EFFECTS OF ADAPTATION TO WINTER STRESS ON BIOMASS PRODUCTION AND MORPHOLOGY OF THREE CONTRASTING WHITE CLOVER CULTIVARS - Bodil, E. Frankow-Lindberg

SLU, Dept. Crop Production Science, Box 7043, S-750 07 Uppsala, Sweden

Summary. Three white clover cultivars (Grasslands Huia, AberHerald and Sandra), sub-populations of these, and a wild Swedish ecotype were grown in a pot experiment in two temperature regimes (24/18 and 12/9°C). Sub-populations of Huia and AberHerald collected after exposure to four winters were less productive than plants from the original populations, and their internode lengths were reduced. The values of these characteristics converged towards the values for Sandra and the wild ecotype. DM partitioning above- and below-ground and individual leaf size did not differ between the original populations and their sub-populations.

Keywords: white clover, biomass production, morphology, winter stress

INTRODUCTION

Poor winter survival of white clover is perceived as a major drawback of the crop in marginal areas of Europe. Indigenous ecotypes of the species in these areas are often characterized by their small stature and poor biomass production. It is therefore of interest to identify which characteristics of the plant are involved in adaptation to winter stress, and to establish whether good adaptation is correlated with inherent poor yielding ability.

MATERIALS AND METHODS.

The experiment included nine populations of white clover, namely the cultivars Grasslands Huia (New Zealand, H), AberHerald (bred in UK from material of Swiss origin, AH) arid Sandra (Sweden, S1) and sub-populations of these collected from a field experiment performed in Uppsala (59°49'N, 17°39'E) during the years 1993-1996 (see Frankow-Lindberg and von Fircks (1998) for details of this experiment). Stolon tips from plots with Huia and AberHerald were collected in October 1995 (after exposure to two winters), and seeds from these were multiplied by the Legume Breeding Group at IGER in Aberystwyth. In spring 1997 (after exposure to four winters) stolon tips were again collected, this time from all plots, and these were grown as stock plants from which experimental plant material was taken. In addition, a wild ecotype (WE) from the area was also collected from a field that had been mechanically mown, with similar intervals as were used in the field experiment, during the past 15 years. Stock plants were also raised from seed of the original cultivars and from the plant material collected in 1995. Field collections are later referred to as survival populations, and these are denoted H2 and H4, AH2 and AH4 and S4 for sub-populations of Huia, AberHerald and Sandra, respectively. There were nine populations in all, and eight genotypes of each population were used for the experiment.

The experiment was conducted during November and December 1997 (when the external photoperiod was <12 h) in two illuminated glasshouses in which control of the temperature regime was possible. The supplementary light provided c. 300 micro moli m^-2 s^-1 PAR and the photoperiod was 12 h. Two temperature regimes were imposed, 24/18°C (day/night, T1) and 12/9°C (day/night, T2). A total of 144 stolon tips containing one fully expanded leaf were planted in Perlite in plastic containers 26.5 x 16.5 x 14 cm³; a few of these failed to establish. The plants were regularly supplied with a low N nutrient solution containing all essential elements. Additional water was added daily, and the containers were leached weekly to prevent any salt accumulation. The plants were inoculated with a slurry containing Rhizobium a week after planting. There were never any signs of mineral nutrient deficiencies and all plants developed healthy nodules on the roots.

Harvesting was done on two occasions after 36 and 50 days of growth at T1 and T2, respectively. At these respective points in time, the plants had developed some secondary branches and the oldest leaf on the main stolon had begun to show some signs of senescence; however, there were fewer nodes on the main stolons in T2 than in T1. The plants were washed free of Perlite and were dissected into eight categories: the stolon part present at the beginning of the experiment, main stolon, branches, apices, petioles, leaflets, roots and flowers with their peduncles. The following characteristics were recorded: main stolon length, branch stolon length, number of main stolon nodes with an expanded leaf, rooted nodes of main stolon, total leaf number, total branch number, petiole lengths (subsample) and leaf areas (subsample). All component parts were dried at 105°C for 24 hours and weighed. The data were analysed by ANOVA as a completely randomized design.

