388 EFFECTS OF DEHAULMING ON CANOPY GROWTH AND TUBER YIELD OF `YUKON GOLD'

Seed tubers of `Yukon Gold' (Solanum tuberosum L.) exhibit strong apical dominance, resulting in relatively poor stem emergence. Cutting seed tubers to overcome apical dominance in `Yukon Gold' results in irregular, uneven stem emergence. In 1992 and 1993, experiments were conducted to evaluate the effects of dehaulming (excising stems to ground level after field emergence) whole, cross- and longitudinal-cut seed tubers of `Yukon Gold' on canopy growth and tuber yield. For all seed piece types, dehaulming during the first week of field emergence produced uniform plant stands and increased yields of U.S. no. 1 tubers by 16 and 42% and large tubers (> 6.4 cm dia.) by 340 and 64% in 1992 and 1993, respectively. Our data indicate that tuber bulking rate was increased by dehaulming. The possible causes and implications of increased tuber bulking rates in dehaulmed potato plants are discussed.

Analysis of growth and tuber yield in sweet potato (Ipomoea batatas) cultivars

SUMMARYGrowth and tuber yield of local sweet potato cultivars were compared during the 1974 and 1975 growing seasons. Three cultivars significantly outyielded the others. The cultivar effect was associated with differences in individual tuber weight, rate of bulking and percentage of total dry matter diverted into the vine or stem. Mean crop growth rate (C) and percentage of total dry matter diverted into the tubers were greater in the higher yielding cultivars and there was positive and significant correlation between Cand final total dry weight. Leaf area duration (D) was greater in 1975 than in 1974 but the differences between cultivars in D was not consistent. Higher yielding cultivars had significantly greater tuber/leaf ratio than the other cultivars.Bulking rate and tuber weight were positively correlated with tuber yield, while the percentage of total dry matter diverted into the vine was negatively correlated with tuber yield.

Effect of shading at different times on the growth and yield of the potato

The effect of shading during the periods of new tuber initiation and subsequent bulking were studied with three crops of field-grown potatoes cv Sebago Plots were shaded (34% reduction on full sunlight) up to or after the stage when the young tubers had formed (3 weeks after the onset of initiation), receiving full sunlight for the other half of growth In other treatments plots were shaded throughout growth or not shaded at all In two experiments plots were split for two planting densities (7.14 and 14.28 tubers planted m-2), but density had no effect on the response to shade. Shade during the first part of growth did not affect the numbers of tubers initiation but, compared with unshaded plots, did reduce the numbers which reached a size of 1 cm in diameter or more, which was the same as the final numbers harvested at maturity Shade also delayed by a few days the onset of the rapid tuber-bulking period. Shade during later growth had some additional effect In reducing the maximum bulking rate achieved, so that the final yields from the differentially shaded plots were intermediate between those from the plots either unshaded or shaded throughout Plants receiving late shade had a higher percentage of their total dry weight In the haulm than did unshaded plants.

Some effects of seed size and spacing on the yield and grading of two maincrop potato varieties: II. Bulking rate

SUMMARYData are presented from two experiments which examined the effect of three seed sizes and three within-row spacings on the bulking rate of graded potatoes from the two early mainorop varieties Pentland Crown and Maris Piper.There were significant linear and quadratic regressions of yield on time and there were significant effects of all main treatments on the bulking rate of the crop. Differences in yield between the levels of each treatment depended upon the time of harvest and the size of tubers considered.

Productivity of vegetable crops in a region of high solar input. I. Growth and development of the potato (Solanum tuberosum L.)

Two spring-planted and one summer-planted crops of the potato var. Scbago have been grown under conditions of high solar input (daily averages throughout each experiment 23.1, 22.2, and 13.9 MJ m-2 respectively) in the Murrumbidgee Irrigation Areas of New South Wales. Treatments were a combination of three levels of solar input (0, 21, and 34% shade throughout growth) with two levels of irrigation (soil restored to field capacity at each estimated soil moisture deficit of 2.0 cm or 3.5 cm in two experiments, 3.5 cm or 5.0cm in the other). Growth analyses were made throughout each experiment. Environmental measurements showed little effect of shade covers on air temperature or relative humidity, and occasional measurements of leaf temperature and leaf water potential showed only small differences between shaded and unshaded plants. Differences in leaf and stem dry weights between plants in any treatments were small, but leaf area and stein length increased and specific leaf weight decreased with increasing shade. An increase in shade also decreased the numbers of tubers per stem which developed, and, while it did not influence the time of initiation, the time between onset of initiation and development of maximum bulking rate was greater as shade increased. Bulking rates with no shade were high (up to 1.4 t ha-l day-l) and only slightly less on shaded plots. In the drier of the two irrigation treatments in each experiment bulking rate was only slightly reduced, but the rate of leaf senescence was greater. The treatments did not affect the time of cessation of bulking. The results are consistent with the hypothesis that photosynthesis in the potato is controlled largely by the size of the 'sink' provided by the growing tubers, and it is suggested that the main effect of solar input was in controlling the 'sink' size which developed in the two or three weeks following the onset of initiation. The high growth rates and accelerated ontogeny were probably largely a result of the high temperatures of the region. Final yields are considered in the next paper.

The effects of seed size and spacing on growth and yield of lesser yam (Dioscorea esculenta)

SUMMARYThe effects of seed size and spacing on growth, development and yield of lesser yam and the relationship between growth, development and yield in this yam variety were investigated in the 1967 and 1968 growing seasons. Larger seeds outyielded smaller ones both in the total and ware tuber yields. The seed size effect was associated with a difference in tuber number, leaf area duration and bulking rate. Mean tuber weight at harvest, decreased with the decrease in seed size. Total and ware tuber yields increased with the decrease in spacing distance. The spacing effect was associated with differences in leaf area duration and bulking rate per unit area. Final total dry matter produced per plant increased with the increase in both seed size and spacing distance but the dry matter produced per unit area decreased with the increase in spacing distance. Leaf area index and leaf area duration increased with an increase in seed size and decreased with increase in spacing distance. There was a greater positive relationship between total tuber yield and leaf area duration between tuber initiation and the final sampling period than that between it and absolute leaf area duration.

The effect of the time of application of nitrogen and potassium on the growth of the second early potato, variety Craig's Royal

SUMMARYThe effect of time of application of nitrogen and potassium on the growth, development and yield of the second early variety Craig's Royal was studied by the technique of growth analysis. There was an indication that witholding nutrients led to the earlier initiation of tubers, but this was of no practical consequence and all treatments affected tuber bulking rate rather than the duration of bulking. Compared with early application, late application of nitrogen depressed tuber yield, although total drymatter yield was not affected. Late application of potassium, however, increased tuber yield; this was associated with an improvement in the recovery of applied potassium. Delaying the application of nitrogen also resulted in its more efficient recovery in the crop. Late application of potassium improved the recovery of early applied nitrogen and vice versa. Reasons are put forward to account for these effects.