Series: Effect of nitrogen managment on tuber initiation

Transcription

1 Series: Effect of nitrogen managment on tuber initiation IV Tuber yield and size group distribution Chantel du Raan (Aartappels SA) en prof. Martin Steyn (Universiteit van Pretoria) Background The main priority for the producer is to develop maximum leaf canopy in the first days after emergence. Thereafter nitrogen (N) is transported to the tubers to assist with tuber growth and to build protein content. It is therefore important to establish a large leaf canopy as soon as possible to intercept maximum radiation as yield is directly correlated to the intercepted radiation. Maintaining the leaf canopy is also important as it ensures that the maximum number of iniated tubers bulk adequately and reach maturiy. The lifespan of the leaf canopy, therefore, is also directly related to the yield. A defieciency or excess of N, can lead to a decrease in yield which can cost the producer dearly. When the potato plant experiences a N defieciency, it leads to a reduction in photosyntesis and growth as well as a decrease in leaf surface, which means the plant takes to long to produce an efficient leaf canopy. This can lead to lower light interception and earlier die-off of the plant. The plant therefore depletes its N reserves before the tuber can bulk sufficiently, and by the time it is noticed, it is already too late to repair the damage. An excess N can also affect potato plants negatively, especially cultivars that reach maturity later. Secondary haulm development is stimulated by excess N, in stead of producing maximum leaf canopy, followed by the development of stolons and iniation of tubers. Unnecessary secondary haulms negatvely influence the primary leaf canopy by overshadowing Page 20 CHIPS Julie/Augustus 2018

2 Harvest index = x 100 (Formula 1) Dry tuber yield Total dry material Figure 1: A graphic visualisation of a high and low harvest index. A plant with more leaves in comparison with the tubers has a low harvest index which means the leaves are stimulated at the expense of tuber development and growth. the primary leaves, which causes it to die-off earlier. It normally leads to a lower harvest index. The harvest index (HI) provides an indication of the dry mass of the tubers relative to the total dry material of the plant (stems, leaves and tubers). Formula 1 can be used to determine the harvest index. This allows us to determine how assimilates are partitioned between plant organs and whether leaf growth was stimulated at the expense of tuber development and growth (Figure 1). Tuber size group distribution is another important factor used to grade potatoes for the market, irrespective of whether it is table potatoes, seed potatoes or for the processing industry. It is thus an important quality aspect to ensure an optimum economically marketable yield. Nitrogen management regulates the leaf canopy and tempo of photosynthesis that later on also influences the initiation of tubers whilst tuber initiation is responsible for the growth and development of the crop which will also influence the yield and size group distribution. The study was conducted on three selected potato cultivars to asses the effect of N management (N level and application time) on yield and size group distribution. A summary of all the treatments is indicated in the block below. This information will make the choice of optimal production practices possible and in this instance, N fertilisation management, not only to ensure the efficiency of nutrients and environmentally friendly practices, but also to produce a better product sustainably which will increase profitability. Summary of treatments 3 cultivars: BP1 (C1; standard), Eos (C2) and Lanorma (C3) 3 different N levels, based on the clay content of the soil and potential yield: - Spring season: 160 kg/ha N (R1), 240 kg/ha N (R2) and 320 kg/ha N (R3) - Autumn season: 140 kg/ha N (R1), 220 kg/ha N (R2) and 300 kg/ha N (R3). 3 N application times (identical in both seasons): - 30% N at planting and 70% after tuber initiation (T1); - 50% N at planting and 50% after tuber iniation (T2); and - 70% N at planting and 30% after tuber iniation (T3). More details regarding treatments as well methods, are available in the first article of the series in CHIPS Vol. 32 No.1 January/February 2017 edition CHIPS July/August 2018 Page 21

3 *Bars with the same letter do not differ significantly at P<0.05 according to the Tukey test * CV% = * LSD (Cult) = * SE (Cult) = Figure 2: Marketable yield as influenced by the cultivars BP1, Eos and Lanorma. *Bars with the same letter per cultivar do not differ significantly at P<0.05 according to the Tukey test * CV% = * LSD (Rate) = 2.43 * LSD (Cult x Rate) = * SE (Rate) = 0.85 * SE (Cult) = 0.98 Figure 3: Marketable tuber yield (medium and large) as influenced by N level for the cultivars BP1, Eos and Lanorma. Page 22 CHIPS Julie/Augustus 2018

