Moisture-deficit-induced changes in leaf-water content, leaf carbon exchange rate and biomass production in groundnut cultivars differing in specific leaf area

Field Crops Research 74 (2002) 67±79

Moisture-de®cit-induced changes in leaf-water content, leaf carbon exchange rate and biomass production in groundnut cultivars differing in speci®c leaf area P.C. Nautiyala,*, Nageswara Rao Rachaputib, Y.C. Joshia a

b

National Research Centre for Groundnut, Post Bag No. 5, Junagadh 362 001, India Department of Primary Industries, J.B. Petersen Research Station, P.O. Box 23, Kingaroy, Qld 4610, Australia Accepted 5 November 2001

Abstract Field experiments were conducted during two rainy seasons to study the effect of soil moisture de®cit on total biomass, pod yield, harvest index (HI) and drought tolerance index (DTI) in groundnut (Arachis hypogaea L.) cultivars possessing a wide range of speci®c leaf area (SLA, 144±241 cm2 g 1). There were three soil moisture regimes: adequate irrigation (W1), drought simulated under rain-out-shelter (W2) and rain-fed (W3). This experiment had two parts, in one, ®ve cultivars were exposed to W1, W2 and W3, and in a second, seven cultivars were exposed to W1 and W3. Using the same set of seven cultivars, pot-culture experiments were conducted to study relative water content (RWC), stomatal conductance (gs) and single leaf carbon exchange rate (CER) during increasing moisture-de®cit in two contrasting (rainy and summer) seasons. Variation in DTI was signi®cant, and low SLA types had greater DTI under both W2 and W3. The ranking of SLA among cultivars was consistent between experiments conducted during the two seasons. The rate of reduction in leaf RWC during the progressive moisture-de®cit was related directly to SLA (r ˆ 0:78, P < 0:01). The coef®cient of determination of the slopes calculated between RWC and soil moisture during the experimental period was more in the summer …r 2 ˆ 0:82† than the rainy …r 2 ˆ 0:54† season. Under increasing moisture-de®cit, the low SLA types were able to maintain higher RWC, CER and gs in both seasons. The relationships between RWC and CER (r ˆ 0:91, P < 0:01), and RWC and gs (r ˆ 0:65, P < 0:01) were signi®cant. It is suggested that under water-limited conditions there is a signi®cant inverse relationship between SLA and RWC. The low SLA types (water use ef®cient) were found to be drought tolerant in terms of total dry matter production in the ®eld studies, and maintenance of higher RWC under drought like situations in pot-culture experiments. Thus the ability of the low SLA types (higher water use ef®ciency, WUE) to maintain higher RWC may form the basis for the differences in drought tolerance vis a vis WUE in groundnut cultivars differing in SLA. Suggestions are made to select parents for drought tolerance or WUE, and to initiate breeding to combine traits like high HI, and WUE in terms of lower SLA. Ultimately, selection for both WUE (measured in terms of SLA) and yield traits (HI) should result in cultivars with improved performance in rain-fed agriculture. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Groundnut (Arachis hypogaea L.); Water use ef®ciency; Speci®c leaf area; Relative water content; Gas exchange parameters; Moisture-de®cit stress

1. Introduction * Corresponding author. E-mail address: [email protected] (P.C. Nautiyal).

Groundnut (Arachis hypogaea L.), a major oilseed crop, is cultivated predominantly under rain-fed

0378-4290/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 4 2 9 0 ( 0 1 ) 0 0 1 9 9 - X

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conditions in the semi-arid tropics. The crop is subjected to soil moisture-de®cits of varying degree and duration, which occasionally result in substantial loss of yield. Therefore, the development of drought tolerant cultivars of groundnut has assumed considerable importance. Conventional breeding based on yield as a sole criterion is proving to be slow in achieving gains. Gutschick (1988) has postulated that it would be more ef®cient and successful to attempt improving growth and yield per resource use rather than selecting for yield alone. Water use ef®ciency (WUE) may be one such trait that can contribute to productivity when water resources are limited (Wright et al., 1994). Cultivar variation for WUE and its association with carbon isotope discrimination (C13/C12 (D)) has been shown in groundnut grown in both the greenhouse (Hubick et al., 1986) and in the ®eld (Nageswara Rao et al., 1993). A strong relationship of speci®c leaf area (SLA, cm2 g 1) with D as well as with WUE was found and thus it was suggested that SLA could be used as an economical surrogate tool to select for WUE (Wright et al., 1994, 1996). Nageswara Rao and Wright (1993) reported low cultivar by environment interaction for the relationship between SLA and WUE. The in¯uence, however, of environmental factors such as temperature and soil moisture-de®cit on SLA has been reported (Vivekanandan and Gunesena, 1976). Association between SLA and carboxylation capacity has been studied, and it was suggested that the cultivars with low SLA have greater photosynthetic capacity per unit leaf area (Bowes et al., 1972). Nageswara Rao et al. (1995) has recommended leaf thickness (low SLA) as a selection criterion for enhancing WUE in groundnut. Information on the relationship of WUE with carbon exchange rate (CER), relative water content (RWC) and stomatal conductance (gs) especially under water-limited conditions is, however, lacking. This paper reports a study of biomass production, DTI, leaf RWC, CER in groundnut cultivars differing in SLA and grown under water-de®cit conditions. 2. Materials and methods 2.1. Experimental conditions This programme comprised ®eld and pot-culture experiments conducted at the National Research Centre

