Effect of high air and soil temperature on dry matter production, pod yield and yield components of groundnut

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1 Plant and Soil 222: , Kluwer Academic Publishers. Printed in the Netherlands. 231 Effect of high air and soil temperature on dry matter production, pod yield and yield components of groundnut P.V. Vara Prasad, P.Q. Craufurd and R.J. Summerfield Plant Environment Laboratory, Department of Agriculture, The University of Reading, Cutbush Lane, Shinfield, Reading RG2 9AD, UK Received 22 March Accepted in revised form 17 March 2000 Key words: air temperature, fruit-set, groundnut, heat stress, pod yield, soil temperature Abstract Groundnuts (Arachis hypogaea L.) grown in the semi-arid tropics are commonly exposed to air and soil temperatures above 35 C during the reproductive period causing significant yield losses. The objectives of this study were to determine: (i) whether effects of high air and/or high soil temperature in two contrasting cultivars were similar; (ii) the effects of the timing of imposition of high air and soil temperature; (iii) the effects of high air, high soil and both stresses combined on yield and yield components; and (iv) whether the effects of high air and high soil temperature were additive or multiplicative. Plants were grown at optimum and ambient soil temperature from planting until start of podding at 45 d after planting (DAP) in Experiment 1, and until start of flowering at 28 DAP in Experiment 2. Thereafter, plants of each cultivar were exposed to a factorial combination of two air temperatures (optimum: 28 /22 C and high: 38 /22 C) and two soil temperatures (ambient: 26 /24 C and high: 38 /30 C) until final harvest at 90 DAP. The effects of high air and high soil temperatures imposed from start of flowering or podding were similar. Exposure to high air and/or high soil temperature significantly reduced total dry matter production, partitioning of dry matter to pods, and pod yields in both the cultivars. High air temperature had no significant effect on total flower production but significantly reduced the proportion of flowers setting pegs (fruit-set) and hence fruit numbers. In contrast, high soil temperature significantly reduced flower production, the proportion of pegs forming pods and 100 seed weight. The effects of high air and soil temperature were mostly additive and without interaction. Introduction Groundnut is an important source of oil and proteinrich food and feed for people and livestock in the developing world. It is an integral part of the cropping systems of the semi-arid tropics, which are characterized by high temperature and low and erratic rainfall. High temperature stress is one of the least well understood of all the abiotic adversities that affects crops (Paulsen, 1994) and is one of the major uncontrollable factors affecting plant growth, development and productivity (Marshall, 1982). Groundnut crops grown in the semi-arid tropics are often exposed to air and soil temperatures warmer than 35 C during the reproduct- FAX No: p.q.craufurd@reading.ac.uk ive phase, circumstances which significantly reduce seed yields (ICRISAT, 1994). Groundnut plants are susceptible to both high air and high soil temperature due to their aerial flowering and subterranean fruiting habit, respectively. That said, most research in groundnut has been done either on high air (Ketring, 1984; Ong, 1984) or on high soil temperature (Golombek and Johansen, 1997; Ono et al., 1974); studies on the combined effects of both factors have received far less attention. The optimum mean air temperature range for vegetative growth in groundnut is between 25 and 28 C (Cox, 1979; Wood, 1968), which is slightly warmer than the optimum range reported for reproductive growth, i.e. between 22 and 24 C (Cox, 1979; Wood, 1968). Day temperatures above 35 C

