2024 Volume 93 Issue 2 Pages 160-168
In general, either high or low branching may contribute to the yield potential of a cultivated crop. In this study, we evaluated the varietal differences in branching characteristics in asparagus crowns and examined the relationship between branching types and harvest characteristics among six varieties tested by careful observation. ‘UC 157’ developed numerous tillers and shoot apical meristems 6 months after sowing; thus, it was considered to be a high-branching type variety. In contrast, ‘All-male Gulliver’ and ‘Ryuryoku’ developed few tillers or shoot apical meristems and, were therefore considered to be low-branching type varieties. The final order of sympodial shoots and composition of sympodial shoots by order were similar between the varieties. ‘UC 157’ had fewer scale leaves attached to the underground shoots than other varieties, which may have been advantageous for developing underground shoots more rapidly. The total spear yield of ‘UC 157’ was high, but the marketable spear yield was similar to that of other varieties owing to the large number of thin spears. Although further studies are needed to determine whether high-branching or low-branching type asparagus varieties are more high-yielding, we determined that high-branching type varieties have lower labor productivity under unimproved growing conditions such as non-sparsely planted growing conditions.
Asparagus (Asparagus officinalis L.) is a perennial plant that grows worldwide. Its subterranean structure crown is an aggregate of short underground shoots containing numerous shoot apical meristems (SAMs). Underground shoots branch sympodially and extend rapidly to the surface (Feller et al., 2012). Aboveground shoots are harvested and used as food. Some are allowed to fully develop into stalks and photosynthesize, extending the harvest period (Shou et al., 2007). Therefore, controlling when, where, and how underground shoots branch during harvesting and cultivation is of great importance.
In general, there are two genetically determined types of sympodial branching plants: determinate with low branching and indeterminate with high branching (Pnueli et al., 1998). Tomato (Solanum lycopersicum L.) is a typical example reflecting the characteristics of the determinate and indeterminate types in commercial cultivation. In Japan, determinate-type tomato varieties are often grown for processing because they are suitable for simultaneous harvesting (Sato et al., 2004), whereas indeterminate-type tomato varieties often undergo forced cultivation for fresh consumption because they can be harvested continuously (Higashide, 2010). Various pinching and training techniques have been developed for each tomato branching type (Fukuchi et al., 2004; Ohta and Ikeda, 2015, 2016; Ohta et al., 2022). As another example, among high-yield soybeans (Glycine max (L.) Merr.), determinate-type varieties are shorter than indeterminate-type varieties, making them less susceptible to lodging and more useful for cultivation (Wilcox and Sediyama, 1981). The appropriate sowing time for each branching type has been previously studied (Beaver and Johnson, 1981; Wilcox and Frankenberger, 1987). However, little information is available regarding the branching type of asparagus. The branching pattern of asparagus differs slightly from that of tomato and soybean; whereas in tomato and soybean, shoot elongation stops when SAM turns from vegetative to reproductive growth, in asparagus crowns, when shoot elongation stops, it is apparently not accompanied by SAM reproductive growth. This suggests that TFL1, a gene involved in inflorescence meristem identity and responsible for differences in branching type in tomato and soybean, may not have a similar effect in asparagus. The branching characteristics of the underground shoots of each asparagus variety are unknown. The relationship between branching type and growing characteristics is also unknown, which prevents the development of policies and concrete methods for underground shoot branch control.
One reason that the underground shoot branch and crown growth of asparagus have not been investigated in detail is that they are covered by soil and cannot be observed directly. Another is that the internodes of underground shoots are too short to be observed with the naked eye. Rough observations, after digging up the crown, revealed two alternating rows of above/underground shoots, with many above/underground shoots on the outside of the rows; however, Sawada et al. (1961) pointed out that there are many other small underground shoots hidden at the tips that are not visible because of the short internodes. The only way to observe these small underground shoots is by using a microscope and carefully dissecting the crown with fine tools.
The present study used relatively easy to dig up seedlings to determine varietal differences in underground shoot branching patterns and their relationship with harvest characteristics. Detailed dissection of the asparagus plants was conducted to elucidate the branching patterns. This study addressed two research questions: 1) Whether the branching characteristics of underground shoots in asparagus differ among varieties and 2) How the branching types of underground shoots are related to growth characteristics.
