The effects and residues of maleic hydrazide within the potato crop
This thesis describes the observations of field trials and analytical investigations of the residues of maleic hydrazide (1,2-dihydro-3,6-pyridazinedione) known as MH. MH is marketed in a formulation called `Fazor' and was introduced to the United Kingdom under PSPS (Pesticide Safety Precaution Scheme) in 1985. It is targeted at sprout suppression of ware tubers in store with the added benefits of increased marketable yield and reduced groundkeepers. Two field trials were conducted to investigate the sprout inhibition of MH and its effect on yield. Both field trials used a popular ware seed (c.v. Maris Piper) grown under typical growth conditions and treated with Fazor(5 kg/500 litres/hectare) under optimum conditions. Treated potatoes were observed to have typical characteristics of a break in apical dominance. Every eye on the tubers was active and formed spikelets that developed into growths similar in appearance to a small cauliflower head. It was also noticed that sprout suppression was not complete. A proportion of potatoes had inadequate suppression. Some showed no characteristics of MH, typical of an untreated potato. The inadequate sprout suppression was investigated by analysing the MH distribution and the comparison of sprout suppression. MH treated plots were found to have slightly lower yields than untreated plots. This reduction in yield was not shown to be statistically significant. Two methods to calculate the residues of MH were adapted. The first used the action of zinc in concentrated sodium hydroxide to produce hydrogen, which reduced and hydrolysed MH to hydrazine. The hydrazine was distilled and caught in an acid scrubber containing a colour reagent. The colour reagent and hydrazine combined to produce a coloured chromophore. The colour intensity was proportional to the amount of MH. This method was used to calculate the total MH. The second method calculated the amount of free MH (not metabolised or conjugated) by a methanol extraction, cleanup of the concentrate and analysis by high pressure liquid chromatography (HPLC). The total MH was roughly made up of three factions:- (1) Non-extractable MH (2) Methanol extractable MH (a) free MH (b) metabolised MH The uptake of MH over a four week period, after spraying Fazor, was assessed by harvesting at weekly intervals for four weeks and analysing the tubers by the above methods. The distribution of MH between individual tubers was investigated by calculating the MH in whole tubers. The distribution within individual tubers was calculated by analysing the skin, the outer flesh and the inner core. The distribution of MH between grades was calculated. The carryover of MH into processed foods was calculated by analysis of crisps and boiled potatoes from field treated plots. It was found that the MH accumulated in the potato tubers before one week and remained static up to four weeks. However, there was a fraction, a possible metabolite, that increased in concentration, whereas the free MH concentration decreased. Sprout suppression was found to be dependent on the total MH. Small tubers had lower concentrations than larger tubers. MH was found throughout the whole tuber with a slightly lower concentration in the core. MH was also found in fried and boiled potatoes and was identified as free MH. The metabolites of MH were investigated by applying radiolabelled MH to greenhouse grown potato plants. The potatoes were extracted by methanol and the concentrated extracts separated by thin layer chromatography (tlc). A metabolite was detected after exposing the plates to X-ray photographic plates. However, this metabolite was not successfully identified. This was due to the combination of low specific activity of the radiolabelled material, and the low quantity of metabolite produced. Hydrolytic enzymes were used to cleave possible conjugated metabolites in the potato juice concentrates. The action of a beta-D-glucosidase was found to increase the free MH. This would appear to suggest that a metabolite of MH is a conjugated product between MH and a glucose molecule. However, a conjugate could not be synthesized between uridine diphospho glucose (UDPG) and MH with soluble potato protein. There was no evidence to suggest that a metabolite is produced between MH and glutathione. It was suspected that MH would inhibit wound healing because it inhibits cell division. Cut potatoes treated with various concentrations of MH were examined histologically to assess cell division and suberin production. The rate of water loss from potato discs treated with MH was calculated. If the rate of water loss does not slow down because of chemical inhibition of the process, potatoes will lose weight and condition in store. No difference between control potatoes and MH treated potatoes was found at the beginning of a storage season. At the end of storage (four months), MH treated tubers had better wound healing than controls.