2 232 during the reproductive phase reduce fruit-set, and consequently the number of pods and ultimately seed yields (Ketring, 1984; Vara Prasad et al., 1999a). The optimum mean soil temperature range for seed germination is between 29 and 30 C (Mohamed et al., 1988) and for root growth it is close to 30 C(Suzuki, 1966). Similarly, the optimum soil temperature range for pod formation and development is between 31 and 33 C and soil temperatures above 33 Csignificantly reduce the number of mature pods and seed yields (Dreyer et al., 1981; Ono, 1979; Ono et al., 1974). Golombek and Johansen (1997) found that the greatest number of pods were produced at mean soil temperatures between 23 and 29 C; temperatures of 17 and 35 C were sub- and supra-optimal, respectively. The optimum soil temperature for individual pod growth rate is between 31 and 33 C (Dreyer et al., 1981; Ono et al., 1974). However total and mature pod number decrease as temperatures increase from 22 to 28 C (Dreyer et al., 1981) due to lower pod initiation rates at higher temperatures (Golombek and Johansen, 1997). In the research reported here, the effects of both high air and high soil temperature on dry matter production, partitioning and yield, and flower production and fruit-set, were investigated in two experiments. The objectives were to determine: 1. whether effects of high air and/or high soil temperature in two contrasting cultivars were similar; 2. the effects of the timing of imposition of high air and soil temperature; 3. the effects of high air, high soil and both stresses combined on yield and yield components; and 4. whether the effects of high air and high soil temperature were additive or multiplicative. Materials and methods Two experiments were conducted between May and September 1997 in the controlled environment facilities of the Plant Environment Laboratory, Department of Agriculture, The University of Reading (51 27 lat. and long.). The experiments were carried-out in two adjacent polyethylene-covered tunnels (poly-tunnels) aligned east-west, each 25 m long by 8 m wide by 3 m high at the apex, maintained either at close-to-optimum day/night temperatures of 28 /22 C or at hot days combined with an optimum night temperature of 38 /22 C. The diurnal photo- and thermo-period in both poly-tunnels were coincident and equal at 12 h d 1 ( h). The photoperiod was controlled by a manually operated blackout facility (Vara Prasad, 1999). Air temperatures were measured in each polytunnel with screened and aspirated copper constantan thermocouples positioned 30 cm above the plant canopy. Readings were taken at 10 s intervals and means of successive 10 min periods were stored using a data logger (Delta-T Devices Ltd, Cambridge, UK). Carbon dioxide was at ambient concentration, 350 µmol mol 1, and relative humidity during the day was controlled by automatic water sprinklers and ventilation to give a VPD close to 2 kpa in both poly-tunnels. The poly-tunnels transmitted about 75% of incoming photosynthetically active radiation such that the photosynthetic photon flux density at canopy level averaged 597 and 594 µmol m 2 s 1 during Experiment 1 and 2, respectively. Within each poly-tunnel, the high soil temperature treatments were imposed by placing pots on customised bench 2.75 m long by 1.5 m wide and 0.5 m high. The high soil temperature bench was fitted with five tubular heaters, 2.4 m long, with a total wattage of 2.4 kw. The ambient soil temperature treatment was maintained on a similar bench without tubular heaters. The target high day time soil temperature was set at 10 C above ambient soil temperature and was controlled by automatically switching the heater on and off using a data logger (CR 10, Campbell Scientific Ltd, Shepshed, UK). Soil temperature during the night was not controlled and pots were allowed to return to ambient temperature, typically achieved within 4 h. Soil temperature during in both ambient and high soil temperature regimes were recorded by thermocouples placed at a depth of 5 cm in the soil. Readings were measured at 10 s intervals and mean of successive 10 min periods were stored using a data logger. Plant husbandry Uniform seeds of each cultivar were selected and treated with Apron Combi 453 FS (Ciba Agriculture, Cambridge, UK) as a precautionary measure against seed-borne disease. Seeds were pre-germinated at 25 C on moist filter paper in Petri dishes kept in the dark for 2 d until radicles emerged. The germinated seeds were then sown one per 15 L pot at a depth of 2.5 cm. The sides of the pots were covered with aluminum foil to reduce radiative soil heating. The rooting medium comprised sand, gravel, medium grade vermiculite and Levington M2 compost mixed in proportions of 4:2:2:1, by volume, respectively. Seeds were not in-