Six asparagus varieties were used in this study: ‘All-male Gulliver’ (Pioneer EcoScience Co., Ltd., Tokyo, Japan), ‘Mary Washington 500W’ (Tsurushin Co., Ltd., Nagano, Japan), ‘Ryuryoku’, ‘Super Welcome’, ‘UC 157’, and ‘Welcome AT’ (Sakata Seed Corporation, Kanagawa, Japan). The sex of individuals of dioecious varieties (‘Mary Washington 500W’ and ‘UC157’) was not determined.
Fifteen seeds of each variety were sown in black plastic pots (9 cm in diameter) filled with a soil mixture (Yosaku N-150; JCAM Agri. Co., Ltd., Tokyo, Japan) and placed in a growth chamber (TGH-3; ESPEC MIC Corp., Aichi, Japan) at 25°C/15°C day/night temperature under a 14-h photoperiod on November 18, 2020. Irrigation was performed once daily using online drippers (20500-001100 dripper; Netafim Japan Co., Ltd., Tokyo, Japan).
Seedling dissection and branching pattern observationsThree months (February 25 to March 5, 2021) and 6 months (May 20 to 27, 2021) after sowing, 5 and 3 seedlings of moderate growth per variety, germinated within 2 to 3 weeks of sowing, were carefully dissected. Seedlings were dissected as follows: shoot-scale leaves on each primary shoot (PSM) or sympodial shoot (SYM) were carefully removed individually from the lower leaves using precision tweezers and needles. The development of the leaf axils was simultaneously observed under a stereomicroscope. Above/underground shoots were dissected by removing the scale leaves until the SAMs were exposed (Fig. 1). The number of SAMs (equal to the number of above/underground shoots) per seedling, number of tillers, number of scale leaves on each above/underground shoot (including leaves on the ferns), order number of each above/underground shoot, number of ferns, fern diameter, and the node position and height at which the cladodes developed were recorded. The number of stored roots (> 5 mm long) was recorded 6 months after sowing. SAMs with internode lengths of 5 mm or more were counted as elongated SAMs, while others were counted as non-elongated SAMs. Regarding the number of scale leaves, leaves with an internode length less than 5 mm and those with an internode length greater than 5 mm were counted separately to distinguish between those that remained underground and those that emerged aboveground. Figure 2 shows the definitions of each term used in this study.
Dissection procedure for asparagus above/underground shoots. Shoot-scale leaves on each primary shoot (PSM) or sympodial shoot (SYM) in the crowns were carefully removed from the lower leaves using precision tweezers and needles. Simultaneously, the following were recorded: (1) the number of scale leaves on each above/underground shoot and (2) the branching occasion (development of the leaf axils). Each PSM or SYM was examined until the shoot apical meristem (SAM) was exposed. In this figure, the 4 SAMs (S1, S2, S3, and S4) encased in the scale leaves on the underground shoot were progressively distinguished as the scale leaves were removed in the order A, B, and C.
Schematic illustration of an asparagus crown and the definition of each term. Primary shoot (PSM), sympodial shoot (SYM), and shoot apical meristem (SAM).
Two seedlings per pot were transplanted into plastic pots (66.4 cm in diameter, NP pot #100; DIC Plastics, Inc., Saitama, Japan) filled with unsterilized sand and compost (Midori Organic, JA Kagawa, Japan) on June 9, 2021. All pots were placed in a glass greenhouse at NARO, Zentsuji City, Kagawa Prefecture, Japan. The pots were irrigated five times a day using online drippers (20500-001100 dripper; Netafim Japan Co., Ltd.) until November 17, 2021. The ferns were cut just above the ground and allowed to remain dormant.
The spears were harvested from February 25 to November 4, 2022. The weight of 25-cm long spears was measured, and any thin (< 7 g), bent spear, or loose tip was recorded as an unmarketable spear. Irrigation was resumed three times a day using the same drippers on March 1, 2022. To promote photosynthesis and extend the harvest period, three spears of approximately 1 cm in diameter per seedling were selected and allowed to develop fully into stalks on March 16, 2021. A chemical fertilizer of 10-8-6 (N-P2O5-K2O, FTE Aspara-Organic 086; Nihon Hiryo Co., Ltd., Gunma, Japan) was applied monthly during harvesting at 20 g per plant. Three to five seedlings were used per variety (three ‘Ryuryoku’, five ‘Mary Washington 500W’, and four of the other varieties).
Statistical analysisAll data were subjected to analysis of variance (ANOVA) and Tukey’s honest significant difference (HSD) test at P < 0.05 using JMP® 8.0.2 (SAS Institute, Cary, NC, USA). The relationships between underground shoot branching characteristics of asparagus crowns and other traits were evaluated by calculating Spearman’s rank correlation coefficients between the number of tillers and SAMs and the node position at which the cladodes developed, the number of ferns, and fern diameter in each variety.
