Effect of different concentrations and application periods of p- hydroxybenzoic acid on development, yield and yield components of
spring wheat
by Olaf Christen* and Christiane Theuer
Institute of Crop Science and Plant Breeding, Christian-Albrechts-University Kiel, 24118 Kiel, Germany
*corresponding author. Tel.:+49 431 8803474; Fax: +49 431 8801396
Key words: allelopathy, grain yield, phytotoxicity, phenolic compounds, p- hydroxybenzoic acid, spring wheat, yield components, yield losses
Abstract
Information on the effect of allelochemicals on grain yield of cereal crops is scarce in the literature. Therefore, the aim of these experiments was to evaluate the effect p- hydroxybenzoic acid on development, yield and yield components of spring wheat.
The development of spring wheat was considerably affected by a 2 or 4 week exposure to p-hydroxybenzoic acid right after germination. At growth stage 31 (tillering) the phenolic compound had significantly reduced the single plant dry weight, the plant length and the number of tillers per plant. The strongest inhibition was recorded after a four week exposure to p-hydroxybenzoic acid. This effect diminished until the next sampling date at growth stage 76 (ripening). At maturity, the grain yield was significantly reduced by all p-hydroxybenzoic acid treatments.
All yield components were affected. The reduced number of kernels had the largest impact on grain yield, whereas the number of tillers per plant and the thousand grain weight had smaller effects. The reduction in grain yield was more pronounced in the higher category tillers. Although, the development of spring wheat was more affected by the early treatments with p-hydroxybenzoic acid, the larger yield reduction was recorded after an exposure at later stages during the development. We argue that this effect was caused by the great sensitivity of the cereal crop during the transition from the vegetative to the generative development.
The implications of our results for the interpretation of allelopathic experiments are:
(i) though, early stages of crop development were more sensitive to an exposure to a phytotoxic chemical, the yield reduction was more severe after the later application and (ii) more agronomic factors have to be considered in allelopathic research, since different tiller categories showed a distinct quantitative response and results of such experiments will be affected, for example, by the seed rate.
Introduction
Despite substantial evidence for allelopathic effects in no-till systems, short cereal rotations or cereal monocultures, caused by phytotoxic substances released during the decomposition of plant residues agronomy research and standard agronomy textbooks tend to ignore these results (McCalla and Norstadt 1974; Müller-Wilmes et al. 1977; Wolf and Höflich 1983). One of the reasons might be, that the most common methods of identifying phytotoxicity have been germination or seedling growth bioassays with little or no relation to yield response under field conditions (Guenzi et al. 1967; Kimber 1973; Leather and Einhellig 1986; Mason-Sedun and Jessop 1989 ).
Allelochemicals associated with decomposing cereal residues are mainly phenolic and short chain aliphatic acids (Chou and Patrick 1976; Guenzi and McCalla 1966;
Tang and Waiss 1978; Wojcik-Wojtkowiak et al. 1990). In addition, Norstadt and McCalla (1963) isolated the antibiotic patulin produced by a number of different soil fungi as another cause of toxicity problems in mulch farming.
Siqueira et al. (1991) concluded that most phenolic acids present in crop residues have also been isolated from soils, however, their concentration varies considerably depending on soil type, amount of organic matter, previous cropping, type and amount of retained residues and extraction method. Based on the reviewed literature Siqueira et al. (1991) suggested that the amount of p-hydroxybenzoic acid might peak at 1400 nM in the soil solution. Other phenolic acids will contribute to phytotoxic effects, since in a soil environment there is normally a mixture of different phenolic acids.
Despite this evidence the yield response of wheat to phytotoxic substances has not been adequately established. In a series of experiments McCalla and associates (Ellis and McCalla 1973; McCalla and Norstadt 1974; Norstadt and McCalla 1971) quantified the effect of a single application of the microbial product patulin on wheat grown to maturity. Christen and Lovett (1993) investigated the effect of a short term application of p-hydroxybenzoic acid on spring barley and reported yield reductions of nearly 20 % depending on the concentration of the phenolic substance. Other experiments with short term exposure of crops to a phytotoxic chemical were either
conducted with other plant species, different chemicals, or the plants were not allowed to grow to maturity. Cochran et al. (1983) compared the influence of various concentrations of acetic acid on the average number of first stem tillers of wheat and described an almost 30 % reduction after a 4 day exposure with 10 mM acetic aceid compared with the control, but lower concentrations of a short term exposure with acetic acid did not cause long-term effects in their experiment. In contrast, Waters and Blum (1987) report that Phaseolus spp. bean plants recovered from two day treatment with 1.0 mM or 2.0 mM ferulic acid at seedling or flowering sateg. Only an exposure at podfill caused a significantly lower leaf area and plant dry weight at maturity due to water stress and a loss of turgidity. Using cucumber in a series of experiments, Blum and Rebbeck (1989) observed a rapid recovery of roots after a short-term exposure with ferulic acid but did not relate this result to later growth, development or yield parameters.