3 233 oculated with rhizobia and so plants were dependent on inorganic nitrogen. This was done to remove any confounding effects of air or soil temperature on rhizobia. All pots were soaked with tap water and allowed to drain for 24 h before sowing; thereafter, all pots were irrigated as necessary through an automatic drip irrigation system. All plants were healthy and there were no serious pests or disease problems. Releases of predators (Phytoseiulus persimilis Athias-Henriot) and foliar sprays of Torque (a.i. Fenbutatin Oxide) controlled a mild incidence of red spider mite (Tetranychus urticae Koch). Thrips (Thrips tabaci Lindeman) were controlled by release of predator Amblyseius cucumeris Oudemans. Cultivar and temperature treatments Experiment 1 Two cultivars, one each of Spanish botanical type (ICGV 86015) and Virginia botanical type (ICGV 87282) were grown in the poly-tunnel at optimum day/night temperature of 28 /22 C from sowing until the appearance of the first pod at 45 DAP. Thereafter, one-half of the plants of each cultivar were transferred to the adjacent poly-tunnel maintained at a hot day/optimum night temperature of 38 /22 C, where they remained until final harvest at 90 DAP. Within each air temperature regime, two soil temperature environments, ambient and 10 C above the ambient (high) were imposed from first pod initiation at 15 d after first flowering until final harvest. Plants were given a commercial controlled-release fertiliser (0.15 kg kg 1 N, 0.10 kg kg 1 P, 0.12 kg kg 1,0.02kgkg 1 MgO plus trace elements; Osmocote Plus, Scotts UK Ltd, UK), incorporated into the mixture at the manufacturer s recommended rate of 5 gl 1. Experiment 2 This experiment was nearly identical to Experiment 1, except that only one cultivar, ICGV , was grown and high air and soil temperature treatments started at flowering, i.e. 28 DAP. Also, plants in Experiment 2 were supplied with a complete nutrient solution containing 100 ppm inorganic N (Summerfield et al., 1977) rather than a controlled-release fertilizer. Observations and data analysis Durations (d) from sowing to appearance of the first fully opened flower (R1; Boote, 1982) and then the first peg (R2) were recorded on all plants. Thereafter, the number of flowers opening each day was counted until final harvest. At the final harvest, all the plants were removed from each pot without damaging the root systems and separated into roots, leaves, stems (including petioles), pegs and pods. The roots were washed with water to remove the potting medium. The respective weights of roots, leaves, stems, pegs and pods per plant were recorded after oven-drying at 80 C to a constant weight. Total dry matter (inclusive of senesced leaves and roots), pod harvest index (ratio of pod total weight), shelling percentage (ratio seed pod dry weight) and root shoot ratio (ratio of root to leaf and stem dry weight) ratio were calculated from the weights of individual components. Values of pod dry weight were adjusted by a factor of 1.65 to allow for the oil content of the seeds (Duncan et al., 1978). At final harvest, the total number of pegs and pods (reproductive numbers) per plant were counted. The proportion of flowers setting pegs (fruit-set) was calculated as the ratio of total cumulative flower number to reproductive number. Similarly, the proportion of pegs forming pods was calculated as a ratio of reproductive number to number of pods. The data on fruit-set and the proportion of pegs forming pods were subject to angular transformation before analysis to ensure homogeneity of variances. Experiment 1 was analysed as a split-split plot design with air temperatures as unreplicated main plots, soil temperature as unreplicated sub-plots and cultivars as sub-sub plots, with five replicated pots. Experiment 2 was analysed as a split-plot design with air temperatures as unreplicated main plots and soil temperature as sub-plots with five replicated pots. Analysis of variance for all the variables was performed using Genstat 5 (Genstat 5 Committee, 1987). Results Target temperatures in both experiments were with in close tolerances (SD<1.3 C). The mean (day/night) air temperature in the optimum and high air temperature poly-tunnels were 25 C (27.9 /22.1 C) and 30.3 C (38.3 /22.3 C), respectively, in Experiment 1 and 25.1 C (28.2 /22.0 C) and 30.4 C (38.4 /22.4 C), respectively, in Experiment 2. Mean (day/night) ambient and high soil temperatures were 25.4 C (26.4 /24.4 C) and 33.8 C (37.7 /29.9 C), respectively, in Experiment 1 and 25.3 C (26.2 /24.4 C) and 33.7 C (37.5 /29.9 C), respectively, in Experiment 2.