There were varietal differences in the number of tillers and SAMs in the asparagus crowns 3 and 6 months after sowing (Table 1). Three months after sowing, the number of tillers and SAMs was higher in ‘UC 157’, with 12.3 tillers and 40.0 SAMs, including 30.0 unelongated SAMs, than in ‘All-male Gulliver’ and ‘Ryuryoku’, with 4.6 tillers and 16.6 SAMs, including 9.8 unelongated SAMs, and 5.6 tillers and 19.4 SAMs, including 11.2 unelongated SAMs, respectively. The remaining varieties were somewhere between these groups. Similarly, 6 months after sowing, the number of tillers and SAMs was higher in ‘UC 157’, with 40.7 tillers and 128.0 SAMs, including 99.3 unelongated SAMs, than in the other five varieties, with 12.0–23.0 tillers and 39.3–67.0 SAMs, including 28.0–50.0 unelongated SAMs.
Number of tillers and shoot apical meristems (SAMs) in asparagus crowns at 3 or 6 months after sowing.
Figure 3 shows the above/underground shoot composition according to the order. Three months after sowing, the highest number of above/underground shoots per seedling was in the 5th–7th SYM for all varieties. In the 1st–4th and 7th SYM, there were varietal differences in above/underground shoot number; the number of SYMs in these orders was higher in ‘UC 157’ than in the other five varieties (1st–3rd SYMs) or in ‘Welcome AT’, ‘Ryuryoku’, and ‘All-male Gulliver’ (4th SYMs) or in ‘All-male Gulliver’ (7th SYMs). Six months after sowing, the highest number of above/underground shoots per seedling was in the 7th–10th SYM for all varieties. In the 6th–13th SYM, there were varietal differences in the number of above/underground shoots; the number of SYMs in these orders was higher in ‘UC 157’ than in ‘Welcome AT’, ‘Ryuryoku’, and ‘All-male Gulliver’ (6th, 7th, and 10th SYMs) or in ‘Super Welcome’, ‘Welcome AT’, ‘Ryuryoku’, and ‘All-male Gulliver’ (8th, 9th, and 13th SYMs) or in the other five varieties (11th and 12th SYMs). There were no varietal differences in the maximum order of the above/underground shoots (P < 0.05); it was 9.0–11.0 and 13.7–17.3, 3 and 6 months after sowing, respectively.
Shoot composition by order at 3 or 6 months after sowing. Total aboveground and underground shoots are shown. NS: Varietal differences were not significant. **: Varietal differences in the number of shoots for each order were significant at P < 0.01 by ANOVA. Different letters within the same sympodial shoot (SYM) and months after sowing indicate significant differences between varieties (Tukey’s HSD test, P < 0.05).
The number of scale leaves remaining underground on each above/underground shoot ranged from one to several on the PSM and the 1st and 2nd SYMs, whereas it increased on the 3rd and higher SYMs (Fig. 4). It then decreased again because of immature underground shoots on the 5th or higher SYMs 3 months after sowing, and on the 5th–13th or higher SYMs 6 months after sowing. Generally, it was higher in ‘Ryuryoku’ and ‘All-male Gulliver’ and lower in ‘UC 157’ and ‘Mary Washington 500W’. In contrast, there were no varietal differences in the number of scale leaves above the ground on each fern, except for the 6th SYMs 3 months after sowing and the 1st and 2nd SYMs 6 months after sowing; it was approximately 30 to 40 on the PSM and 1st to 4th SYM and decreased on the 5th or higher SYM owing to immature shoots.
The number of scale leaves on each shoot at 3 or 6 months after sowing. Total aboveground and underground shoots are shown. The primary shoot (PSM) and all sympodial shoots (SYMs) combined were tested by ANOVA, followed by Tukey’s HSD test as the post hoc test. NS: Varietal differences were non-significant. **, *: Varietal differences were significant at P < 0.01, 0.05 by ANOVA. Different letters within the same column indicate significant differences (Tukey’s HSD test, P < 0.05). n = 5 (3 months after sowing) or 3 (6 months after sowing).
Figure 5 shows a typical sample of the branching pattern of each variety 6 months after sowing.
