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Item A study of the effects of temperature and photoperiod on vegetative growth and seed production of leaf lettuce (Lactuca sativa L.) : a thesis presented in partial fulfilment of the requirement for the degree of Doctor of Philosophy in Seed Technology(Massey University, 1985) Sukprakarn, SuteveeThree leaf lettuce (Lactuca sativa L.) cultivars. Thai, Grand Rapids and Slobolt were grown in a controlled environment at either 25/l5°C or 30/20°C day/night temperatures and at 10, 12 or 14 hours daylength. Leaf fresh weight, dry weight, leaf area and leaf number all increased with increasing temperature and photoperiod. Slobolt produced a higher vegetative yield than Thai and Grand Rapids. Long days and high temperatures hastened stem elongation. Flowering in Thai and Grand Rapids was mainly influenced by high temperature whereas Slobolt required both high temperatures and long days. Slobolt however, was considerably delayed in bolting and flowering compared with the other two cultivars. Lettuce plants showed two definite flowering peaks over a 50-day flowering period. Slobolt produced only one peak at 25/l5°C due to its slow bolting character. Seed development studies were carried out on plants grown in both the 25/15°C and 30/20°C temperature regimes and at 12 hours daylength. The higher temperature regime hastened seed maturity, germination and shattering. Lettuce seed reached physiological maturity 11 days after anthesis at 30/20°C and 13 days after anthesis at 25/15°C, when the seed head had just begun to turn brown-green. Germination capacity also reached its maximum at this stage. Shattering however, occurred about two days after seed maturity at 20% moisture content at 30/20°C and 4 - 5 days later at 25/15°C. The three lettuce cultivars used in this study produced seed successfully in all treatments. Optimum time of harvest was found to occur when the majority of seed heads had turned slightly brown in colour and was also reduced by higher temperatures and longer daylengths due to earlier bolting and flowering. Highest seed yield was obtained under long days (14 hours). Grand Rapids produced higher seed yields than Thai and Slobolt. High seed yield was related to increased branch and flower numbers, percentage of seed set or seed numbers per head and time of harvest. Good seed set was obtained only under longer daylengths at high temperature while at 25/15°C, daylength was relatively unimportant. Final seed germination was unaffected by temperature or daylength. Practical application of the results which are relevant to Thailand conditions are discussed.Item Growth studies with lettuce : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at Massey University(Massey University, 1970) Nichols, Michael AdairGrowth studies were made in the field using two cultivars of head lettuce, Webbs Wonderful (a crisphead type) and Cobham Green (a butterhead type). In a fertiliser and density experiment using a systematic spacing design superimposed on a rotatable fertilizer design evidence is presented to suggest that the 'normal' reciprocal yield-density model W -1 = Aρ + B (when W is the mean plant weight at density ρ, and A and B are constants) is only applicable when there is competition at all densities. A 'modified' model is proposed which includes an additional parameter C, the density at which competition begins. The modified model is: W -1 = Aρ + B if ρ > C W -1 = AC + B if ρ ≤ C The plant weights in a non-competitive situation were fitted to a logistic model using a 'heat unit' environmental time scale, and an analysis of the logistic parameters showed a response only to serpentine superphosphate. This quadratic response was due to an increased relative growth rate (due mainly to an increased net assimilation rate) from the use of serpentine superphosphate up to 40 cwt./acre. At low plant densities Webbs Wonderful hae a higher relative growth rate compared with Cobham Green due to a slower rate of leaf production, and a higher net assimilation rate. This net assimilation rate difference is attributed to the heavier leaves of Webbs Wonderful being light saturated at a higher radiation level than the leaves of Cobham Green. This theory is supported by the similarity in the yields from the two varieties at high densities. The optimum marketable yield spacing for Cobham Green was found to be 1.4 plants/sq.ft. and for Webbs Wonderful 1.1 plants/sq.ft. In spite of a lower plant density the marketable yield from Webbs Wonderful was approximately double that from Cobham Green (at their respective optimum densities) due mainly to the later maturity of Webbs Wonderful, but also due to its higher growth rate. In an experiment carried out in England, and later in New Zealand, successive sowings (over a total period of 22 months) were sampled at regular intervals from emergence until past maturity. The dry weight per plant data were then fitted to a logistic model, with a single set of parameters for each variety over all the sowings, using chronological time, end a number of environmental time scales. All the environmental time scales tested provided a better fit than chronological time, with solar radiation being superior to 'heat units'. A further improvement with the solar radiation time scale was obtained by valueing all radiation above a certain daily integral at only 50%. In spite of the marked improvement when using environmental time scales, the results have little commercial application at present as a predictive tool because substantial differences were found in the logistic parameter estimates for the two sites, and also in the estimates of the asymptotes for the different sowings. It is essential that the asymptotes be the same over all sowings, or that the reason for any variation be known, because being based on a log. scale even a small variation would result in a large difference in absolute weight.