for Groundnut, Junagadh (latitude 218310 N, longitude 708360 E) Gujarat, India. In the ®eld experiments (1A and 1B) a range of cultivars varying in their SLA were grown under various soil moisture regimes in two consecutive rainy seasons in order to make observations on growth, biomass and pod yield. In the potculture experiments (2A and 2B), the same cultivars used in experiment 1 were exposed to a short period of soil moisture-de®cit. Observations were made on leaf RWC, and gas exchange parameters in the control and stressed plants. 2.2. Experiments 1A and 1B (®eld) The soil at the experimental site was a Verticustochrept (pH of 8.5) with low available organic matter, nitrogen (N) and phosphorus (P) (Table 1). The treatments to create different soil moisture regimes using seven groundnut cultivars (Table 2) where W1 is the irrigation at 5-day intervals throughout the crop growth period to fully replenish cumulative pan evaporation; W2 the drought simulated between 40 and 80 days after sowing (DAS) using manually operated rain-out-shelters and replenishing only 25% of the Table 1 Mineralogical, physical and chemical properties of the soils of the experimental site at Junagadh (Gujarat), India Soil characteristics

Soil depths (cm) 0±15

15±30

Mineralogical Sand (%) Silt (%) Clay (%)

22.40 14.00 63.60

20.03 15.77 64.20

Physical Field capacity (%) Permanent wilting point (%) Bulk density (g cm 3)

30.35 14.4 1.44

30.25 13.95 1.46

8.5 1.91 0.02 112 178 10906 656 313 4 0.16

8.5 1.64 0.02 74 142 10764 658 346 8 0.14

Chemical pH (1:2.5) Organic matter (%) N (%) P (ppm) K (ppm) Exchangeable Ca (ppm) Exchangeable Mg Exchangeable Na S (ppm) EC (1:2.5, d S m 1)

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Table 2 SLA, adjusted biomass, adjusted pod yield, HI, DTI-PY and DTI-BIO of groundnut cultivars under control (W1), rain-out shelter (W2), and rain-fed (W3) during the rainy seasons in experiment 1A Treatment

Variety (1995) ICGV 86031

Variety (1996)

TMV2 NLM

TAG 24

CSMG 84-1

ICG 476

ICGV 86031

TMV2 NLM

TAG 24

CSMG 84-1

ICG 476

SLA (cm2 g 1) W1a 150 144 135 136 W2b W3c 148 137 S.E. Variety: 3.33

163 151 157 Treatment:

190 185 178 2.58

241 185 201

160 151 150 138 155 144 Variety: 8.10

167 204 163 193 160 188 Treatment: NS

214 218 187

Adjusted pod weight (g m 2) W1 361.4 205.8 W2 251.8 164.6 W3 306.5 163.3 S.E. Variety: 8.34

435.9 281.7 334.8 Treatment:

295.4 181.9 235.5 6.46

303.5 224.1 252.8

268.2 253.9 206.2 206.7 223.0 219.2 Variety: 12.38

395.1 305.5 287.2 191.4 352.4 254.4 Treatment: 9.59

284.4 210.3 217.5

Adjusted biomass (g m 2) W1 844.1 651.2 712.4 563.6 W2 W3 778.1 561.2 S.E. Variety: 12.46

706.2 561.0 595.5 Treatment:

795.2 571.8 618.2 9.65

627.3 488.2 525.4

741.5 671.2 640.9 582.2 654.2 592.4 Variety: 23.80

626.0 787.1 508.1 586.7 559.8 634.4 Treatment: 18.43

577.9 442.0 478.6

HI W1 W2 W3 S.E.

