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Thirteen experiments conducted near Tenby, Dyfed, between 1973 and 1980, examined the effects of seed-tuber weight and within-row spacing on tuber yields of seven potato varieties grown for early production. These data have been used in an attempt to develop an approach to the determination of optimum tuber planting densities in early crops which will provide agronomists with logically applicable principles. Tuber planting densities, which maximized the net crop return (ware value minus seed cost), were estimated for eight ratios of seed cost to ware value ranging from 0-25 to 80. Optimum tuber planting densities decreased with the use of higher ratios of seed cost: ware value and heavier seed, except for ratios of 0-25 and 0-50 applied to yields of tubers > 25 mm harvested in June. In general, later harvesting together with the use of larger riddle sizes give a lower optimum density with ratios < 20. Optimum densities for Red Craigs Royal, Ulster Sceptre and Vanessa were higher than for the other varieties.The results provide an illustration of how to approach the problem of selecting tuber planting densities in order to maximize net crop returns in early potato varieties.

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S U M M A R YThirty-two experiments examining the effects of the weight and within-row spacing of potato seed tubers on graded tuber yields of five varieties were conducted on eight sites from 1980 to 1985. A complex analysis technique was used to combine these data and estimate the optimum tuber planting densities for different ratios of seed cost to small (40-60 mm) and large (60-80 mm) ware value. The same technique could be applied to any other combination of seed cost, ware size and ware value.The optimum tuber planting density decreased with increasing seed-tuber weight. Differences in optimum planting density between varieties were much greater with small (35 g) than with large (105 g) seed tubers and decreased as the cost of seed increased relative to the value of ware. As large ware became worth more than small ware the influence of increasing seed cost on the optimum density was reduced. As the value of large ware increased, net returns increased and the effect of seed cost on net returns was reduced. Mean tuber size decreased with increasing stem density at harvest and at the same stem density was lower in varieties producing more daughter tubers/stem. Changes of mean tuber size (ji) and the spread of yield across size grades (

Crisp lettuce plants cv. Saladin were grown from the time they started flowering, at 20/10 "C (16 h day, 8 h night), 25/15 "C and 30/20 "C in glasshouses on two occasions in 1985.Yields of seed increased from, on average, 15 g to 27 g and then fell to 20 g per plant with progressive increases in temperature. The number of mature florets per plant increased with temperature but the number of seeds per mature floret was lower at 20/10 "C and 30/20 "C than at 25/15 "C. An increase in temperature reduced mean seed weight by up to 45%, seed volume by 15%, cell numerical volume density (N,) by 27% and the number of cells per seed by 39%. Percentage seed germination reached a maximum early in seed development at the stage when the pappus appeared through the involucral bracts. Differences in percentage germination and vigour of seeds (slope test) from different temperatures were accounted for largely by the effects on mean seed weight. However, when germinated at 30 "C seeds produced at 30/20 "C germinated more readily than those produced at 25/15 "C or 20/10 "C. Seed vigour gradually increased with an increase in the length of storage after harvest, reaching a maximum after 260 days. In general, seeds produced at 25/15 "C exhibited a greater variation in numbers of seeds per floret, N,, seed weight, times of seedling emergence, seedling and mature head weight than seeds produced at lower or higher temperatures.