CAMBIOS EN LAS PRÁCTICAS HABITUALES DEL PROFESORADO EN SUS AULAS

The fact that starch accounted for approximately 55 % of tuber dry weight at planting (Figure 6. 7), and the correlation of subsequent changes with growth of the tuber, indicates that starch is the primary form of storage carbohydrate in Zantedeschia 'Best Gold� This result was similar to that reported for other members of the A.raceae, i.e. , taro and tannia

(Xarithosoma sagittifolium Schott) (Hashad et al. , 1956; Brouk, 1975; Onwueme, 1978). I

Once tuber growth had recommenced, the existence of a constant relationship between tuber starch and structural dry weights, under all treatment regimes (equation (6.6)), indicates that starch deposition within the tuber was a controlled process. However, prior to tuber growth the alteration of this relationship under the low PPF regime (equation (6. 7)) indicates that while this process was controlled, in situations where the availability of newly-produced photoassimilate is unable to meet sink demand, the rate of

remobilization of stored starch can be increased. A similar situation has been reported in

soybean where only under conditions of limited assimilate availability was starch from the pod wall remobilized to enable continued

seed growth (Fader and Koller, 1985). Also

Davies (1984) was able to manipulate starch concentration of sprouting potato tubers by manipulating sink activity by varying shoot number. Both storage organ starch degradation and synthesis have been correlated with enzyme activity and orthophosphate concentration (Mares et al. , 1981; Preiss, 1982; Ou-Lee and Setter, 1985). Future examination of changes in these components may provide new insights to the mechanisms controlling growth and development in Zantedeschia, in particular changes in the

Under the low PPF regime the bi-phaseal relationship between tuber starch concentration and tuber structural dry weight (Figure 6.8), is indicative of the tuber's transition from a source to a sink. However, there is no biological relevance to determining the point of intersection of the bi-phaseal relationship as a region of overlap of data points exists. Clearly, while separating the data into two phases of development has allowed the determination of two independent relationships, the extent of overlap of data points coupled with the inability to detect the bi-phaseal relationship under the high PPF regime, indicates that it would be inappropriate to infer the existence of any abrupt switch in the onset of tuber growth being related to the attainment of a specific starch concentration and/or vice versa. Rather than looking to the tuber for the point of control, it is more likely that the onset of tuber growth, reflected by increases in both structural and starch dry weights, was dictated by the establishment of a photosynthetic leaf area large enough to provide an adequate supply/flux of photoassimilates.to meet the demands for continued leaf and root development, and respiration. The sink-source transition of leaves commences at 40 % to

50 % leaf expansion (Wardlaw, 1968; Giaquinta, 1978). However, tuber growth in

Zantedesclzia 'Best Gold' did not commence until one or more leaves had reached maturity. In addition, plants grown at low PPF and increasing temperature carried a greater leaf number and associated leaf area, at the time of onset of tuber growth, than plants grown at high PPF and cooler temperatures. This illustrates the greater demand for assimilates by organs other than the tuber. In addition the negative relationship between RGRT and LWP (refer Chapter 4) also supports the hypothesis that tuber, growth receives a lower priority for photoassimilates than foliage growth at this early stage (Loomls et al. , 1979;

Ho, 1988).

i

I

During tuber growth the increase in the proportion of starch dry weight relative to structural dry weight and/or total tuber dry weight, was simil� to that reported for potato tubers (Plaisted, 1957), tulip bulbs (Aung et al. , 1973) and corms of other members of the

Araceae (Hashad et al . , 1956; Ching, 1970) . However, this change in partitioning with development was different from that reported for sugar beet where no change in the distribution of assimilates between storage root growth and storage carbohydrate (sucrose) occurred (Das Gupta, 1969; Watson et al. , 1972; Milford and Thorne, 1973; Milford et al. , 1988) . Watson et al. (1972) suggested this constancy in distribution disputed the earlier proposed hypothesis (Ulrich , 1952, 1955) that carbohydrate stored in the tap root is photosynthate in excess of that which can be used for the growth of the rest of the plant.

