Plant and Soil 1551156: 355-358, 1993 © 1993 KluwerAcademic Publishers. Printed in the Netherlands.

Pod development of groundnut (Arachis hypogaea L.) in solution culture G. E. ZHARARE, C. J. ASHER, F. P. C. BLAMEY and P. J. DART Department of Agriculture, The University of Queensland, Brisbane, Queensland 4072, Australia

Key words: Arachis hypogaea L, nutrient solution culture, pod development, 'root hairs', Mg, Mn, Zn

Abstract

Normal pods (containing seed) of groundnut (Arachis hypogaea L.) (cv. TMV-2) were successfully raised in darkened, aerated, nutrient solution, but not in the light. The onset of podding was evident 7 to 8 d after gynophores were submerged in the darkened nutrient solution. An examination of pods and submerged portions of gynophore surfaces by scanning electron microscopy showed the presence of two distinctly different protuberances: unicellular root-hair-like structures that first developed from epidermal cells of the gynophores and developing pods; and branched septate hairs that developed later from cells below the epidermal layer. The septate hairs became visible only after the epidermal and associated unicellular structures had been shed by the expanding gynophore and pods. Omission of Mn and Mg from the podding environment increased pod and seed weight, whilst omission of Zn reduced pod and seed weight.

Introduction

The developing groundnut (Arachis hypogaea L.) pod derives part of its nutrient requirements from the root system via the vascular tissue and part by direct absorption from the podding medium. Detailed quantitative studies of direct absorption processes would be facilitated by culturing attached gynophores and developing pods in chemically-defined media such as well-stirred nutrient solutions. Successful groundnut fruit development requires a mechanical stimulus in addition to moisture and darkness (Zamski and Ziv, 1976; Ziv and Zamski, 1975). The penetration of soil by the gynophore is generally considered to provide the conditions necessary for pod development. Studies which separated the rooting and podding zones have showed the i m p o r t a n c e of adequate Ca in the podding medium (Bledsoe et al., 1949), but the role of other essential elements is less well established. Campbell et al. (1975) showed that despite the

poor phloem mobility of B, low B in the podding medium did not impair pod development or seed production. On the other hand, several studies have shown that addition of Mg to the podding environment of field-grown groundnut may depress seed production (Brady and Colwell, 1945; Colwell and Brady, 1945; Chesney, 1975). The present paper reports the results of an experiment designed to test the feasibility of producing groundnut pods in nutrient solutions. A further experiment was designed to examine the effects of Mg, Mn and Zn in the podding zone on pod development.

Materials and methods

Experiment 1: Effect of darkness on pod development in nutrient solution Plants of groundnut (cv. TMV-2, a Spanish cultivar) were raised in p o l y s t y r e n e boxes

356 Zharare et al. containing 18 kg of potting mix in a glasshouse. A total of twelve gynophores, 4 to 5 cm long, were led into 100 mL polyethylene bottles filled w i t h n u t r i e n t s o l u t i o n o f the f o l l o w i n g composition (~tM): 2500 Ca; 2600 N; 250 K; 196 S; 100 Mg; 7 Na; 5 Si; 3 B; 2 Fe; 2 P; 0.5 Zn; 0.25 Mn; 0.15 Cu; 0.04 Co; 0.02 Ni and 0.02 Mo. E i g h t o f the b o t t l e s w e r e w r a p p e d in a l u m i n i u m foil to e x c l u d e light. Four of the b o t t l e s w e r e left u n w r a p p e d . S u r f a c e tissue samples of gynophore and pod were studied by scanning electron microscopy (SEM).

