El Estado Liberal Democrático: La Constitución de 1925 en Chile

Results from both glasshouse studies (Chapter 3 and Chapter 4) showed that B application had a significant influence on the concentration of plant-available soil B, plant B uptake and subsequently plant growth. Plant-available soil B increased as a function of increasing the rate of B fertilisation.

Review of the available literature (as described in Chapter 2) showed that a CaCl2- extractable plant-available B concentration less than 0.5 mg/kg can be defined as critically deficient, and will therefore detrimentally affect plant growth. However, soil B greater than 14 mg/kg can be considered toxic. Results from the growth trials showed that B toxicity was apparent at a B application fertilisation rate of 32 kg/ha. All treatments above 16.7 mg B/kg soil (equivalent to a field application rate of 2 kg/ha) overcame B deficiency that was apparent for the control soil in glasshouse trial 1. But only a rate somewhat between 4 and 16 kg/ha overcame deficiency in trial 2.

Boron application increased the magnitude of plant growth in both glasshouse studies (Chapter 3 and Chapter 4). Boron application in the equivalent range 4-8 kg/ha) positively increased the plant growth parameters height and fresh and dry weight, however with a further increase in soil B level the performance of these plant growth parameters declined. The results of Chapter 4 on B adsorption onto a range (seven) of soils collected from around the North Island of New Zealand showed that both Langmuir and Freundlich isotherms can be successfully used to model B adsorption in all soils tested.

Although B adsorption to soil will reduce the potential for loss of nutrient out of the soil through leaching, particularly in high rainfall areas, adsorbed B is not linearly related with plant B uptake. Therefore the addition of B in the form of B fertiliser is considered as an

with a corresponding relative decrease in that associated with the specifically adsorbed fraction, which can be considered as plant unavailable.

Boron adsorption was a function of pH across the range of 2-9, with adsorption increasing with pH. Maximum adsorption occurring at pH 9.0 implies that liming of acidic soil will increase B retention in such soils.

6.3.1 The availability of B to clones and the subsequent distribution of B within the clones

The B concentration throughout plant organs of all clones tested in this work was significantly increased with B fertilisation during both greenhouse trials, from a level of deficiency in the control plants to toxicity at the highest application rate (32 kg/ha) (both Chapter 3 and 4). Boron distribution between plant parts was in the order, needle > stem > root. Needles alone accounted for 52-85% of total plant B and can be classified as the main B sink. Among needle ages of year-old to current-year needle class, one-year-old needles were the primary sink for plant B.

Boron translocation from roots to needles behaved differently under different B fertilisation rates. Boron was confined to source organs (roots) under both the control and the higher fertilisation rate (32 kg/ha), but was distributed to sink organs (needles) under optimum B application rates (4-16 kg/ha) presumably in response to demand for new plant growth. These results suggest that B translocation from roots to shoots is restricted under conditions of both low and excess B supply. Such an uneven distribution between plant parts may result from B toxicity-or deficiency-induced impairment of vesicular tissues in P. radiata.

Needle tip yellowing was observed at the highest B application rate used in the current study (32 kg/ha). Boron in excess of 32 kg/ha will likely promote toxicity symptoms, and hence impair plant growth. The apparent effect on plant growth at both low levels of soil B (deficiency) and high levels (toxicity), confirms the narrow range of optimal B concentration reported in literature. The relationship between soil B (Plant available CaCl2-

extractable B) and the deficiency/toxicity status of P. radiata under different B application rates is summarised in Table 6.1.

Table 6.1 Effect of soil B on plant deficiency and toxicity symptoms

Notes: [a] the rate of 2 kg/ha was not used in trial 1, [b] the rate of 32 kg/ha was not used in trial 2.

[c] toxic for one-year-old needles]

The major difference in the plant-available B concentration in soil for the same fertilisation rate between greenhouse trial 1 and 2 is ascribed to the difference in B absolute fertiliser application due to surface area difference, difference in depth, distribution of fertiliser

B rate (kg/ha)

Plant health indicator (deficiency and toxicity)

Concentration is the CaCl2-extractable plant available

B concentration in soil (mg/kg)

Glasshouse trial 1 Glasshouse trial 2

0 0.30 (deficient) 0.09-0.10 (deficient) 2 [a] 0.20-0.24 (deficient) 4 3.04 (sufficient) 0.42-0.45 (deficient) 8 5.39 (sufficient) [c] 0.84-0.85 (deficient) 16 5.95 (sufficient) 1.45-1.60 (sufficient) 32 14.05 (toxic) [b]

Table 6.1 implies that fertilisation with ulexite at rates between 2-16 kg/ha will maintain plant-available B in pumice soils at a rate sufficient to maintain the optimal plant nutrition of P. radiata plantations under glasshouse studies.

The concentration of B in soil responded to B fertilisation, and increased with the B application rate (Chapter 3). The observed B distribution between the surface soil (0-10 cm) and sub-surface soil (10-20 cm) showed that B is more plant available in the surface soil relative to the deeper soil (Chapter 4).

Both clones used in trial 2 (Clone 37 and Clone 18) were sensitive to a low soil B concentration and responded to B application. Needles represent the dominant site of B accumulation regardless of clones. Although both clones showed no difference in stem and root B concentrations across B treatments, the needle B concentration in Clone 37 was greater that in Clone 18. As Clone 37 showed a greater response to B applied at higher application rate and was more susceptible to lower B application, these results implies that Clone 37 is a faster growing clone having a relatively higher demand for B than Clone 18.

6.3.2 The effect of variable B fertiliser rates on soil microbes with particular focus on ectomycorrhizae

Soil dehydrogenase activity and mycorrhizal colonization were used in this work as indices of soil microbiology and responded to B application rates in both glasshouse trials of the study (Chapter 3 and Chapter 4).

Boron application has an effect on soil dehydrogenase activity (DHA). Activity increased up to a fertilisation rate of 8 kg/ha, but decreased beyond. The inference from this result is that B is toxic to microbiological activity at a lower rate of application than is apparent for P. radiata itself. The level of DHA activity was elevated in the rhizosphere soil when compared to the bulk soil (Chapter 4) suggesting that B plays a more influential role on the functioning of soil microbes in the rhizosphere than in the bulk soil. This study showed that instead of having a direct effect on DHA, B had an indirect effect on soil microbes. Low photosythate partitioning manifest for a plant either deficient in B of subject to toxic levels of soil B triggered competiton between plant and soil microbes for soil nutrients, leading to a decline in the soil micorbe population measured in terms of DHA.

Both inadequate and excess B concentration may therefore be deleterious to mycorrhizal colonization in P. radiata. Boron application at the rate of 8 kg/ha resulted in maximum colonization of P. radiata roots in greenhouse trial 1. However, the maximum colonization in trial 2 occurred for a B rate of 2-4 kg/ha. The reason for this variable result is unclear, but may be a function of clone-specific mycorrhizae relationships and/or differences in the experimental conditions such as plant age, soil volume, and method of fertiliser application.

6.3.3 Response of plant photosynthesis to variable B fertiliser rates

A significant relationship between plant B concentration and photosynthesis was recorded throughout the current study. Chapter 3 showed that B application in excess of 8 kg/ha reduced photosynthesis in P. radiata. However, Chapter 4 showed that B-effected