This constancy in distribution was not found with Zantedeschia 'Best Gold; and while not necessarily supporting Ulrich' s hypothesis, the data presented here also does not dispute the validity of this hypothesis. The additional finding that the relationship can be

manipulated under periods of high competitive sink demand (Figure 6. 8), adds some validity to the application of Ulrich 's hypothesis to Zantedeschia.

In reviewing the literature on starch storing organs, Jenner (1982) confirmed the intimate linkage between growth of storage organs and their capacity for storing starch. As found in the current experiment, Jenner concluded •that for any given genotype the proportion of dry matter contributed by starch does not vary with the size of the organ as much as does the size of the organ itself'; i.e. , large organs contain more starch than small ones.

If correct, this would suggest that cell number will be a major determinant of tuber growth and the amount of starch stored in Zantedeschia 'Best Gold! With phytohormones being implicated in the control of cell division in storage organs such as potato tubers (Lovell and Booth, 1967; Mares and Marschner, 1980), future investigations may develop a greater understanding of the mechanisms controlling tuber growth in Zantedeschia if consideration is also given to rates of cell division and hormonal status.

6.4.4 Carbohydrate and structural dry weight concentration as predictors of growth and yield

The inability to detect significant correlations between source activity, as quantified by

NAR and RLS

WR

, and the relative rates of growth of the entire plant and/or its subcomponents (RGRw, RGRy, RGRTotn RGRyJ was not surprising. Warren Wilson (1972) suggested that more accurate determination of sink activity was gained by examining the relative growth rate of structural material, but with Zantedeschia 'Best Gold' no correlation was evident between the structural material -of the tuber and the various measures of source activity. This inability to detect a correlation may have arisen from

I

tw� sources. Firstly, time did not permit the structural dry weights of all organs of the

plants in the current experiment to be quantified. Since roots, petioles and unexpanded leaves are likely to contribute towards the total sink activity, it is possible that their inclusion as the measure of sink activity was critical. -Secondly, while leaves were the primary sites of synthesis of new carbohydrates, quantification of carbohydrate concentrations within the leaf and other organs, fails to account for respiratory losses and the rates of assimilate flux. Both have been shown to be important determinants of growth of the total plant and/or organs, and not just the actual concentration of carbohydrates and/or their rates of change (Borchers-Zampini et al. , 1980; Fader and Koller, 1985; Ho, 1988, Farrar, 1990). Hence while not negating the importance of photosynthetic rate,

photosynthetic area, and total photoassimilate supply, the methods of

quantification/description of assimilate used here do not describe the rate of flux, nor respiratory losses once initially fixed .

. The future development of mechanistic metabolic models to explain growth and

development of Zantedeschia will therefore require the additional definition of the structural/carbohydrate concentrations of all organs, in addition to carbohydrate flux data

and respiratory losses.

6.4.5 Conclusions

As with many plant genera, sucrose, fructose, and glucose, accounted for the majority of

soluble carbohydrates in

Zantedeschia

'Best Gold�

The primary form of storage

carbohydrate was starch, and changes in storage organ growth occurred together with

changes in starch.

It is suggested that the inability to correlate growth with carbohydrate concentration is as

a result of the inability to quantify carbohydrate fluxes.

The maintenance of

concentrations of both soluble carbohydrate and starch was highly controlled, presumably

involving enzymatic activity associated with storage and flux. While Morrell and ap

Rees

(1986) suggested that control of the carbohydrate concentration of developing potato tubers

was achieved via fine control, i.e. , enzymatic synthesis and activity, F

arrar

(1990)

suggested a modified form of coarse control, i.e. , regulation of enzymatic synthesis and

activity

via

supply of carbohydrate. Future examination of changes in these components

may provide new insights to the mechanisms controlling growth and development in

Zantedeschia.

In

the absence of evidence suggesting the existence of a trigger for tuberization, rather than

looking to the tuber for the point of control it is more likely that the onset of tuber growth,

as incr

eases

in both structural and starch dry weights, is dictated by the establishment of

a photos�thetic leaf

area

large enough to provide an adequate supply/flux of

photoassimilates beyond the demands for continued leaf and root development, and

respiration.

In document La formación permanente del profesorado a través de la investigación acción Análisis a través del proyecto de innovación docente sobre educación inclusiva en Educación Infantil (página 104-109)