Typical sample of the branching pattern of each variety at 6 months after sowing. —: primary shoot (PSM) and sympodial shoots (SYMs), ●: shoot apical meristems (SAMs) that elongated into ferns and spears, ○: unelongated SAMs. Scale leaves were omitted from the drawings.
No varietal differences in the number of ferns were observed 3 months after sowing. However, 6 months after sowing, ‘UC 157’ had more ferns than all other test varieties. There were no varietal differences in the fern diameter, their coefficients of variation, or the node position and height at which cladodes developed. Six months after sowing, the numbers of tillers and SAMs in asparagus crowns were significantly correlated with the number of ferns (r = 0.94**, P < 0.01) and fern diameter (r = −0.89*, P < 0.05; Table 2).
Number, diameter, node position, and height where asparagus fern cladodes developed at 3 or 6 months after sowing.
There were varietal differences in the number of storage roots per seedling 6 months after sowing; it was higher in ‘UC 157’ than in other varieties. However, the number of storage roots per SAM was higher in ‘Ryuryoku’ and lower in ‘UC 157’ and ‘Mary Washington 500W’ (Table 3). By naked eye observation, ‘UC 157’ and ‘Mary Washington 500W’ had a complex 3-dimensional intertwining of adjacent SAMs and storage roots, which was not observed in the other varieties. The numbers of tillers and SAMs in the asparagus crowns were significantly correlated with the number of storage roots per SAM (r = −0.94**, P < 0.01).
Number of storage roots at 6 months after sowing.
Varietal differences were observed in the total number of spears. There were more in ‘UC 157’ and less in ‘All-male Gulliver’. In contrast, the total weight of spears, marketable number/weight of spears, and salability rate did not significantly differ among varieties. The number of tillers and SAMs in asparagus crowns 3 months after sowing was significantly correlated with the total spear yield (both r = 0.94**, P < 0.01) and salability rate in terms of weight (both r = −0.83*, P < 0.05). Similarly, the number of tillers and SAMs in asparagus crowns 6 months after sowing significantly correlated with the total spear yield (both r = 0.83*, P < 0.05; Table 4).
Yield and marketable rate of 2-year-old asparagus.
In some plant species, the translocation of photosynthates between the tillers is poor (Carvalho et al., 2006; St-Pierre and Wright, 1972). In asparagus, most photosynthetic products are translocated from ferns to storage roots and then to new shoots (Downton and Torokfalvy, 1975). Although the effect of the morphological position of storage roots and above/underground shoots on translocation efficiency in asparagus is unknown, assuming that the translocation efficiency is low between storage roots and above/underground shoots belonging to different tillers, high-branching type varieties with more tillers may have inferior growth and yield. Additionally, excessive tillers in high-branching type asparagus varieties may result in a lack of photosynthetic products, and some tillers may not grow well and may not be harvested because asparagus usually grows only a few photosynthetic ferns during mother fern cultivation. Therefore, identifying the underground shoot branching characteristics of asparagus crowns of each variety is important.
A comparison of the number of tillers and SAMs suggested that ‘UC 157’ was more branched (Table 1), and closer to the determinate-type variety. ‘UC 157’ continued to develop new tillers frequently without growing many scale leaves during the test period (Fig. 4). A more plausible hypothesis is that the low number of scale leaves on underground shoots contributes to the trait of ‘UC 157’ to differentiate many tillers and SAMs; underground shoots with fewer scale leaves may be less costly to develop. ‘All-male Gulliver’ and ‘Ryuryoku’ were less branched (Table 1) and considered closer to indeterminate-type varieties than ‘UC 157’. In these varieties, underground shoots contain more scale leaves (Fig. 4), making underground shoot differentiation costlier; thus, they may not produce extra tillers. In strawberries, a sympodial branching species, a decrease in the number of shoot leaves and an increase in the number of tillers are simultaneously observed (Ishihara et al., 1994; Yano et al., 2021). Further studies are needed on the cost of SYM formation in asparagus, as its scale leaves are tiny, and their characteristics are not understood to be similar to those of strawberries.