0.42 0.27 0.39 0.23 0.41 0.29 Treatment: 0.007

0.25 0.26 0.22 0.25 0.23 0.26 Variety: 0.182

0.44 0.41 0.42 Treatment: NS

0.23 0.20 0.21 Variety: 0.009

0.27 0.25 0.27

0.34 0.32 0.31

0.27 0.23 0.28

0.37 0.35 0.33

DTI-PY W2 W3

0.70 0.85

0.80 0.79

0.65 0.77

0.82 0.80

0.70 0.83

0.76 0.83

0.81 0.86

0.72 0.89

0.62 0.83

0.73 0.76

DTI-BIO W2 W3

0.84 0.92

0.86 0.86

0.79 0.84

0.71 0.77

0.77 0.86

0.83 0.86

0.87 0.81

0.81 0.74

0.74 0.76

0.76 0.82

a

Control, irrigation given to replenish 100% pan evaporation at 5-day intervals. Drought simulated between 40 and 80 DAS, crop received irrigations equivalent to 25% of W1. c Rain-fed. b

cumulative pan evaporation during the period of stress and subsequently 100% as in W1; and W3 the crop solely dependent on rainfall. Experiment 1 was conducted in two parallel trials. In experiment 1A, ®ve cultivars belonging to the Spanish, i.e. A. hypogaea, ssp. fastigiata (ICGV 86031, TAG 24, ICG 476) and the Virginia, i.e. A.hypogaea, ssp. hypogaea (TMV2NLM, CSMG 84-1) botanical groups were exposed to W1, W2 and W3, whereas in experiment 1B, seven cultivars belonging to the Spanish (ICGV 86031, TAG 24, ICG 476) and the Virginia (TMV2 NLM, CSMG 84-1, and Kadiri 3), and the Valencia, i.e. A. hypogaea, ssp.,

vulgaris, (ICG 4747) botanical groups were exposed to W1 and W3. The experiments (1A and 1B) were laid out in a split plot design and conducted during two consecutive rainy seasons (July±October, 1995 and 1996). Each plot comprised three rows of 4 m length (3.6 m2). Row spacing was 30 cm and plant spacing within rows was 10 cm. Urea (25 kg N ha 1) and single super phosphate (17.5 kg P ha 1) was applied at the time of sowing. After sowing the crop in the ®rst week of July, two irrigations were given uniformly to the treatments to facilitate emergence and subsequent irrigations were scheduled accordingly to treatments.

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2.3. Experiments 2A and 2B (pot-culture) Pot-culture experiments were conducted in two contrasting seasons, i.e. rainy (July±October, 1997) (2A) and summer season (February±June, 1998) (2B), using seven cultivars as described in experiment 1B (Table 4). Earthen pots (10 kg capacity) were ®lled with a mixture of farm soil (black calcareous), sand and farmyard manure (2:1:1). Eight to ten seeds were sown in each pot. About 25 DAS, extra seedlings were removed to leave four uniform seedlings per pot. The Virginia types were sown 15 days in advance of Spanish and Valencia types to have all cultivars at similar developmental stages at the time of initiation of the water stress treatments. At 60 DAS, pots were arranged in a split-plot design, with soil moisturede®cit stress as the main treatment and cultivars as the

sub-treatment. One set of pots was irrigated daily (C) while the other was subjected to progressive soil moisture-de®cit (S) by withholding irrigation. Before initiation of the stress (at 60 DAS) ®ve plants of each cultivar were harvested to determine total biomass (BIO) and SLA. On each day of observation, two pots of each cultivar were randomly selected and leaf samples from ®ve to six plants taken for studying gas exchange parameters and leaf RWC. The leaf RWC and gas exchange observations were recorded on the stressed (S) and control (C) plants. In rainy season (experiment 2A), the observations were recorded for 7 days, i.e. 0, 1, 3, 5, 7, 9 and 11 days after initiation of stress. In the summer season (experiment 2B) there was a rapid decrease in the soil moisture content compared to that in rainy season (experiment 2A) due to high evaporative demand (Figs. 1 and 2).

Fig. 1. Total rainfall, RH, minimum and maximum temperatures during the crop growth period in the rainy seasons of the years 1995 (a), 1996 (b) (experiments 1A and 1B), 1997 (c) (experiment 2A) and summer season of 1998 (d) (experiment 2B) at Junagadh. There was no rain in 1998 summer season (d).

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harvest to the total biomass at the ®nal harvest. Due to shedding of leaves prior to physiological maturity in groundnut, there is considerable loss in vegetative weight and hence the vegetative weight recorded at mid pod-®ll (80±90 DAS) was used for determining total biomass production (Wright and Redden, 1995). 2.5. Carbon exchange and RWC measurements

Fig. 2. Soil moisture in pots during progressive moisture-de®citstress in experiments 2A and 2B during the rainy (1997) and summer (1998) seasons.