Agronomy research on cereal development and organogenesis with respect to reaction towards fertilizer (Langer and Liew 1973; Thorne et al. 1988), herbicide applications (Tottman 1977) or water stress (Christen et al. 1995) has focused on stage specific responses and identified differences in susceptibility depending on the developmental stage of the shoot apex. In particular the transition from the vegetative to the generative stage, the so called 'double ridges stage', seems to be a period with a particularly high sensitivity towards external factors, but so far only Christen and Lovett ( 1993) have addressed this.
The objective of the present study was to compare the effect of an application of p-hydroxybenzoic acid, as an example for a phenolic compound which has been frequently isolated from soil or soil water, on development, grain yield and yield components of spring wheat.
Methods
The experiment was conducted during 1995 with the spring wheat variety "Devon"
in sand-water culture using 5 l pots filled with coarse sand. In each pot 20 plants were sown after germination on tissue paper. The design was a multifactorial experiment with the following factors:
1) 5 Concentrations of p-hydroxybenzoic acid (control, 0.5, 1, 2.5 and 5 mM) 2) 3 Treatment periods (1-2 week, 1-4 week and 3-4 week)
3) 3 Sampling dates 4) 4 replicates
Therefore this design had a total of 180 pots. The p-hydroxybenzoic acid (Sigma Chemical Company, St Louis, Missouri) was applied with a standard nutrient solution three times a week with 200 ml each time giving a total of 600 ml per week.
In order to avoid a pH effect of the acid, the pH of all solutions regardless of the p- hydroxybenzoic concentration was adjusted to pH 5.8 with 0.01 M Na OH. The pots were flushed after treatment periods in order remove p-hydroxybenzoic acid from the solutions. The pots were kept outside and therefore the growing conditions for all treatments were similar.
The sequential harvests were conducted at GS 31 (12.6.95) and GS 76 (10.7.95), growth stages according to Zadoks et al. (1974). At these dates single plant dry matter, plant length and the number of tillers per plant were determined.
After the last application of p-hydroxybenzoic acid the wheat was allowed to grow to maturity and was harvested on 1 August (GS 91). At maturity all shoots having fertile ears were harvested individually and, after drying at 65 °C for 48 h, the grain weight and ear structure (kernels per ear) were determined. The thousand grain weight was calculated.
All data were subjected to a statistical analysis using PROC GLM (Generalized Linear Models), option LSMEANS of the SAS package (SAS 1985; Searle 1987).
The statistical procedure GLM was applied to allow for the unequal numbers of observations in the cells.
Results
All three parameters, viz dry matter, plant length and number of tillers per plant were negatively affected by the application of p-hydroxybenzoic acid compared with the untreated control (Table 1). Averaged over the different application periods, the single plant dry matter was reduced up to 43 % following an exposure to 5 mM of the phenolic compound. The number of tillers per plant showed a smaller response because there are only 14 days between the last application and the sampling date.
With respect to the different application periods, all three parameters were more affected by application during the first two or the first four weeks of the experiment.
This effect occurred more pronounced at the higher concentrations 2.5 and 5 mM.
At the sampling date GS 76, the differences in single plant dry matter (and the other parameters which are not shown) resemble the results of the first sampling (Table 2).
There was no difference in dry matter production with the application periods 1-2 week and 1-4 weeks however, these were significantl less thenn that from the 3-4 week.
Averaged over the different application periods, the single plant yield of spring wheat showed a clear response to p-hydroxybenzoic acid (Table 3). Even a concentration of only 0.5 mM caused a reduction of 32 % in single plant yield compared with the control treatment. This difference increased to 52 % following an application of 5 mM p-hydroxybenzoic acid. On average, the largest yield reduction occurred after an application of p-hydroxybenzoic acid from the 1-4 week. In order to explain the described differences in the single plant yield caused by the application of the phenolic compound, the different yield components have to be considered. The number of kernels per plant resembles the previously described data for the single plant yield, i.e. a clear negative response to the higher concentrations of the compound as well as the strongest effect of the late application. The two other yield components thousand grain weight and number of ears per plant did respond less to the application. Especially the thousand grain weight showed some compensational effect after an application from the 3-4 week, causing an increase in this parameter.