4 234 Table 1. Total flower number, proportion of flowers setting pegs (angular transformed), number of pegs and pods (reproductive number), total dry matter, pod harvest index, root shoot ratio, and pod yield, and the standard error of the difference (SED) of cv. ICGV (Spanish) and cv. ICGV (Virginia) exposed to high temperature from start of first flowering until reproductive maturity Trait Cultivar SED (1,39 df) ICGV ICGV Total flower number (plant 1 ) Fruit-set (%) Reproductive number (plant 1 ) Total dry matter (g plant 1 ) Pod harvest index (%) Root to shoot ratio Pod yield (g plant 1 ) ,,, Significant at P<0.05, P<0.01 and P<0.001, respectively. Cultivar responses In Experiment 1, there were significant differences (P< ) between Spanish cv. ICGV and Virginia cv. ICGV for most traits measured (Table 1). Reproductive number (RN), i.e. the number of pegs and pods, was greater (P<0.05) in the Spanish than in the Virginia cultivar due to the greater flower production of the Spanish cultivar. Although the Virginia cultivar produced slightly (P<0.05) more dry matter than the Spanish cultivar, the Spanish cultivar partitioned twice as much dry matter to pods. Similarly, root shoot ratio was also significantly (P<0.05) greater in the Spanish cultivar. There were no interactions between air temperature, soil temperature and cultivar for any traits shown in Table 1. There were, however, significant interactions (P<0.05) between cultivar and air temperature and between cultivar and soil temperature for the proportion of pegs forming pods, shelling percentage and pod yield. The Virginia cultivar was much more sensitive to both high air and soil high soil temperature than the Spanish cultivar; in the Virginia cultivar pod yield was reduced by 49 59%, compared with only 21 24% in the Spanish cultivar (Table 2). Effect of timing of high temperature High air and soil temperature had similar effects, both in terms of direction and magnitude, on most measured traits irrespective of whether temperatures were imposed from flowering or from podding (Figures 1 and 2). The only exception, in terms of the direction of the response, was for reproductive number; high soil temperature at flowering reduced reproductive number more than air temperature, whereas at podding, high Table 2. Effects of cultivar and air temperature, and cultivar and soil temperature interaction on pod yield (g plant 1 ) of ICGV and ICGV grown at high temperatures from start of podding until reproductive maturity Cultivar Mean air temperature Mean soil temperature 25 C 30 C 25 C 34 C ICGV ICGV Significance P<0.05 P<0.05 air temperature reduced reproductive number more than soil temperature. The magnitude of the effects at flowering and podding were also broadly similar (Figures 1 and 2). For example, the combined effects of air and soil temperature reduced fruit-set and pod weight by 58 and 57% and 49 and 52% at flowering and podding, respectively. However, the proportion of pegs forming pods (peg pod ratio) was affected by the timing of high temperature; the proportion of pegs forming pods was increased by 8% by high temperature at flowering but reduced by 22% at podding. Effect of air temperature High air temperature from flowering and from podding increased flower numbers, but reduced fruit-set, resulting in reduction in reproductive number, pod number and pod yield (Figures 1 and 2). The proportion of pegs forming pods was not affected by air temperature. The effects of air temperature were greater at podding than at flowering, and pod numbers were reduced by 32 and 22%, respectively.

5 Figure 1. Richard s diagrams of the effects of high air temperature ( ) and high soil temperature ( ) imposed from start of flowering until maturity and from start of podding until maturity on total dry matter, root to shoot ratio, pod yield, shelling percentage and 100 seed weight. Vertical bars show SE for comparing treatment means. Key:, 25 /25 C;,30 /25 C;,25 /34 C;,30 /34 C air/soil temperature. 235

6 236 Figure 2. Richard s diagrams of the effects of high air temperature ( ) and high soil temperature ( ) imposed from start of flowering until maturity and from start of podding until maturity on total flower number, proportion of flowers setting fruits (angular transformed, fruit-set), total number of pegs and pods (reproductive number), proportion of pegs forming pods (angular transformed, peg to pod ratio) and pod number. Vertical bars show SE for comparing treatment means. Key:, 25 /25 C;,30 /25 C;,25 /34 C;,30 /34 C air/soil temperature.

