Effects of modifying assimilate supply to developing tubers
4.1 Introduction
In starch-storing sink organs such as potato tubers, incoming sucrose is degraded predominantly by sucrose synthase as described in the previous chapter and by others (Morrell and ap Rees, 1986; Sung et al., 1989). The importance of this enzyme in sucrose cleavage en route to starch biosynthesis has been clearly shown in the maize sh mutants, which are characterised by shrunken kernels due to a reduced starch content in the endosperm (40% less than in the normal endosperm, [Chourey and Nelson, 1976]). Sucrose synthase activity in a number of these sh mutants was less than 5% of that in normal endosperms (Chourey and Nelson, 1978). The significant positive correlations found between sucrose synthase activity and starch levels in tuberising potato (see Chapter 3) provide further evidence that high enzyme activity is required to maximise starch synthesising potential. As potato tubers mature on the plant and the rate of starch synthesis decreases, a significant decline in sucrose synthase activity occurs (Pressey, 1969a). In mature, stored tubers, the level of sucrose synthase is barely detectable and there is an apparent switch in the pathway of sucrose cleavage to one dominated by acid invertase (Pressey, 1970; Richardson et al.,1990; see Chapter 3). Oparka et al., (1990) showed that the gradual decline in sucrose synthase activity found during tuber maturation could be accelerated by excising the tuber from the mother plant. Following tuber excision there was a rapid loss in the starch-synthesising capacity of
storage parenchyma cells which was accompanied by a 3-fold reduction in sucrose synthase activity. Acid invertase activity decreased 4-fold. In this Chapter, this work has been extended to examine, in detail, the consequence of excising tubers at various stages of development on sugar balance and the activities of sucrose-cleaving enzymes. The effect of genotype on the responses to tuber excision has also been examined.
The work of Claussen et al. (1986) implied that the concentration of sucrose in the cytoplasm of sink leaves of Solanum melongena regulated the rate of sucrose cleavage by sucrose synthase, by changing both the activity and amount of the enzyme. The concomitant rise in sucrose synthase activity and sucrose concentrations in tuberising potato also implies that such a mechanism operates in starch-storing sinks. The possibility that sucrose flux regulates sucrose synthase in potato tubers is examined in this Chapter.
4.2 Materials and Methods
4.2.1 Plant material
4.2.1.1 Tuber excision and postharvest storage experiments
Seed tubers of potato (Solanum tuberosum L.) cvs Cara, Record and Brodick were planted in the field on 6th May 1988 at the Scottish Crop Research Institute at a density of 4 plants m’2. Plots were fertilized prior to planting with N, P and K at rates of 187, 187 and 262 kg ha’1. Four sequential harvests were taken between 1st August and 26^ September to provide tubers of varying maturity. Mean individual tuber weights were about 70 g fresh weight at the first harvest and 125 g fresh weight at the end of September. After tuber excision, five tubers of each genotype were selected at random for immediate
analysis (within 2 h). The remainder were stored at 10 ± 1°C and analysed at - regular intervals over a period of up to 18 d.
4.2.1.2 Effect of minimising tuber assimilate supply by light exclusion
experiments
Unsprouted seed tubers of cv. Maris Piper were planted in the field on 14th June 1989 in a randomised block design. Tuberised plants (tubers approximately 90 g FW) were used to test the effect of manipulating sucrose flux to the tubers. On the 18th September, 40 potato plants, 10 from each of 4 plots, were selected at random and covered with two black polythene sacks to exclude, completely, any light from the plants. Each plot contained 60 plants at a density of 7.5 plants m"2. Ten uncovered plants (controls) were harvested on the 19th September and two tubers from each plant used for measurements of sucrose, hexoses and the activities of sucrose synthase and acid invertase. Similar harvests from both covered and uncovered plants were taken after 7, 15, 22 and 28 days.
4.2.1.3 Exogenous application of sugars to intact, detached tubers
Seed tubers of cvs Cara and Record were planted in a peat/sand compost (University of California formulation [Baker, 1957]) in 30 cm square pots in an unheated glasshouse. Developing tubers with a mean fresh weight of 40 g (±10%) were excised at the point of attachment of the stolon with the stem and used in experiments to test the effect of supplying sugars exogenously over a period of up to 12 days. The effectiveness of two methods for supplying sugars was compared. 1) The excised stolons, each attached to a small tuber (Plate 4.IA), were immersed immediately in distilled water or in solutions of either
Plate 4.1 A) The two methods used to supply sugars to developing tubers either via the attached stolon (left) or via a channel cut through the tuber (right); B) Experiment showing the method of supplying sugar solutions to tubers via their stolons; C) Experimental set-up used for supplying solutions via a channel cut through the tubers. Adjustable syringes were used to maintain a constant level of solution in each of the tubes supplying the tubers.