The previously described response of the single ear weight after an application with p-hydroxybenzoic acid is confirmed for all tiller categories, but the magnitude of the reaction differed between the categories (Table 4). On average, the highest concentration of p-hydroxybenzoic acid caused a yield decrease of 41 % in the main stem tillers compared with the untreated control, whereas ears of the third category tillers suffered a 66 % reduction in weight compared with the untreated control. The yield of the second tillers averaged 54 % less at the highest concentration of p- hydroxybenzoic acid. In general, higher tiller categories suffered more severely after an application of p-hydroxybenzoic acid and therefore were not able to compensate for the detrimental effects of the phenolic acid. Apart from a few exceptions, this observation can be confirmed for all application periods and different concentrations of p-hydroxybenzoic acid. The different components of yield showed the same behaviour previously described for the single ear yield of the different tiller categories (data not shown).
Discussion
The evidence presented demonstrates the potential of p-hydroxybenzoic acid to substantially reduce grain yields of spring wheat. We believe this to be the first report of deleterious effects of a phenolic acid application for different periods on development, grain yield and yield components of spring wheat. These results, however, have to be interpreted very carefully in relation to yield effects observed in field experiments, since the growth medium used here, sand with only negligible organic content, as we have used here, does not completely reflect the situation in the field. Additionally the concentrations of p-hydroxybenzoic acid chosen in this experiment were generally higher than concentrations found in the soil. However, this concentration of phenolic acids in the bulk soil and the soil solution is affected by various parameters and estimates of the active part are still subject of considerable dispute and speculation in the literature (for review see Siqueira et al.
1991). On the other hand, in the field plants are exposed to a great number of different phenolic acids at the same developmental stage, which in combination,
according to Einhellig et al. (1982), affect germination and early growth well below their separate thresholds.
An important result of this experiment is the decreasing effect of the p- hydroxybenzoic acid application on the parameters of the plant development in relation to the effect on the yield and yield components of spring wheat. This finding extends conclusions drawn by Christen and Lovett (1993) in a comparable experiment with spring barley. The reduction in dry weight, plant length and number of tillers per plant does not completely correspond with the yield reduction, i.e. in the treatment which only received p-hydroxybenzoic acid from the 3-4 week a smaller reduction was observed during the development, but the yield was more severely affected. The finding that both yield components, i.e. kernels per ear and thousand grain weight, were affected by an exposure to p-hydroxybenzoic acid in the development experiment confirms reports from Ellis and McCalla (1973) applying patulin to wheat in a similar experiment. Also the response of the different tiller categories confirms reports by Christen and Lovett (1993) with spring barley and is in general comparable with tiller responses to stress of various kinds (Christen and Hanus 1993, Christen et al. 1995).
Higher tiller categories proved to be more sensitive and showed a larger yield decrease. It could be argued that the development of the main stem tiller was enhanced after the application of p-hydroxybenzoic acid at double ridge and, subsequently, the higher tiller categories suffered a yield reduction due to interplant competition. Another possibility would be a larger sensitivity of the higher tiller categories at early stages of the apex development which consequently increased the single ear weight of the main stem tiller. Both effects might also appear in combination. The so-called "sensitive stages" and, especially, the transition from the vegetative to the generative development of the apex, are solely based on experiments comparing herbicide or fertilizer treatments and the cereal crop will not necessarily show sensitivity at similar stages towards allelochemicals.
The specific response of different tiller categories in respect to grain yield following an exposure to p-hydroxybenzoic acid clearly demonstrates the need for considering more agronomic and husbandry factors in the design of experiments investigating
allelopathy. Since higher tiller categories showed a larger yield decrease after short- term stress caused by p-hydroxybenzoic acid, the yield depression in a field situation interact with seed density, thus modifying the ability of different tiller categories to compensate for the detrimental effects of allelochemicals.
The great sensitivity of cereal plants to stress at the transition of the shoot apex from the vegetative to the generative development ('double ridges stage'), which has been confirmed in our experiment with p-hydroxybenzoic acid, is not fully understood.