Hence, during the summer season experiment the observations were recorded for only ®ve sampling days, i.e. 0, 1, 3, 5 and 7 days after initiation of stress. Soil moisture between 5 and 15 cm depths was measured gravimetrically, in each pot for ®ve sampling days after recording the gas exchange parameters. 2.4. Growth measurements In the ®eld experiments (1A and 1B), seven to eight plants (from an area of 0.30 m2 of each plot) were uprooted 80 DAS for Spanish and Valencia types and 90 DAS for Virginia type. Plants were separated into leaf, stem and pod and the root was discarded. Leaf area was measured using an LI 3000 leaf area meter (LICOR, Lincon, USA). Plant samples were dried at 80 8C to constant weight and weighted. SLA, the ratio of fresh leaf area to leaf dry weight, was calculated. Drought tolerance index (DTI), the ratio of the total dry matter …leaves ‡ stems ‡ pods† under stress treatments (W2 and W3) to that under irrigated conditions (W1) was calculated. The ®nal harvest of Spanish and Valencia types was made at 110 DAS and for the Virginia type at 120 DAS. Pod weights recorded at ®nal harvest at a moisture content between 7 and 8% were adjusted for their energy content using a multiplication factor of 1.65 (Duncan et al., 1978). The harvest index (HI) was calculated as the ratio of total pod weight at the ®nal

In experiment 2 (pot-culture), three lea¯ets of the second or the third leaf from the apex of the main stem were used for various measurements (Nautiyal et al., 1999). All measurements were made between 0900 and 1000 h. Single leaf CER and stomatal conductance (gs) were measured using an LI 6200 portable photosynthesis system (LI-COR, Lincoln, USA). For measurement of leaf RWC, the ratio of the water content of leaf sample to the water content of fully turgid leaf sample (Barrs and Weatherley, 1962) was recorded and expressed as per cent. Coinciding with the time of recording the gas exchange parameters, soil sample from each pot (100 g each) was collected from a depth between 5 and 15 cm for the determination of gravimetric moisture content. About 10 days after withholding water, when the RWC values in most of the cultivars was less than 50%, the experiment was terminated. 2.6. Statistical analysis Statistical analysis of the data followed the procedures was described by Gomez and Gomaz (1984). 3. Results 3.1. Weather Rainfall (mm), relative humidity (RH, %), and minimum and maximum temperatures (8C) during two cropping seasons were recorded. In ®eld experiments during two rainy seasons, most of the total rainfall (845 mm in 1995 and 750 mm in 1996) was received in the beginning of the season (Fig. 1a and b). A scanty rainfall during the latter part of the season resulted in increased VPD during the reproductive phase of the crop and hence the drought patterns experienced by the crop during the two seasons were

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almost similar. In contrast, the weather conditions for the pot-culture experiments, i.e. experiments 2A and 2B, differed in the summer, air temperatures were high with low RH whereas in the rainy season air temperatures were relatively low with RH high (Fig. 1c and d). The average VPD was 1.62 kPa in the rainy season and 2.08 kPa in the summer season and accordingly the evaporative demand, seasonal air temperatures, rate of loss of soil moisture from the pots (during the treatment period in pot-culture experiments) also differed for experiments 2A and 2B. Within a season, there were little differences in the soil moisture contents of individual pots (cultivars) during the period of treatment and hence the data on soil moisture was averaged over the cultivars. The reduction in soil moisture during the experimental period was greater in the summer (experiment 2B) than in the rainy season (experiment 2A) (Fig. 2). There was no any rain in both the seasons during the treatment period (Fig. 1c and d). Recommended agronomic and plant

protection practices were followed to maintain the healthy crop. 3.2. Biomass production and DTI In ®eld experiments the effect of moisture and cultivars treatments were signi®cant for the parameters studied whereas the effect of season was non-signi®cant (Tables 2 and 3). Interactions between cultivars and treatment were signi®cant for pod yield in both seasons. The reduction in total dry matter was highest in CSMG 84-1 (23 and 20% in 1995 and 1996, respectively) and lowest in ICGV 86031 (6 and 10% in 1995 and 1996, respectively). Reduction in total pod weight was higher in ICG 4747 (30% in 1995 and 33% in 1996) and lower in ICGV 86031 (16%) in 1995 and in TAG 24 (11%) in 1996 in rain-fed crop (W3), compared with adequately irrigated control (W1). Mid-season drought, simulated using the rain-outshelter (W2), resulted in a reduction of total dry matter

Table 3 SLA, adjusted pod yield (adj-pod), adjusted biomass (adj-bio), HI, DTI-PY and DTI-BIO of groundnut cultivars under control (W1), and rainfed (W3) during two rainy seasons in experiment 1B Variety

1995 ICGV 86031 TMV2 NLM TAG 24 CSMG 84-1 ICG 4747 Kadiri 3 ICG 476 S.E. Variety (v) Treatment (t) 1996 ICGV 86031 TMV2 NLM TAG 24 CSMG 84-1 ICG 4747 Kadiri 3 ICG 476 S.E. Variety (v) Treatment (t)

SLA (cm2 g 1)

Adj-pod (g m 2)

Adj-bio (g m 2)