RESULTS

There were no interactions between temperature treatment and populations in the data presented here; population comparisons are therefore based on 15-16 replicates.

Biomass production and partitioning

There were no significant differences in biomass production between AberHerald populations (Table 1), and these, with the exception of AH4, produced more dry matter (DM) than the Swedish populations (p<0.001). DM production of H and H2 were intermediate between those of AH and S, while H4 produced significantly less than both H and H2 (p<0.001). The root/shoot ratio was lower in plants grown in T1 (p<0.001), and also generally so in plants of AberHerald (p<0.001), while there were no significant differences in DM partitioning above- and below-ground between Huia and Sandra (Table 1).

Table 1. Plant weight (g), dry matter partitioning, individual leaf area (cm²), individual petiole length (cm) and internode length (cm).

Morphology

The average individual leaf area was somewhat larger in T2 compared with T1 (p<0.001), or 3.61 cm² and 3.12 cm², respectively, while the average individual petiole length was reduced from 6.23 to 5.07 cm in T2 compared with T1 (p<0.001). The average internode length was significantly greater in T1 compared with T2 (1.12 vs. 0.83 cm, p<0.001). AberHerald plants generally had the largest leaves (Table 1), and this was particularly the case with leaves from AH2 (p<0.001). WE plants also had larger leaves than plants from both Huia and Sandra (p<0.001). The most obvious morphological difference between survival populations and original populations was the reduced internode length (Table 1) in plants from both H4 and AH4 compared with H and H2, and AH and AH2, respectively (p<0.001). The internode length of H4 did not differ significantly from that of both S1 and S4, and that of AH4 was not significantly different from that of WE.

DISCUSSION

The temperature response of the characteristics presented here corresponds well with previously published results (e.g. Eagles and Othman, 1988). Differences in characteristics between H2 and H, and AH2 and AH, respectively, were small, while plants from H4 and AH4 showed changes in both productivity and morphology. Productivity was reduced in these latter populations and was similar to that of the Swedish populations, which suggest that adaptation to winter stress is indeed negatively correlated with a high inherent production potential. DM partitioning within the plants was highly characterized by the cultivar and remained unaffected by exposure to winter stress. It is interesting to note that individual leaf size also remained unaffected, which implies that it should be possible to combine good winter hardiness with a high yield of leaves. The fact that WE had larger leaves than the Swedish cultivar suggests the same. However, the effect of photoperiod was not investigated and an adapted white clover population might exhibit a greater reduction of leaf area as a response to a shorter photoperiod. This aspect has to be considered as developing leaves of S were consistently smaller than those of H and AH during autumn in another study where the genotypes used were from the same populations as in the study reported here (Frankow-Lindberg, 1997). However, a short internode appears to be involved in the adaptation to winter stress since the value of this characteristic converged towards a common value for the survival populations of H4, AH4 and the Swedish populations.

ACKNOWLEDGEMENTS

I thank the Legume Breeding Group at IGER in Aberystwyth for developing and supplying me with seed of H2, AH and AH2.

REFERENCES

Eagles, C.F. & Othman, O.B. 1988. Variation in growth of overwintered stolons of contrasting white clover populations in response to temperature, photoperiod and spring environment. Annals of Applied Biology, 112:563-574.

Frankow-Lindberg, B.E. 1997. Assimilate partitioning in three white clover cultivars in the autumn, and the effect of defoliation. Annals of Botany, 79:83-87.

Frankow-Lindberg, B.E. & von Fircks, H.A. 1998. Population fluctuations in three contrasting white clover cultivars under cutting, with particular reference to overwintering properties. Journal of Agricultural Science, Cambridge, 131:143-153.