The observed effects of p-hydroxybenzoic acid on wheat yield and yield components are the integrated results of specific biochemical and cytological reactions of the plant. It has been demonstrated that phenolic acids might affect processes like cell division, cell elongation, membrane permeability and mineral uptake. Tottman (1977) investigating the effect of herbicides on wheat, argued that the yield decrease caused by a stress at 'double ridges' is due to interference with the extreme sensitive process of primordium formation on the shoot apex. It, therefore, seems also possible that phenolics like p-hydroxybenzoic acid, which might interact with the hormone synthesis will affect a cereal plant at this developmental stage by changing its hormonal balance.
Apart from Christen and Lovett (1993) only McCalla and colleagues have investigated the response of a cereal crop to the application of a phytotoxic substance in the context of yield decline caused by crop residues. They compared germination bioassays with the response of wheat grown to maturity after a single application with the microbial product patulin in order to quantify yield losses caused by phytotoxic substances. The response in tillering revealed an interaction with the medium, thus patulin reduced the number of tillers in sand but increased tillering of wheat in soil, an effect which the authors attribute to the ability of soil microorganisms to inactivate patulin in soils. In both treatments, however, the patulin application caused a reduction in the kernel weight, due to a lower number of kernels per head. Based on further experiments, McCalla and co-workers (Ellis and McCalla 1973; McCalla and Norstadt 1974) concluded that spring wheat was most sensitive to an application of patulin at germination or at heading. Other application dates based on the external development of the wheat plant at the second node stage
(GS 32) and the flag leaf sheath opening (GS 49) were less susceptible. Due to the different chemicals used in the bioassays, a direct comparison is restricted to the general approach; however, it is confirmed that an exposure with a phytotoxic substance during the early development of the cereal crop might cause substantial losses in grain yield.
In order to relate these findings to yield depressions described in field situations, quantitative analyses of the phenolic content in soils and/or soil water based on sequential sampling dates are required. But so far most analyses have been restricted to soil samples taken only once or twice during the growing season (Whitehead et al.
1981; Whitehead et al. 1983). Few reports have attempted to relate changes in concentrations of phytotoxic substances in the soil to the crop development in field experiments.
Müller-Wilmes et al. (1977) found highest concentrations of phenolics in an experiments with winter barley grown in monoculture or following potatoes from early spring until ear emergence. They explain the yield depression of 10 % to 23 %, depending on the level of nitrogen application, to differences in the chemical composition of the phenolic compounds rather than to the actual concentration in the soil. In contrast, Wolf and Höflich (1983) compared the allelopathic potential of soil collected in a wheat monoculture with soil from wheat grown in rotation. They used using a radish germination bioassay and reported the largest degree of toxicity in October and April. However, they did not isolate or identify the chemical nature of the effects. These results indicate a shift in phytotoxicity during the growing season, although the flow rates between the different pools of phenolics in the soil and conditions affecting these processes are little understood (Blum et al. 1991).
A quantification of the yield decrease caused by phytotoxic substances released from crop residues under field conditions is only possible if sequential soil sampling is accompanied by a detailed observation of the crop development, since differences in the apex development are most important for an understanding of yield differences in respect to environmental conditions and husbandry factors (Kirby and Appleyard 1984; Landes and Porter 1989).
On the evidence presented this is an approach which has to be considered in allelopathic research.
Acknowledgements
This project was financially supported by a grant of the German Research Council (Deutsche Forschungsgemeinschaft).
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Tab.: 1 Effect of different concentrations [mM] and application periods of p- hydroxybenzoic acid on dry matter [g] per single plant, plant length [cm]
and number of tillers per plant of spring wheat at GS 31
Dry matter per single plant
Control 0.5 1 2.5 5 ∅
1-2 Week 1.59ab 1.29de 1.16ef 1.03fg 0.70h 1.15b 1-4 Week 1.65a 1.25de 1.15ef 0.98g 0.68h 1.14b 3-4 Week 1.52b 1.72a 1.58ab 1.50bc 1.37cd 1.54a
∅ 1.59a 1.42b 1.30c 1.17d 0.92e
Single plant length
Control 0.5 1 2.5 5 ∅
1-2 Week 50.1ab 45.8d 43.3e 39.4f 31.9h 42.1b 1-4 Week 50.9a 42.3e 40.8f 36.9g 31.1h 40.4c 3-4 Week 48.2c 51.0a 48.9bc 48.1c 43.5e 47.9a
∅ 49.7e 46.6d 44.3c 41.5b 35.5a
Number of tillers per plant
Control 0.5 1 2.5 5 ∅
1-2 Week 6.0a 4.7de 5.0cde 4.5e 3.9f 4.8b
1-4 Week 5.7ab 5.1cd 5.1cd 4.6e 3.5g 4.8b
3-4 Week 6.0a 5.4bc 5.6ab 6.0a 5.3bc 5.7a
∅ 5.9a 5.1c 5.2b 5.0c 4.2d
Values followed by the same letter within the same feature are not significantly different at p<0.05 level.