HI

W1

W3

W1

W3

W1

W3

W1

W3

150 144 164 190 184 195 241

184 137 157 178 167 176 201

361.4 206.8 435.9 295.4 316.6 369.7 303.5

306.5 164.6 334.8 235.5 220.8 267.7 252.8

844.1 651.2 706.2 795.2 1157.0 1054.0 627.3

778.1 561.2 595.5 618.2 932.2 885.3 525.4

0.23 0.27 0.42 0.27 0.17 0.21 0.34

0.21 0.27 0.41 0.28 0.14 0.22 0.31

5.32 2.84 160 151 167 204 170 172 214 9.05 4.83

13.46 7.19 155 144 160 188 187 158 201

268.3 253.9 395.2 305.5 286.4 318.5 284.4 19.17 10.24

23.99 12.82 223.0 219.2 352.2 254.4 192.1 281.1 217.5

911.1 671.2 626.0 787.1 741.5 882.4 577.9 32.29 17.26

0.009 0.005 787.6 592.4 559.8 634.4 654.2 706.6 478.6

0.25 0.26 0.44 0.29 0.22 0.24 0.37 0.021 0.011

DTI-PY

DTI-BIO

0.85 0.79 0.77 0.80 0.70 0.82 0.83

0.92 0.86 0.84 0.77 0.84 0.85 0.83

± ± 0.23 0.26 0.42 0.28 0.18 0.25 0.33

0.83 0.86 0.89 0.83 0.67 0.88 0.76 ± ±

0.88 0.88 0.89 0.80 0.82 0.80 0.82

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by about 12±13% in TMV2 NLM and ICGV 86031, and 24±26% in CSMG 84-1, during the two seasons. Reduction in pod yields was higher in CSMG 84-1 (34 and 37% in 1995 and 1996, respectively), and least in TMV2 NLM (20% in1995 and 18% in 1996). The HI of the cultivars was reduced signi®cantly under both W2 and W3. The HI under control conditions (W1) was highest in TAG 24 (0.42±0.44) and least in ICGV 86031 (0.23±0.27). TAG 24 also had highest HI under all treatments (0.39±0.42) (Tables 2 and 3). Drought toleranceindex basedonbiomass(DTI-BIO) calculated for total dry matter and drought tolerance index based on pod yield (DTI-PY) showed signi®cant cultivars variation in drought tolerance capacity (Tables 2 and 3). The DTI-BIO was related inversely (r ˆ 0:71, P < 0:01) to SLA (Fig. 3a), but such a relationship was not evident between DTI-PY and SLA. This ®eld study revealed that SLA had a signi®cant role in imparting drought tolerance in terms of TDM productivity. SLA recorded at 75 DAS was greater in W1 than that in W2 and W3 (Table 2). A positive correlation of SLA in W1 with that in W2 and W3 (r ˆ 0:90, P < 0:01) suggested a very low G  E interaction for this parameter. 3.3. Plant±water relations and gas exchange (pot-culture) 3.3.1. Rainy season In experiment 2A, the RWC in the leaves of wellwatered plants ranged from 93 to 95%. A decrease in the RWC was noticed even on the 1st day following cessation of watering, and was least in TMV2 NLM (1%) and highest in ICG 4747 (7.8%). With progressive soil moisture decline, RWC decreased gradually, and on 7th day was lowest in CSMG 84-1 (55%) and highest in TMV2 NLM (65%), followed by ICGV 86031 (64%). At the peak of the stress (11th day), RWC in TAG 24 (49%) and ICGV 86031 (41%) was higher than in other cultivars (range: 31±38%). The mean RWC (measured over the treatment period) in low SLA types was higher than in the high SLA types (Table 4). On the 5th day of the stress, average RWC was 75% at 1.8 MPa cl (water potential data not presented) when soil moisture in the pots was around 8.5% (Fig. 2). To study the relative performance of the cultivars with respect to the control plants, the ratio

Fig. 3. Relationship between drought tolerance index based on biomass (DTI-BIO) and SLA in groundnut cultivars grown in simulated drought conditions under rain-out-shelters (ROS) in experiment 1A (a), and between RWC (average RWC measured over the treatment period) and adjusted-SLA (adjusted for prevailing VPD in experiments 2A and 2B) (b).

of the observations recorded under stress to that of control was calculated (Figs. 4 and 5). Maximum difference in the cultivar ratio of RWC was found on the 9th day, and was lower in Kadiri 3 (0.49) and higher in TMV2 NLM (0.66), followed by ICGV 86031 (0.64) and TAG 24 (0.59). Maintenance of higher ratios by cvs. TMV2 NLM, ICGV 86031 and TAG 24 (all low SLA types) under soil moisture-de®cit indicated the ability to maintain relatively higher RWC during leaf desiccation under soil drying conditions (Fig. 4a). Average conductance (recorded during the stress period) was higher in TMV2 NLM and TAG 24 than in the other cultivars (Table 4). The cultivar gs ratios were distinct on 3rd day and ranged

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Table 4 CERa (mmol m 2 s 1), stomatal conductancea (gs, mol m 2 s 1) and leaf RWCa (%) of groundnut cultivars with a range of SLA (cm2 g 1)b under control (C) and progressive soil moisture-de®cit stress (S) simulated in earthen pots in experiments 2A and 2B Parameters

ICGV 86031

Rainy season (experiment 2A) SLA 140 CER Cc Sd gs

C S

RWC C S

12.8 7.0 0.36 0.17 93.8 73.1

Summer season (experiment 2B) SLA 116 CER C S gs

C S

RWC C S

12.7 6.0 0.33 0.16 94.7 68.3

TMV2 NLM

TAG 24

CSMG 84-1

ICG 4747

Kadiri 3

ICG 476

S.E.