4 Results Marketable yield Cultivar effect The marketable yield (medium and large tubers) was significantly influenced by the cultivars. As indicated in Figure 2, cultivar BP1 had a substantially lower yield of medium and large tubers in comparison with Lanorma, which produced the highest marketable yield. Eos did not significantly differ from Lanorma and BP1. Effect of nitrogen levels The marketable yields for the cultivars Lanorma and Eos treated with an N level of 320 kg/ha N, increased significantly in comparison with the 160 kg/ ha N treatment (Figure 3). A trend was observed that the highest N level of 320 kg/ha delivered the highest yield of medium and large tubers. It is also evident that the cultivar Lanorma produced a higher yield of medium and large tubers than the cultivar BP1, whereas Eos did not significantly differ from the other two cultivars. Marketable yield (medium and large) reflected a higher yield tendency with an increase in N level. Effect of nitrogen application times A nitrogen application time of 70% N at planting and 30% N after tuber initiation, as well as 50% N at planting and 50% N after tuber initiation (Figure 4), reflected a significant increase in marketable yield in comparison with 30% N at planting and 70% N after tuber initiation. There was, however, no significant difference in marketable yield between the treatments of 70% N at planting and 30% N after tuber initiation and 50% N at planting and 50% N after tuber initiation. Total tuber yield Effect of nitrogen levels Total tuber yield was significantly influenced (P <0.05) by N levels. Total tuber yield has reflected a progressive trend as the N level increased from 160 to 320 kg/ha N (Figure 5). These results correspond with other studies that also reported that the highest N level led to the highest total tuber yield, whereas other studies again have shown that yield even out if more N is applied. Effect of nitrogen application times As far as N application timing is concerned, all three treatments (30% N at planting and 70% N ± 3 weeks after emergence, 50% N at planting and 50% N after emergence and 70% N at planting and 30% N after emergence) also differed significantly from one another. A trend was observed that final tuber yield increased as the volume of N applied at planting increased. Thus, the treatment that received 70% N at plant and 30% N after tuber initiation, attained the highest total tuber yield in respect of all three cultivars (Figure 6). Interaction effect: Cultivar x N level x N application time The interaction effect of cultivar x N level x N application time for total tuber yield was highly significant (p <0.01) (Table 1). The cultivars BP1 and Eos performed best in respect of final tuber yield when the recommended and highest N levels were applied in combination with 70% N with planting and 30% N after tuber initiation. However, in respect of the cultivar Lanorma the N level x N application time was not significant, although the trend was the same for the other cultivars. Lanorma has also reacted less upon the N applications, especially upon the N level, which indicates that each cultivar has its own unique optimum nutrient needs. As cultivars differ in respect of morphology and development characteristics, each cultivar therefore reacts differently to various external factors, such as fertilisation Discussion The timing (onset and duration) of tuber initiation has an important role to play in Lanorma s high marketable yield as the cultivar initiated most of its tubers within two weeks after emergence. This extended the bulking period of initiated tubers in comparison to the other cultivars, and which resulted in a higher percentage medium and large tubers. In contrast BP1 iniated tubers throughout the season which shortened the bulking period of tubers that initiated late in the season, and consequently a high percentage small tubers were produced (this aspect was addressed in detail in a previous article). The highest (320 kg / ha N) and recommended (240 kg / ha N) N level treatments already reached an effective leaf canopy early in the season. There was also sufficient N reserves to maintain the leaf canopy, which ensured sufficient tuber bulking and a high total yield with a high percentage large tubers. On the CHIPS July/August 2018 Page 23

5 *Bars with the same letter per cultivar do not differ significantly at p<0.05 * CV% = * LSD (Time) = 2.43 * LSD (Cult x Time) = 4.58 * SE (Time) = 0.85 * SE (Cult) = 0.98 Figure 4: Marketable tuber yield (medium and large) as influenced by N timing for the cultivars BP1, Eos and Lanorma. *Bars with the same letter per cultivar do not differ significantly at P<0.05 according to the Tukey test * CV% = 7.1 * LSD (Rate) = * LSD (Rate x Cultivar) = * SE (Rate) = Figure 5: Total tuber yield as influenced by N level for the cultivars BP1, Eos and Lanorma. Page 24 CHIPS Julie/Augustus 2018

6 *Bars with the same letter per cultivar do not differ significantly at P<0.05 according to the Tukey test * CV% = 7.1 * LSD (Time) = * LSD (Time x Cultivar) = * SE (Rate) = Figure 6: Total tuber yield as influenced by the N application n timing for the cultivars BP1, Eos and Lanorma in the spring season. Table 1: The interaction effect between the cultivars (BP1, Eos and Lanorma), N levels and applicaion times on total tuber yield. Totale opbrengs (t/ha) Timing of N application 30% atplant; 70% 3-4 weeks after emergence 50% at plant; 50% 3-4 weeks after emergence 70% at plant; 30% 3-4 weeks after emeregence BP1 N-rate 160 kg/ha bcd bcd ab 240 kg/ha 44.1 cd abc abc 320 kg/ha abcd abcd abc Eos N-rate 160 kg/ha d 51.5 abcd ab 240 kg/ha abc abc abc 320 kg/ha bcd abc a Lanorma N-rate 160 kg/ha abcd abcd abcd 240 kg/ha abcd abcd 55.8 abc 320 kg/ha abcd abc a Lowest value Highest value *Values followed by the same letter do not differ significantly at P<0.05 according to the Tukey test * SE = 2.169; LSD = 6.168; CV% = 7.1 CHIPS July/August 2018 Page 25

