Abstract

Media summary

Key words

Water deficit, irrigation, growth rate, transpiration, clover, annual legume

Introduction

Drought is the main environmental constraint limiting yield under Mediterranean conditions and it develops progressively during the last part of the crop cycle. Crops grown in these areas display varied responses to water stress. A dominant response is the reduction of water loss by plants. Thus, plants close the stomata apparatus and modulate their leaf area, and thereby adjust the loss of water from the canopy (Passioura 1997; Tardieu 1997).

The efficient water use by the plants plays a predominant role particularly in the areas where the water is a limiting factor of production. In addition, the water use efficiency (WUE) could be used as selection criterion, to improve yield in a dry environment (Tardieu 1997). The estimation of the WUE can be based either on the evaporotranspiration or on crop transpiration; however, the transpiration based WUE provides a more useful indication of plant performance (Tanner and Sinclair 1983; Davies and Pereira 1992). Moreover, for forage crops that cover the ground most of the year and have abundant litter, the evaporation is generally very small and can be ignored as a variable (Hanks 1983; Thornley 1996).

Berseem clover is an annual legume grown in Mediterranean regions. It is well adapted to various environments but its yield is related to irrigation (El-Bably 2002). However, there are few reports on morphological and functional plant traits when it is grown under water limited conditions. A more detailed knowledge of these traits in an annual clover species may be important for the improvement of management practices, as well as for future selection of genotypes for drought prone areas.

The objective of this study was to identify the effect of drought on water use efficiency, estimated on plant basis, as well as on the parameters affecting WUE.

Materials and Methods

The Greek origin cultivar Pinias of berseem clover was used in this study. The experiment was conducted in the glasshouse of Technological Educational Institute of Drama (41°09΄ N, 24°09΄ E, 130 m a.s.l.) in Macedonia of Greece. Plastic pots (30 cm diameter by 30 cm deep containing 20 L soil) were filled with medium texture soil in which 15% organic fertilizer was added. The pH in the pots was 7.8. Five seeds were sown in each pot on March 16, which first germinated on March 22 in the year 2002. The plants were irrigated uniformly until April 15. After that, two water regimes were applied; in the first, plants were irrigated frequently to maintain field capacity (H, irrigated treatment); in the second, half the quantity of water was used (Ho, drought treatment). The pots were placed on a platform and rotated regularly so that the plants received the same quantity of irradiation.

Leaf transpiration rate was measured by steady-state porometer (Li1600, LiCor Nebraska U.S.A.) at midday, before other measurements. At each measurement, a plant from each pot was removed and separated into stem, leaves and inflorescences. Leaf area was measured by a portable leaf area meter (Li-300). The plant parts were oven dried at 75⁰C for 48 hours and weighed.

The yield and growth rate of the above ground dry biomass of each plant was estimated at each growth stage. We estimated the water used per plant as plant transpiration (PT) (Lazaridou and Noitsakis 2003) following the next formula:

PT=TxLA, in mmol s^-1,

where T is transpiration rate (in mmol m^-2 s^-1) and LA (in m²) is the leaf area per plant. The evaluation of plant water use efficiency (PWUE) was based on the relation between produced above ground dry biomass per day during respective growth stage (GR, in g day^-1) and the water transpired by plant (PT) as:

PWUE= GR/PT in g day^-1 mmol^-1s.

A completely randomized design with six replications of each treatment was used. Five measurements were made in various phenological stages: early vegetative, vegetative, bud, early flower, and full flower.

Results and Discussion

The growth rate (GR) of irrigated berseem clover increased progressively, being lower at early vegetative stage and higher at the beginning of flower stage (Figure 1a). It decreased at full flower stage, although it still remained at high levels. In contrast, GR remained low under drought and showed statistically significant (p ≥ 0.05) differences from irrigated treatment after bud stage. A reduction in GR at full flower stage was also observed in this treatment.

Yield of berseem clover, namely the above ground dry biomass per plant, is presented on Figure 1b. Under irrigation yield increased progressively as a result of high growth rate (Figure 1a) and reached to 6.8 g/plant. When berseem clover was grown under drought, the yield reached to 2.3 g/plant, namely about one third of irrigated plants. Obviously, these values were due to GR differences observed between treatments. Yield reduction of berseem clover subjected to drought has also been reported by Iannucci (2001) and El-Bably (2002).

The leaf area per plant is directly associated with dry matter accumulation, as well as with water used by plant as it forms the transpiring surface (Blum 1997). In the irrigated treatment the LA increased as plants grew and was significantly different from the drought treatment after vegetative stage (Figure 1c). The difference increased during the next stages, as plants grown under drought lost a part of their leaf area at flower stage. Reduction of leaf area, as a response to drought, has also been reported by other researchers (Noitsakis et al. 1991; Tardieu 1997) and could be one of the parameters resulting in a gradual decrease of yield (Passioura 1997).

The transpiration rate of leaves was significantly lower in plants under drought than under irrigation in all stages (Figure 1d). The effect of water deficit on transpiration occurred right after irrigation differentiated. This reduction is due to stomata closure (results are not shown). Therefore, the berseem clover seems to control water loss to atmosphere by reducing leaf area and transpiration rate. The combined effect of these two parameters was indicated by plant transpiration (PT). We consider PT being a better integrator of plant performance than transpiration rate. PT, as an expression of total water transpired per plant, indicated substantially lower values under drought than under irrigation (Figure 1e). Indeed, PT was significantly lower in plants under drought after the early vegetative stage. PT steadily increased in the irrigated treatment, while it remained about constant under drought.

Figure 1. Effects of irrigated (H) and drought (Ho) treatments, on a) growth rate, b) yield (above ground dry biomass), c) leaf area, d) transpiration rate, e) total plant transpiration, f) plant water use efficincy (PWUE). Bars indicate the standard error of mean of six replicates.

The relationship between growth rate and plant transpiration gives an estimation of the water use efficiency of the plant. The obtained values of PWUE were higher under drought than under irrigation right after the early vegetative stage (Figure 1f). These differences were statistically significant (p ≥ 0.05) only at the two measurements of flower stage. Similar results have been reported for berseem clover (El-Bably 2002), alfalfa (Metochis and Orphanos 1981; Lazaridou and Noitsakis 2003), and peanut (Craufurd et al. 1999), although they mostly used different methods of estimation.

The question deriving from these results is how beneficial are the observed high PWUE values under drought for generating higher plant productivity. A further analysis on this is needed.

Conclusion

The results confirm the high effect of drought on berseem clover function and structure in all growth stages, finally integrated as yield production. The growth rate was lower in early vegetative stage and higher in early flower stage regardless of water regime. The difference in GR between the two water treatments resulted in yield under drought being one third of irrigated.

The transpiration rate reduced right after irrigation differentiated, while leaf area reduced at bud stage. Thus, the plant transpiration declined under drought after early vegetative stage. Berseem clover substantially reduced plant water losses by reducing the transpiration rate and the leaf area. However it reduced less the yield, resulting in higher values of PWUE. The PWUE obtained the highest value under drought at the beginning of the flower stage.

References

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