Experiment 2: Effect of omission of Mg, Mn and Zn from nutrient solution on pod development T w o p l a n t s o f cv. T M V - 2 w e r e r a i s e d as in E x p e r i m e n t 1. Four replicates of g y n o p h o r e s were cultured for 35 d in solutions lacking Mg, Mn or Zn, or in a c o m p l e t e s o l u t i o n w i t h a similar basal c o m p o s i t i o n as in E x p e r i m e n t 1 except that Ca and N were each reduced to 100 ~tM. Because of a change in the nutrient salts selected, the S concentration varied from 196 to 296 ~tM. All nutrient solutions were continuously fed by gravity to the respective bottles from 23 L r e s e r v o i r s t h r o u g h p o l y e t h y l e n e tubing. The r e s e r v o i r s w e r e p o s i t i o n e d 0.7 m a b o v e the bottles. The rate of nutrient flow into the bottles was regulated at 3.5 mL min -1 by use of 26 gauge h y p o d e r m i c n e e d l e s f i t t e d at the end of the t u b i n g and i n s e r t e d into the b o t t l e s . W a s t e nutrient solution left the bottles by a plastic tube inserted 2 cm below the neck of the bottle. At harvest, the pods were washed successively in 1000 ~tM SrC12 and distilled w a t e r to d e s o r b c a t i o n s h e l d on the e x c h a n g e s i t e s on p o d surfaces. Thereafter, the pods were separated into seeds and shells, wet ashed in a mixture of nitric and perchloric acid, and the concentrations o f Ca, M g , K, P, S, Fe, Mn, Zn, Cu and B analysed by inductively coupled plasma atomic emission spectroscopy.

Results and discussion Contrary to the conclusions of Ziv and Zamski (1975) and of Zamski and Ziv (1976), the results

o f the p r e s e n t e x p e r i m e n t s i n d i c a t e d t h a t a mechanical stimulus was not necessary for pod initiation and development in groundnut, at least for cv. TMV-2.

Experiment 1 W h i t e r o o t - h a i r - l i k e s t r u c t u r e s a p p e a r e d in patches on submerged portions of the gynophores 14 h after the gynophores were introduced into the darkened aerated nutrient solutions. These outgrowths progressed with time to cover most of the submerged portions of the gynophores. Pod development was visibly evident on gynophores submerged in the darkened nutrient solutions at 7-8 d as swellings of the gynophore tips. In three g y n o p h o r e s a l l o w e d to d e v e l o p f u r t h e r , the swellings p r o g r e s s e d into normal pods bearing one or two seeds at 20 d. The pods were also covered with the white outgrowths for most of their surface except the apices during the early g r o w t h stages. N o n e of the f o u r g y n o p h o r e s introduced in aerated nutrient solution without light exclusion had formed pods by 20 d. The culture of gynophores and pods in nutrient s o l u t i o n p r o v i d e d an o p p o r t u n i t y , w i t h o u t complications associated with podding in soil, to e x a m i n e s u r f a c e s t r u c t u r e o f p o d s and gynophores. The SEM study showed two anatomically distinct forms of epidermal o u t g r o w t h s : slender, n o n - s e p t a t e , u n b r a n c h e d r o o t - h a i r - l i k e structures (Fig. 1A) and septate branched outgrowths (Fig. 1B). The two types of outgrowths were spatially separated. The nonseptate outgrowths were found to be associated with the primary epidermis. These had swollen bases similar to root hairs o b s e r v e d by Brady ( 1 9 9 2 ) in r o o t a x i l s o f g r o u n d n u t cv. R e d Spanish. The septate o u t g r o w t h s were m o s t l y branched and variable in shape and size. Each septate hair a p p e a r e d to be made up of 3 or 4 segments attached to each other with either one or more short stalks, or one long stalk. The sizes of the s e g m e n t s v a r i e d , but the b a s e s e g m e n t a p p e a r e d to be m o s t l y short and round. The uppermost segments were mostly sausage-shaped and l o n g , b u t s o m e w e r e r o u n d and had a tendency to branch. They were densely clustered and were only visible after the e p i d e r m a l and associated unicellular structures were shed by the

Pod

development

of groundnut

351

Table 1. Effect of the omission of Mg, Zn or Mn from solution in the podding zone on pod dry weight, seed dry weight and shelling percentage Treatment

Mean pod weight (8)

Thousand seed weight(g)