Tab. 2: Effect of different concentrations [mM] and application periods of p- hydroxybenzoic acid on dry matter [g] per single plant of spring wheat at GS 76
Control 0.5 1 2.5 5 ∅
1-2 Week 7.97bcd 5.88j 7.20fgh 7.43cfg 7.02gh 7.10b 1-4 Week 8.01bcd 7.22fgh 6.93h 7.45cfg 6.39i 7.20b 3-4 Week 8.57a 8.14abc 7.84cde 8.46ab 7.59dcf 8.12a
∅ 8.19a 7.08cd 7.32c 7.78b 7.00d
Values followed by the same letter are not significantly different at p<0.05 level.
Tab. 3: Effect of different concentrations [mM] and application periods of p- hydroxybenzoic acid on single plant yield [g], number of kernels per plant, thousand grain weight [g] and number of ears per plant of spring wheat at GS 91
Single plant yield
Control 0.5 1 2.5 5 ∅
1-2 Week 3.03a 1.69ef 1.75de 2.31c 2.00d 2.16a
1-4 Week 2.74ab 2.36c 1.51fg 1.66ef 1.26g 1.91a
3-4 Week 2.82ab 1.82de 1.83de 1.80de 0.92h 1.84b
∅ 2.86a 1.96b 1.70c 1.92b 1.39d
Number of kernels per plant
Control 0.5 1 2.5 5 ∅
1-2 Week 81a 58de 56defg 71b 61cd 65a
1-4 Week 76ab 68bc 46hi 61cd 45hi 59b
3-4 Week 73ab 53efgh 53efgh 49ghi 43i 54c
∅ 77a 60b 52c 60b 50c
Thousand grain weight
Control 0.5 1 2.5 5 ∅
1-2 Week 37ab 29e 30d 32d 32d 32b
1-4 Week 36ab 35bc 32d 26f 27ef 31b
3-4 Week 38a 33cd 35bc 38a 21g 33a
∅ 37a 32b 32b 32b 27c
Number of tillers per plant
Control 0.5 1 2.5 5 ∅
1-2 Week 2.3a 1.5f 1.7ed 2.2ab 2.4a 2.0a
1-4 Week 2.1abc 1.9cde 1.6ed 2.0bcd 1.7ed 1.9b
3-4 Week 2.0bcd 1.8de 1.9cde 2.2ab 1.8de 1.9b
∅ 2.1a 1.8c 1.7d 2.1a 2.0b
Values followed by the same letter within the same feature are not significantly different at p<0.05 level.
Tab. 4: Effect of different concentrations [mM] and application periods of p- hydroxybenzoic acid on single plant yield [g] of spring wheat on first, second and third category tillers at GS 91
Single ear yield of first category tillers
Control 0.5 1 2.5 5 ∅
1-2 Week 1.69ab 1.42c 1.32cd 1.38c 1.39c 1.44a
1-4 Week 1.66b 1.71ab 1.17e 1.15e 1.01f 1.34b
3-4 Week 1.79a 1.19e 1.23de 1.20de 0.66f 1.21c
∅ 1.71a 1.44b 1.24c 1.24c 1.02d
Single ear yield of second category tillers
Control 0.5 1 2.5 5 ∅
1-2 Week 1.16a 0.51de 0.58d 0.86b 0.51de 0.73*
1-4 Week 1.04a 0.80bc 0.54de 0.60bcd 0.37e 0.67
3-4 Week 1.06a 0.74bc 0.62bcd 0.59d 0.33f 0.67
∅ 1.09a 0.68b 0.59c 0.68b 0.40d
Single ear yield of third category tillers
Control 0.5 1 2.5 5 ∅
1-2 Week 0.78a 0.28fg 0.49cdef 0.63bc 0.32fg 0.50*
1-4 Week 0.76bc 0.42def 0.46cdef 0.45def 0.24g 0.46
3-4 Week 0.83a 0.61bcd 0.55cde 0.39efg 0.24g 0.52
∅ 0.79a 0.44b 0.50b 0.49b 0.27c
Values followed by the same letter within the same feature are not significantly different at p<0.05 level.
* Not significantly different at p<0.05
