Tissue analysis

Chemical analysis of plant tissue is also an important technique in the diagnosis of nutritional disorders. In annual crops, tissue analysis is most often used in ‘trouble-shooting’ rather than in the recommendation of fertiliser rates. However, if the tissue samples are collected early in crop growth and analyses are completed quickly, a corrective fertiliser application may be possible within the same season.

The interpretation of tissue analyses is based on established relationships between crop yield and nutrient concentrations in plant tissue (Figure 1). Critical concentrations are those which separate the sufficient (healthy) range from the deficient or toxic range. For practical purposes, critical concentrations are defined as those concentrations associated with 90% of maximum yield. Between these concentrations is the range of concentrations required for healthy growth.

The relationship between crop yield and concentration of a particular nutrient in plant tissue may be determined by means of nutrient solution culture experiments, glasshouse pot experiments, or field experiments. Generally, field experiments are considered the best (Bates, 1971), but are considerably more expensive than solution culture and pot experiments. They also depend on the availability of sites which are deficient in each of the nutrients to be studied. The values given in this publication have been derived from solution culture experiments. Where possible, their reliability has been validated under field conditions.

A certain part of the plant rather than the whole plant, is usually collected for analysis. Leaves are usually considered the most satisfactory parts. Because leaves continue to accumulate some nutrients with age, it is important that nutrient concentrations in leaves of the same physiological age are compared. In many annual crops, the blade of the youngest fully expanded leaf is selected as the ‘index’ tissue for analysis. However, sweetpotato leaves may continue to expand throughout much of their life, so their physiological maturity cannot be judged easily on the basis of full expansion. The many studies referred to in this book have selected different leaves or parts of the vine for analysis, which makes the information difficult to compare. For the critical concentrations quoted here, the blades of the 7th to 9th youngest leaves have been selected as the index tissue. They were selected as being sufficiently responsive to disorders of both mobile and immobile nutrients, and having less variable nutrient concentrations than younger leaves.

Apart from the physiological age of the leaf, as indicated by its position on the plant, tissue composition may vary with the age or stage of development of the crop. For instance, some researchers have found the critical concentration for N deficiency to decline with crop age. Caution should therefore be exercised in applying the critical concentrations established for young plants to tissue taken from older crops. With regard to the values given in the above table, it should be noted that the sweetpotato plants grew more rapidly under experimental conditions than could be expected in the field. These plants may be equivalent to crops from 6 to 10 weeks of age, depending on the temperature and water supply experienced by the crop. In any case, they represent crops prior to the stage of rapid growth of the storage roots. From the point of view of applying corrective measures to the crop on the basis of the diagnosis, it is preferable to sample the crop at as early a stage as possible. However, in many instances, symptoms of a disorder may not appear until advanced stages of crop development. In such cases, tissue analysis will still be valuable, but discrepancies arising from plant age should be borne in mind when interpreting the results.

Environmental conditions may further affect the concentrations of nutrients found in leaves. The concept of critical nutrient concentration requires that the nutrient of interest is the only factor limiting growth when the plant material is sampled. It has been shown that water stress can change the nutrient concentrations in leaves, and that plants take some time to recover normal concentrations after restoration of adequate water supply. It has been recommended that a period of several weeks without water stress should precede the sampling of tissue. This is not practical in many situations, but users should be mindful of this potential source of error.

To collect leaf samples, the blades are removed without the petiole, and should be dried as soon as possible after sampling, using gentle heat (eg. 60-70^oC for 48 hours) or microwaving. If samples must be stored for more than a few hours before drying, it is preferable to keep them cool (eg. in an ice box, but not in contact with water) to minimise the weight loss due to respiration of the living tissue. It is important to sample leaves that have not been contaminated with soil. If the leaves are dusty, they may be gently rinsed and blotted dry, but extended immersion in water and rubbing should be avoided. Only distilled or deionised water should be used.

When sampling a crop, it is best to take a composite sample of leaves from several plants which are equally affected by the symptom of concern (Reuter and Robinson, 1986). If the crop is not uniformly affected, several samples could be taken, each from small, uniform areas of the crop, from the most to the least affected. The symptoms observed, and the level of severity, should be recorded for each sample, and the samples clearly labelled.

Source: O’Sullivan, J.N., Asher, C.J. and Blamey, F.P.C. (1997) Nutrient Disorders of Sweet Potato. ACIAR Monograph No. 48, Australian Centre for International Agricultural Research, Canberra, 136 p.

Correcting nutritional disorders