Shelling percentage

Complete Minus Mg Minus Zn Minus Mn

0.19 0.23 0.15 0.24

124.0 153.0 71.4 176.0

66.0 66.6 52.6 71.8

LSD (5%)

0.02

20.6

8.3

grown and the two discarded. Experiment

Fig. 1. Scanning electron micrograph of the gynophore surface 4 d after introduction into darkened, aerated, nutrient solution showing: (A) the primary epidermis (e) with a nonseptate hair (n-sh) typical of root hairs found in root axils of groundnut; (B) a cluster of septate hairs (sh); and (C) epidermal cells (e) from which have developed non-septate hairs (n-sh) that were being shed, exposing the branched septate hairs (sh). (Horizontal white bars = 0.1 mm.)

expanding gynophore and pods (Fig. 1C). Waldron (1919) observed that the branched hairs did not appear on the fruit until it was well

outer

layers

had been

2

Omission of Mn from the podding solution resulted in significantly higher pod weight, seed weight and shelling percentage compared to the complete solution. The opposite effects were evident on the omission of Zn (Table 1). In the absence of Mg, pod and seed weights increased, but the shelling percentage was not different from that of pods growing in the complete nutrient solution. The detrimental effect of the presence of Mg in the podding solution on pod and seed yield is in agreement with results of field experiments obtained by Colwell and Brady (1945), Brady and Colwell (1945) and Chesney (1975). Omission of Mn from the podding zone did not affect seed or shell Mn concentration (Table 2). With respect to other nutrients, omission of Mn significantly increased both the seed and shell B concentration, but had no effect on seed or shell Zn, Fe, Cu, P or S concentration. Omission of Zn from the podding solution significantly reduced the Zn concentration in the shell compared to that in the complete nutrient solution. In relation to other nutrients, omission of Zn significantly increased seed B, Cu and S concentration, and shell Mg, Fe, Mn, Cu, S and B concentration. The concentration of Mg in the seed, but not in the shell, was significantly reduced by omission of Mg from the podding zone. The omission of Mg significantly

358 Zharare et al. Table 2. Effect of omission of Mg, Zn or Mn from the solution in the podding zone on seed and shell nutrient concentrations (dry weight basis) Treatment

Ca

Mg

K

P

S

Fe

Mn

% Seed Complete Minus Mg Minus Zn Minus Mn Shell Complete Minus Mg Minus Zn Minus Mn LSD (5%)

Zn

Cu

B

mg kg -~

0.11 0.18 0.15 0.14

0.43 0.20 0.40 0.28

0.71 0.73 0.93 0.64

0.51 0.54 0.76 0.50

0.23 0.24 0.34 0.24

21.9 23.7 54.1 26.7

14.0 23.4 23.4 14.4

68.0 89.5 57.2 66.4

4.3 7.9 6.8 4.4

8.6 37.2 37.6 76.7

0.24 0.66 0.52 0.25

0.20 0.08 0.38 0.19

1.00 1.09 1.96 0.99

0.13 0.13 0.26 0.11

0.19 0.19 0.36 0.19

217 266 377 185

82.3 70.3 182.5 40.6

84.1 153.2 44.0 87.4

6.2 10.3 22.5 7.4

2.7 77.3 29.0 33.2

0.11

0.13

0.35

0.12

0.08

117

51.7

20.8

0.6

11.3

increased the concentrations of Zn, B, and Cu in both seed and shell. For each of the three nutrients omitted from the podding zone, appreciable amounts reached the seed from the parent plants. Reductions in concentration resulting from omissions tended to be greater in the shell than in the seed suggesting that imports from the parent plant particularly f a v o u r the seed. With the e x c e p t i o n o f Zn, imports of Mg and Mn appeared to be sufficient for f r u i t and seed d e v e l o p m e n t . T h e r e was greater accumulation of B in the seed than in the shell, except in the Mg omission treatment. This was particularly evident when Mn was omitted. None of the omissions significantly affected the concentration of Ca in the seed, but omission of Mg or Zn increased the concentration of Ca in the shell.

Brady D J 1992. Effects of aluminium on early growth and

References

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