144

163

185

193

205

210

1.15

13.9 7.9 0.36 0.24 91.23 71.4 120 12.2 6.1 0.31 0.13 93.3 64.9

11.6 6.5 0.38 0.19 91.6 73.3 142 14.9 6.4 0.33 0.16 93.5 63.3

10.8 6.1 0.33 0.15 93.4 70.0 150 14.8 5.0 0.35 0.11 94.8 62.2

8.5 5.2 0.21 0.12 92.6 67.2 157 10.8 5.8 0.28 0.10 93.2 56.0

13.4 6.3 0.31 0.15 91.85 65.1 172 12.42 5.5 0.29 0.11 93.0 56.4

11.1 5.9 0.39 0.19 92.1 68.4 170 11.6 5.4 0.27 0.13 92.7 60.3

0.28 0.12 0.003 0.04 0.78 0.27 0.86 0.41 0.15 0.001 0.01 1.11 1.52

a

Values are mean of the observations recorded for 5 days in the summer and 7 days in the rainy seasons. Values are mean of 5 plants at 60 DAS, i.e. before starting the stress treatment. c Control, plant received irrigation daily. d Soil moisture-de®cit stress during observation period. b

between 0.76 in ICGV 86031 and 0.35 in ICG 4747 (Fig. 4c). Thus, low SLA types under moisture-de®cit conditions were able to maintain higher gs than the high SLA types. CERs were between 9.49 mmol m 2 s 1 in ICGS 4747 and 13.93 mmol m 2 s 1 in TMV2 NLM in control plants. On the ®rst day of stress the relative decrease in CER was highest in ICG 4747 (23%). On the 3rd day, reduction in CER in general was around 25%, but for ICGV 86031, it was 15%. At peak stress (11th day), most of the cultivars showed about 80% reduction in CER, when average RWC was around 40%. Average CER (measured over the stress period) was higher in the low SLA types, i.e. ICGV 86031 (7.05 mmol m 2 s 1) and TMV2 NLM (7.88 mmol m 2 s 1) than high SLA types (Fig. 4c), when the soil moisture was around 10% (Fig. 2).

3.3.2. Summer season In experiment 2B, a decrease in RWC was noticed immediately after withholding water, and on the 1st day (after withholding water) cultivars varied in their RWC. Low SLA types maintained higher RWC than in the high SLA types. The RWC on the 7th day was very low and ranged between 34% in CSMG 84-1 and 49% in TMV2 NLM followed by ICGV 86031 (48%) (Table 4). The stress/control ratio indicated the relative capability of cultivars to maintain hydration under soil drying conditions. The cvs. CSMG 84-1 and Kadiri 3 showed lower ratios under stress, indicating poor hydration tolerance, whereas TAG 24 (in the early phase of stress), and ICGV 86031 and TMV 2NLM (in the later phase of stress) showed higher ratios, thus indicating better hydration tolerance. Average gs (measured over the stress period) was

P.C. Nautiyal et al. / Field Crops Research 74 (2002) 67±79

Fig. 4. Cultivar ratio (stress/control) of the physiological parameters like RWC (a, S.E., variety: 0.003; treatment: 0.003; and variety  treatment ˆ 0:009), stomatal conductance (gs) (b, S.E., variety: 0.008; treatment: 0.015; variety  treatment ˆ 0:041), CER (c, S.E., variety: 0.011; treatment: 0.011; variety  treatment ˆ 0:030) under progressive soil moisture-de®cit stress in pot culture experiment (experiment 2A) during rainy season of 1997.

higher in TAG 24 followed by TMV2 NLM (Table 4). In control plants, gs ranged between 0.34 and 0.47 mol m 2 s 1 in TMV2 NLM and TAG 24, respectively, and decreased gradually thereafter. On

75

Fig. 5. Cultivar ratio (stress/control) of the physiological parameters like RWC (a, S.E., variety: 0.005; treatment: 0.004; variety  treatment ˆ 0:12), stomatal conductance (gs) (b, S.E., variety: 0.019; treatment: 0.060; variety  treatment ˆ 0:042), and CER (c, S.E., variety 0.012; treatment: 0.010; variety  treatment ˆ 0:028) under progressive soil moisture-de®cit stress in pot culture experiment (experiment 2B) during summer season of 1998.

the 7th day, gs ranged between 0.002 and 0.062 mol m 2 s 1. Ratios of gs among the cultivars were distinct even in control plants, and under stress cv. CSMG 84-1 maintained lower ratios (Fig. 5c).

