not basically different nutrient-content in these groups, but rather their different soils. So I think that the reason why the foliar ratios of potassium/nitrogen in nitrogen-fixing plants are less than those of grasses and daisies is a difference in nitrogen rather than in potassium.

LEGUMES

legumes in southwestern Australia (Acacia and Papilionoideae)

mesophyllous (soft-leafed) spp.:

sample 21
foliar potassium mean 1.06%
foliar nitrogen mean 1.9%
potassium/nitrogen ratio 0.56

extremely sclerophyllous spp.:

sample 21
foliar potassium mean 0.675%
foliar nitrogen mean 1.24%
potassium/nitrogen ratio 0.54

My commentary:
The foliar concentration of potassium in mesophyllous spp. of legumes is similar to that in the Karoo (about 1%). The ratios do not differ between mesophyllous and extremely sclerophyllous legumes, despite the fact that the latter are the poorer in both elements.

Eutrophic-type legumes do seem to exceed oligotrophic legumes in foliar concentrations of nitrogen (1.9% vs 1.24%). A similar relationship applies to potassium (1.1% vs 0.7%). In both cases the difference is 1.5-fold.

However, the ratios of potassium/nitrogen do not differ: about 0.55 in both cases, which is also similar to that for legumes in general in southwestern Australia. These plants, although diverse phylogenetically and morphologically, are homogeneous in terms of the foliar ratios kf potassium/nitrogen.

particularly sclerophyllous Proteaceae (Banksia and Hakea) in southwestern Australia:
sample size 79
foliar potassium mean 0.4%
foliar nitrogen mean 0.61%
potassium/nitrogen ratio 0.66

My commentary:
The ratio is small, comparably so to nitrogen-fixing legumes. This is because extreme sclerophylly is associated with extreme foliar poverty of potassium.

PARASITES

Parasites with fleshy foliage (few spp. available, restricted to mistletoes and orobanche)
Amyema 15.5/6.5, Lysiana (is the foliage really fleshy?) 12.9/14.7, Orobanche 25/8.1
potassium mean 1.78%
nitrogen mean 0.98%
ratio 1.8 (a surprisingly great value)

My commentary:
Note the extreme potassium-richness in Orobanche
Parasites with fleshy foliage: ratio is 3-fold greater than for casuarina and zamia (both of which are nitrogen-fixers) and drosera.

The sample is small. However, these preliminary results do seem in line with plants in general, w.r.t. foliar concentrations of potassium relative to foliar texture. It seems that parasites do not have greater foliar concentrations of potassium than expected for their foliar textures; it is just that their foliage is fleshier than expected for the soils on which their hosts grow.

So, if the foliar ratios of potassium/nitrogen parasites are boosted, this is possibly because of foliar poverty in nitrogen.

GRAMINOIDS (grasses, restios, and cypes)

grasses

sample 15
potassium 1.32%
nitrogen 0.98%

restios and cypes
sample 22
potassium 0.65%
nitrogen

cypes
sample 18
potassium 0.7%
nitrogen 0.8%
ratio 0.86

My commentary:

Surprisingly, restios are much poorer than grasses in potassium, but only slightly poorer in nitrogen.

It is clear that all graminoids have 'foliar' concentrations of nitrogen of about 0.8-1% - less in the sclerophyllous spp., in which it is the stems, not the leaves, that have been analysed. However, the values for potassium vary greatly: restios have less than half the concentrations of potassium found in grasses. Is this simply explained by foliar hardness/softness, an extension of my rationale w.r.t. succulence?

DAISIES

sample 31
potassium 1.6%
nitrogen 1.25%

SUCCULENTS

Potassium/nitrogen ratios:

• plants with succulent foliage about 1.44 (recalculate); those without succulent fruits 1.13, those with succulent fruits 1.88
• nitrogen-fixing plants and carnivorous plants 0.5 up to 0.7 (generally 0.55-0.65)
• legumes (large sample, so value is reliable) 0.54
• casuarinas 0.55 (good agreement)
• zamia (one species, 4 samples) 0.62
• drosera (one species, 3 samples)

My commentary:

All plants with succulent foliage have great ratios of potassium/nitrogen, but those with succulent fruits as well (e.g. Chenopodium/Rhagodia) have the greatest ratios.

Plants with succulent foliage, none of which are nitrogen-fixers, have foliar ratios of potassium/nitrogen >2.5-fold those of nitrogen-fixing plants (and a carnivorous plant).

Compare parasitic plants with plants with succulent foliage (small sample) 1.8, which agrees with succulent plants with succulent fruits. The mistletoes sampled do not have truly succulent foliage, and the orobanche lacks succulent fruits, but these plants have supplementationnof potassium, boosting their ratios of potassium/nitrogen nearly to the level of non-parasitic plants combining succulent foliage and succulent fruits.

Publicado el 12 de julio de 2023 a las 11:15 AM por milewski