7 237 High air temperature also reduced total dry matter when imposed at flowering, but not at podding. However, pod weights were reduced by high air temperature at both flowering (18%) and podding (26%). Partitioning of dry matter to roots was increased slightly when high air temperature was imposed from flowering, and significantly (P<0.05) reduced when temperatures were imposed from podding. Similarly, partitioning of dry matter to pods was also significantly (P<0.05) reduced by high air temperature. Seed number and 100 seed weight were not affected by high temperature from flowering, but were reduced by high air temperature from podding. Effect of soil temperature High soil temperature, in contrast to high air temperature, reduced flower number, particularly when high soil temperature was imposed at flowering (Figures 1 and 2). High soil temperature also reduced fruit-set at flowering (22%), but only slightly at podding (11%). However, these reductions were much smaller than those caused by high air temperature (41 and 51%, respectively). The proportion of pegs forming pods was not affected by soil temperature at flowering, but was reduced by high soil temperature at podding. The reduction of flower number and fruit-set when high soil temperature was imposed at flowering resulted in a reduction of 52% in pod number (compared with only 22% for air temperature). At podding, effects of soil temperature were less marked and pod number was reduced by 33% (comparable to the effect of air temperature). High soil temperature reduced total dry matter and pod yield by about 30% at both flowering and podding, and the proportion of dry matter partitioned to pods was, therefore, not affected by soil temperature. The proportion of dry matter partitioned to roots was, however, increased by soil temperature in contrast to the effects of air temperature. Shelling percentage and seed size, particularly at podding, were also reduced by high soil temperature and these effects were much greater than those of air temperature. Combined effect of air and soil temperature The effects of air and soil temperature were clearly additive for all the traits except shelling percentage when high temperatures were imposed at podding (Figures 1 and 2). For example, high air, high soil and the combination high air and high soil temperatures at podding reduced fruit-set by 49, 11, and 57%, respectively, and total dry matter by 4, 27 and 32%, respectively. Even for flower number and root shoot ratio, where effects of air and soil temperature were in opposite directions, the net effects were additive or neutral. The effects of high temperature imposed at flowering were also additive for a number of traits, for example fruit-set and peg pod ratio, while for flower number the positive and negative effects of air and soil temperature, respectively, canceled each other out. However, for traits such as reproductive number, pod number and total dry matter, the effects of air and soil temperature treatments were not additive and air and soil temperature combined had no greater effects than soil temperature alone. Discussion It is clear from this research that high air (38 /22 C; mean 30 ) and/or high soil (38 /30 C; mean 34 C) temperatures from the start of flowering or podding to maturity significantly reduced total dry matter, pod yield and yield components in groundnuts. On average, pod yields were reduced by 18 26% by high air temperature, 30 39% by high soil temperature and 49 52% by high air and high soil temperature. Similar results have been reported by Ketring (1984), Ono et al. (1974), Wood (1968), Golombek and Johansen (1997) and Vara Prasad et al. (1999a,b). There were significant differences between the Spanish and Virginia cultivars and interactions between cultivar and air temperature and cultivar and soil temperature. The Virginia cv. ICGV 87282, which had lower pod yields than the Spanish cv. ICGV due to lower rates of flower production and lower rates of partitioning of dry matter to pods, was also more susceptible to high air or soil temperature than the Spanish cultivar. There are reports of genotypic variation in sensitivity to high temperature (Greenberg et al., 1992; Wheeler et al., 1997), though whether these are associated with differences in botanical type (Vara Prasad et al., 1999a) rather than tolerance or susceptibility per se needs to be determined. Previous studies in groundnut (Ketring, 1984; Ong, 1984; Vara Prasad, 1999 a, b), and other legumes such as cowpea (Vigna unguiculata (L.) Walp; Hall, 1992) and common bean (Phaseolus vulgaris L.; Gross and Kigel, 1994), have shown that the principal deleterious effect of high air temperature is on fruit-set, i.e. the proportion of flowers setting fruits (pegs). Our results confirm this finding, and also show clearly that the