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The CER in fully turgid leaves ranged between 15.20 and 11.63 mmol m 2 s 1 being highest in ICGV 86031 and lowest in ICG 4747. CER in general decreased gradually with the progressive moisturede®cit. The cultivars differed signi®cantly for the CER ratios from the beginning, the differences widened during the build up of the stress and narrowed at the peak stress. During ®rst three days, CER ratios were higher in TMV2 NLM and lower in CSMG 84-1 (Fig. 5c). The average CER during the stress period was highest in TAG 24 followed by ICGV 86031 (Table 4). 3.4. Relationship between RWC and SLA In the pot-culture experiments, the RWC and SLA showed signi®cant cultivar and seasonal variations in their response to prevailing climatic conditions and water-de®cit. For example, SLA of cultivars was

Fig. 6. Relationship between the slope of RWC, calculated with soil moisture during the progressive stress, and SLA in: (a) experiment 2A and (b) experiment 2B.

higher in the rainy season (experiment 2A) than the summer (experiment 2B), though the cultivar ranking was maintained (Table 4) (r ˆ 0:90, P < 0:01). In experiment 4, higher VPD and temperature not only in¯uenced the rate of development of stress but also the SLA values. SLA, in general, was higher in the rainy than in the summer season (Table 4). Most of the variation, however, in the relationship between SLA and RWC between experiments was nulli®ed when the SLA values were multiplied with the mean VPD values for the respective seasons (adj-SLA) (Fig. 3b). Cultivar difference in maintaining RWC under stress was quanti®ed for individual cultivar by regressing the RWC against the soil moisture content measured on a daily basis. This analysis revealed signi®cant cultivar variation in RWC slope (Fig. 6a and b). 4. Discussion Previous studies indicated that WUE is a desirable trait to utilise in crop improvement programmes for water-limited environments and that groundnut cultivars with high WUE can be selected using carbon isotope discrimination (Hubick et al., 1986) or SLA (Nageswara Rao and Wright, 1993) as selection tools. In this study, seven cultivars used in the ®eld experiment produced different biomass, pod yield, and HI and DTI under the same soil moisture-de®cit, suggesting intrinsic cultivar variation in water uptake, WUE and ability to partition dry matter to pods. A signi®cant relationship between SLA (thus WUE) and DTIBIO (r ˆ 0:71, P < 0:01) highlights the importance of WUE in crop productivity under water-limited conditions. The lack of relationship between DTI-PY and SLA suggested that processes other than WUE control the production of pod yield, but production of higher biomass per unit of water transpired is a critical requirement for achieving sustainable production of groundnut in rain-dependent cropping systems. Strong relationships of SLA between control and stress treatments in experiments 1A and 1B (r ˆ 0:90, P < 0:01), and in between experiments 2A and 2B (r ˆ 0:90, P < 0:01) indicated a low G  E interaction for this trait, as found by other workers (Nageswara Rao and Wright, 1993). In pot-culture experiments, SLA values of the seven cultivars, growing under moisture suf®ciency, were

P.C. Nautiyal et al. / Field Crops Research 74 (2002) 67±79 Table 5 Correlation between values of SLA and each of RWC, CER and gs in groundnut cultivars grown under irrigated and moisture de®cit stress conditions Pair of parameters

Coefficient correlation, r Stress

Irrigated

SLA and RWC Experiment 2A Experiment 2B

0.844* 0.872*

0.084 NS 0.559 NS

SLA and CER Experiment 2A Experiment 2B

0.780* 0.574 NS

0.454 NS 0.207 NS

SLA and gs Experiment 2A Experiment 2B

0.525 NS 0.603 NS

0.316 NS 0.586 NS

*

Signi®cant at 5% level of signi®cance.

correlated with the mean values of RWC, CER and gs recorded during the subsequent 7 days in summer or 9 days in rainy seasons. The relationship, for those plants which continued to grow under adequate moisture, differed from those, which were subsequently subjected to progressive moisture de®cit. It is evident from the values of the correlation coef®cients (Table 5) that under adequate moisture, the SLA of cultivars does not have any de®nite relationship with their RWC, CER or gs. However, the relationship between SLA and RWC acquires signi®cance if the plants are subjected to water stress. Thus a low SLA type may not accrue any bene®t under irrigated conditions but is likely to perform better than high SLA cultivars under water de®cit. The inverse (r ˆ 0:58, P < 0:05) relationship between SLA and mean CER across both experiments suggests that the low SLA cultivars in general maintained higher CERs, and an inverse relationship between the mean RWC, over the treatment period, and SLA in both the experiments (2A and 2B) (r ˆ 0:75 and r ˆ 0:59, P < 0:05) suggests that the cultivars with low SLA were able to maintain relatively higher RWC under stress. Although the regression between RWC and SLA was signi®cant, the coef®cients varied between experiments. Signi®cant cultivar variations in photosynthesis per unit leaf area and gs indicates that the low SLA cultivars such as ICGV 86031 and TMV2 NLM maintained higher CER at comparable rates of gs under stress in both experiments. Large cultivar and seasonal variations in