7 *Values with the same letter per cultivar do not differ significantly at P<0.05 according to the Tukey test Figure 7: Harvest index as influenced by N level for the cultivars BP1, Eos and Lanorma Table 2: Summary of the effect of cultivar, N level and N application time on yield and size group distribution Nitrogen rate Nitrogen timing Parameter R1 (Deficient) 160 kg/ha N R2 (Recommended) 240 kg/ha N R3 (Excess) 320 kg/ha N T1 (30%; 70%) T2 (50%; 50%) T3 (70%; 30%) Canopy Intermediate Early in season Late in season Intermediêr Vroeg in seisoen Laat in seisoen Final yield Size group distribution Harvest index Interaction effect: Cultivar x N rate x N timing Lanorma BP1 Eos R1 (Deficient) 160 kg/ha N R2 (Recommende d) 240 kg/ha N R3 (Excess)320 kg/ha N T1 (30%; 70%) T2 (50%; 50%) T3 (70%; 30%) Increase in yield & size group distribution No effect Page 26 CHIPS Julie/Augustus 2018

8 other hand the lowest N level (160 kg / ha N) resulted in a decrease in photosynthesis and growth tempo and a smaller leaf canopy, which mean that the plants took to long to produce an effective leaf canopy. This, in turn, resulted to lower light interception and early die-off of the plants. Tuber initatiation is limited because stolons died off before they could initiate tubers, whilst the plants N reserves were depleted before tubers could bulk properly, which resulted in the production of a high percentage small tubers and a lower total yield. With reference to the N application time, a large quantity N early in the season (70% N with planting and 30% N after tuber iniation) produced an adequate leaf canopy and the N reserves were sufficient to bulk the initiated tubers. Sufficient N reserves early in the season is thus important to support vegetative and tuber growth. Large quantities N applied late in the season (30% N with planting and 70% N after tuber initiation), have led to an extention of the vegetative stage at the expense of tuber bulking, which negatively influenced the total yield (Figures 6 and 7). Although the highest N level (320 kg/ha N) produced an adequate leaf canopy early and had sufficient N reserves to maintain foliage growth, the harvest index results indicated that the vegetative stage was prolonged and the the leaf canopy was stimulated (unnecessary secondary growth) at the expense of tuber development and bulking. A trend was observed that when more N is applied during planting (70% N at planting and 30% N after tuber initiation), led to the best total tuber and marketable yield. A linear increase in marketable yield in reaction to earlier N application was observed. Adequate N reserves early in the season is therefore important to support vegetative growth, which will positively influence size group distribution and final yield. adequate N reserves to maintain the leaf canopy as well as to ensure sufficient bulking. In contrast therewith the lowest N level led to a decrease in photosynthesis and growing tempo, and a smaller leaf canopy. This delayed the production of an effective leaf canopy which resulted in lower light interception. Optimal growth could therefore not be maintained which resulted in the ealy die-off of plants. Tuber iniation is reduced by a low N level because of stolons dying-off before the tubers can initiate, whilst the plants deplete their N reserves before the tubers could be bulked efficiently, and which could lead to lower yields. Although the highest N level reached a large and efficient leaf canopy early in the season, a lower harvest index was attained, which indicates that excessive leaves stimulate growth at the expense of tuber development and bulking. A trend was observed that the highest N distribution during planting led to the highest final tuber yield of medium and large tubers. This is explained by the fact that a larger quantity N at planting led to adequate leaf canopy development which promoted optimum growth and tuber bulking. The cultivars BP1 and Eos performed best in terms of final tuber yield when the recommended and highest N levels were applied in combination with 70% N with planting and 30% N after tuber initiation. On the other hand, Lanorma, reacted less to the N treatments, especially the N level, which indicates that each cultivar has its own unique nutritional needs. However, these results (yield and size group distribution) cannot be interpreted in isolation, and as to how tuber quality is influenced by the N treatments should also be taken into account. We shall therefore address as to how nitrogen management influences tuber quality, in the next edition of CHIPS. C Summary The effect of N level and N application time on tuber yield and size group distribution of the three cultivars can be summarised as follow (Table 2): Final tuber yield as well as the yield of medium and large tubers, increased significantly from the lowest to the highest N level. This is ascribable to the fact that the highest (320 kg/ha N) and the recommended (240 kg/ha N) N level treatments could already produce an effective leaf canopy early in the season. There were CHIPS July/August 2018 Page 27