Three months after sowing, the varieties ‘Mary Washington 500W’, ‘Welcome AT’, and ‘Super Welcome’ were considered to have characteristics intermediate between ‘UC 157’ and ‘Ryuryoku’ or ‘All-male Gulliver’, i.e., between high-branching type and low-branching type varieties. However, 6 months after sowing, they were considered to be closer to the characteristics of ‘Ryuryoku’ and ‘All-male Gulliver’, i.e., a low-branching type variety. A possible reason for the observed different branching patterns of these varieties depending on the time after sowing could be the growth progression during the test period, and varietal differences in photosynthetic and/or respiration rates may have affected the branching patterns. Genotyping and detailed phenotyping under diverse conditions are necessary to classify these groups of varieties into intermediate branching types. Previous studies have shown that the branching types of tomatoes and soybeans are qualitative; however, in tomatoes, one gene is involved, whereas in soybeans, two genes are involved (Bernard, 1972; Pnueli et al., 1998). The findings in tomatoes and soybeans should be informative for determining the number and identity of genes involved in the branching patterns in asparagus.
Whether high-branching or low-branching type asparagus yields are higher is a practical matter. The yield superiority of high-branching and low-branching type soybean varieties depends on the growing conditions (Beaver and Johnson, 1981; Kato et al., 2015; Wilcox and Frankenberger, 1987). One study has shown that determinate-type sesame (Sesamum indicum L.) varieties have higher yield than indeterminate-type varieties (Uzun and Çağırgan, 2006). However, the cultivation characteristics and yields of soybeans and sesame may be due to specific morphology, that is, the number of pods, number of seeds per pod, and lodging resistance associated with differences in plant height owing to branching type. No pods are produced in asparagus, and the seeds are not edible. Additionally, the internode length is concise and underground, and differences in branching type are unrelated to aboveground appearance. Indeed, in this study, the aboveground growth of asparagus seedlings 3 or 6 months after sowing was similar regardless of the branching type. Our results agree with those of 3-year-old 6 asparagus varieties, including four varieties grown in the present study (Haihong et al., 2017).
Several studies have compared the yield advantages of the high-branching type ‘UC 157’ and the low-branching type ‘All-male Gulliver’, showing that the ‘All-male Gulliver’ is superior (Takamura et al., 2015), while others have found no difference between the two (Haihong et al., 2017; Shinoda et al., 2018). Our survey using one-year-old plants showed that branching characteristics 3 and 6 months after sowing are related to the total number of spear yields, but not the total weight of spear yields, marketable number, or weight of spear yields. Asparagus high-branching type ‘UC 157’ has numerous tillers containing SAMs, which are the source of spears. Nevertheless, the marketable spear yield of ‘UC 157’ is not superior to low-branching type ‘All-male Gulliver’ and ‘Ryuryoku’ for two possible reasons. First, there could be an inadequate sink-source balance, i.e., photosynthesis that is not commensurate with the number of SAMs. In this study, the SAMs with insufficient photosynthates were thin or bent spears and unsalable. Second, it may be that the numerous storage roots compete spatially in the soil and cannot fully absorb nutrients and water.
It is still too early to conclude whether the high-branching or low-branching type of asparagus is superior in terms of yield; however, obtaining high yields from the high-branching type variety would be difficult if grown without appropriate measures. Additionally, high-branching type sprouts contain many unsalable spears, which may result in lower labor productivity. Cultivation efforts are needed to achieve high yields with high-branching type asparagus. For example, we propose extending the growing space above and below ground to promote photosynthesis and root absorption (Motoki et al., 2009; Tanaka, 2004). Alternatively, we propose providing plenty of fertilizer and irrigation to promote shoot growth (Roth and Gardner, 1990). In addition, there are many cropping types of asparagus, e.g., open field culture, semi-forcing culture, forcing culture, and “fusekomi” culture (Yamaguchi and Maeda, 2013). Cropping types that allow SAMs to grow in good sequence and at speed may result in better yields of determinate varieties.
It should also be noted that the effects of asparagus age on branching and harvest characteristics have not been fully investigated. This study used easily observable one-year and two-year-old asparagus seedlings to investigate the crown characteristics. On the other hand, as the crown grows over the years, the effects of subsoil components, including allelopathic substances, space limitations, and microorganisms will likely change, resulting in lower yields in some varieties (Falloon and Grogan, 1991; Falloon and Nikoloff, 1986; Reijmerink, 1973; Yang, 1982). Further studies are needed to investigate asparagus crown branching and harvesting characteristics using older plants.
Our conclusions regarding the research question at the beginning of this paper are as follows. There are at least two types of asparagus varieties: high-branching types that are close to the determinate-type, and low-branching types that are close to the indeterminate-type, of other plant species. High-branching type varieties are currently unsuitable for labor-saving and low-cost cultivation because of their harvesting characteristics, often resulting in unsalable products.
We thank Ms. Chizuko Ono for her assistance with this research.