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CER have been shown in groundnut (Bhagsari and Brown, 1976; Pallas, 1982; Ravindra et al., 1995; Nautiyal et al., 1999). Among the low and high SLA types, the variation in leaf-water retention capacity and CER may be due to the difference in the thickness of palisade and spongy parenchyma layers, and the capacity of water storage cells (Ketring et al., 1982). Under water-de®cit, reduced lea¯et area with corresponding increase in stomatal frequency and number of stomata on the abaxial surface showed relationships with high WUE in cvs. TAG 24 and Somnath (Badigannavar et al., 1999). Earlier studies have also shown a correlation of low SLA with higher carboxylation capacity (Bowes et al., 1972; Nageswara Rao et al., 1995). The higher photosynthetic capacity is achieved by comparable or slightly higher stomatal conductance (Wong et al., 1979) in lower SLA types. This could have allowed the maintenance of lower leaf temperature at high irradiance, resulting in a lower vapour pressure de®cit and lower transpiration ratio. The CER has been shown to depend on thickness of leaves and the size of the mesophyll cells (Wilson and Cooper, 1967). It is also possible that low SLA might result in higher Ci (internal CO2 concentration), which leads to higher WUE or drought tolerance. A lower Ci arises from a greater fractional contribution of stomatal resistance to total resistance in photosynthesis, i.e., greater decrease in transpiration rate than photosynthesis (Nobel, 1991). The overall relationship for both experiments (2A and 2B) between adjusted SLA and RWC (Fig. 3b) recon®rmed the consistency in the above relationship when SLA was corrected for the prevailing VPD effects. Under water-de®cit the ratio of stress and control observations (S/C) has been suggested as the best indicator of the capacity of a cultivar to maintain hydration under drying soil conditions (Ketring, 1985). The rate of reduction in RWC per unit reduction in soil moisture percent (RWC slope) indicates the ability of a cultivar to maintain leaf-water status to support metabolic activities as well as to maintain favourable leaf temperatures. Drought tolerance in groundnut cultivars is characterised by the maintenance of relatively higher RWC under water-de®cit conditions (Joshi et al., 1988; Ravindra et al., 1989; Nautiyal et al., 1995). The regression between SLA and RWC slopes provides conclusive evidence for the role of SLA in maintaining leaf-water status during

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stress as well as signi®cant cultivar variation for this trait. The physiological basis of the relationship between drought tolerance and SLA is still not very clear and needs further studies, especially in relation to the superiority of the low SLA type in maintaining higher RWC under soil water-de®cit. Traits like low SLA (high DTI) and high HI may be combined by making crosses between cultivars with high HI such as TAG 24 and high DTI such as ICGV 86031, and the selection for both the traits should result in cultivars with improved performance under rain-fed cultivation. A negative association between low SLA or WUE and HI has been reported in groundnut (Wright et al., 1996). Thus further research is required to investigate the extent and nature of this association to accelerate the rate of progress in yield enhancement in breeding programmes. Acknowledgements Research work reported here includes some data generated (experiment 1) under ACAIR-ICAR-ICRISAT collaborative research project on ``Selection for Water Use Ef®ciency in Grain Legume'' (PN 9216). First author is highly thankful to ICRISAT, Asia Centre, Patancheru, India, for providing the fellowship as a visiting scholar for data analysis. Authors would like to thank to Dr. A. Bandyopadhyay, Director, NRCG, for encouragements and providing the basic facilities for experimentation. We acknowledge the scienti®c input provided by Dr. J.B. Misra, Principal Scientist, Biochemistry, NRCG, while reviewing the manuscript. Technical assistance in the ®eld and potculture experiments provided by Mr. P.V. Zala and Mr. V.G. Koradia, technical of®cer, is duly acknowledged. References Badigannavar, A.M., Kale, D.M., Bhagwat, S.G., Murthy, G.S.S., 1999. Genotypic and seasonal variation in stomatal characteristics in Trombay groundnut varieties. Trop. Agric. Res. Ext. 2, 10±12. Barrs, H.D., Weatherley, P.E., 1962. A re-examination of the relative turgidity technique for estimating water de®cit in leaves. Aust. J. Biol. Sci. 15, 413±428. Bhagsari, A.S., Brown, R.H., 1976. Photosynthesis of peanut (Arachis) genotypes. Peanut Sci. 3, 1±9.

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