8 238 proportion of pegs forming pods is not affected high air temperature. Reduced fruit-set is due to specific effects of high air temperature at microsporogenesis on pollen number and viability (Vara Prasad et al., 1999b), as opposed to effects on dry matter production. The observed increase in flower production at high temperature is a consequence of the effects of high temperature on fruit-set, rather than on effects of temperature on flower production per se (Vara Prasad et al., 1999a). Research in controlled environments has also shown that an increase in the podding zone temperature by 10 C above an ambient soil temperature of C from start of pegging or podding until maturity also significantly reduced the number of mature pods, 100 seed weight and, therefore, seed yields (Dreyer et al., 1981; Golombek and Johansen, 1997). Our results are consistent with these findings; a 10 C increase in soil temperature above the ambient value of 26 C decreased pod numbers and 100 seed weight by 25 50%, and pod yield by 21 39%. In contrast to the effects of air temperature, however, the deleterious effects of soil temperature were principally on dry matter production, particularly 100 seed weight, and flower production and the proportion of pegs forming pods. High soil temperature reduced vegetative and total dry matter significantly more than air temperature (27 33% cf. 4 17%), and this was probably associated with warmer mean temperatures (c. 34 cf. 30 C), particularly during the night (c. 30 cf. 24 C). Flower production was also significantly lower at high soil temperatures and this was probably due to the overall effect of high soil temperature on growth, rather than a specific effect of soil temperature per se. This supposition is supported by the observation that soil temperature had no effect of fruit-set, again in contrast to air temperature. However, soil temperature did significantly affect the proportion of pegs forming pods, which is to be expected since this process occurs after pegs have penetrated into the soil and is therefore directly affected by soil temperature. Similarly, pod and seed growth are also influenced by soil temperature and soil temperature above 34 C are reported to be supra-optimal for individual pod growth and pod initiation rates (Dreyer et al., 1981; Golombek and Johansen, 1997). The overall effect of soil temperature is therefore due to both fewer pods and lower pod growth rates. Root shoot ratios were also affected by high air and soil temperature. High air temperature decreased the root shoot ratio by reducing root growth but not shoot growth, whereas high soil temperature increased root shoot ratio by decreasing shoot growth twice as much as root growth. These responses conform to the general rule that root shoot ratios shift in favor of the environment (air/soil) with the limiting resource (Squire, 1993). Wood (1968) has also reported substantial effects of air temperature on root growth (65% less root mass at 35 C compared with 20 Cair temperature). Golombek and Johansen (1997), however, reported that root mass and specific root length were greater in groundnut at day/night soil temperatures of 38 /22 C relative to 20 /14 C. Differences in partitioning of dry matter to roots at high temperature among heat tolerant and susceptible groundnut cultivars in response to heat stress have also been found (Wheeler et al., 1997), and these responses may be an important component of adaptation to stressful environments. In general, responses to the combined effects of high air (38 cf. 28 C)andhighsoil(38 cf. 26 C) were additive and without interaction, whether imposed from start of flowering or podding until maturity. As discussed above, high air and soil temperature affected different yield forming processes principally fruit-set in the case of air temperature, and flower production, peg to pod ratio and 100 seed weight in the case of soil temperature, and this is why effects were additive and not multiplicative. The only exception to this generalisation occurred where high soil temperature was imposed from flowering: total dry matter and reproductive number were both significantly reduced by soil temperature, with no further reduction in weight or number when high temperature was also imposed. Nonetheless, even here the effects on pod yields were additive. In conclusion, this research has shown that both high air, and to slightly greater extent high soil temperature, imposed from flowering to maturity reduced pod yield in groundnuts. The effects of air and soil temperature were mostly additive and without interaction. The principal effect of high air temperature was on fruit-set and hence pod number, while those of high soil temperature were on flower production, the proportion of pegs forming pods and 100 seed weight. Acknowledgements We thank Felix Foundation for the financial support, Messrs K.E. Chivers and S. Gill for engineering sup-

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