47 Effect of Water Deficit on the Growth of Bean (Phaseolus vulgaris L,) and Cowpea (Vigna unguiculata L,)

M., Berova^ , v., Kerin\ Tz., Stoilova^ ^ Agricultural University, 4000 Plovdiv, Bulgaria ^ Institute of Plant Genetic Resources, 4122 Sadovo, Bulgaria

Introduction During their ontogenetic development, plants are subjected to the unfavourable effect of environmental factors, water deficit being one of the most common of them (Yordanov et al. 2000). Water deficit has a negative effect on the functional status of plant organisms. It reduces the functional activity of plants, changes their normal functions and induces damages leading ultimately to a decrease in their productivity (Blum 1996). The effect of water deficit results fi-om the intensity and degree of dehydration, as well as fi-om the genetically determined plant capacity to overcome it (Bewley 1979). The objective of the present study was to make a preliminary estimation of water- deficit resistance in bean and cowpea plants.

Material and methods A pot experiment was conducted in a greenhouse with bean {Phaseolus vulgaris L., cv. Dodrudjanski 3) and cowpea {Vigna unguiculata L.,local name Papuda) plants. The soil was taken ñ*om the arable layer of the experimental field of the Agricultural University in Plovdiv, Bulgaria, assigned as fluvisols (Gyurov 1959). The experiment involved control (85% FWC) and drought-stressed (to the wilting point) variants, differentiated at the first true leaf stage. The control plants received daily the necessary water rates. The experimental ones were subjected to a progressive drought up to the wilting point. The droughting was assessed using a physiological criterion - the occurrence of a lasting residual water deficit in the functional plant leaves. Growth parameters were determined on day 20 after the variant differentiation. The dry weight of the separate organs was determined after 48 h oven drying at 70^ .The leaf area was measured with a digital area meter NEO-2 (TU, Sofia, Bulgaria). Growth analyses were made according to Beadle (1993). All data were statistically analyzed. The significance of differences was determined using Student's ^test.

Results Under water-deficit conditions, the growth of beans was more significantly suppressed than that of cowpeas. The fi^esh weight (FW) of the drought-stressed plants was by 54% lower than the control. In cowpeas, the respective difference was 28%. The dry weight (DW) changes followed the same tendency. The more significant inhibition of fresh than dry weight was indicative of the reduced hydration of plant organs. This tendency was more obvious in beans. The root weight ratio (RWR) in the bean plants grown under water-deficit conditions decreased significantly (by 26%). The activation of root growth in cowpeas (a 20% higher RWR) was an evidence of plant adaptation to the unfavourable water regime and the redistribution of photoassimilates in favour of roots. The accelerated growth of the root system led to its reorientation to soil layers with higher moisture content and improved the water status of plants. 48

With a normal water supply, the bean produced greater foliage than did the cowpea. Under water-deficit conditions, the leaf area (LA) of bean plants decreased (by 31%), while in the cowpeas was on the control level The leaf area ratio (LAR) of beans and cowpeas increased at the expense of the specific leaf area (SLA). The SLA of the drought-stressed plants increased significantly (by 23% and 64%, respectively), which proved the changes in the water exchange of plants.

Table 1. Effect of water deficit on the growth of bean and cowpea plants Parameters Phaseolus vulgar is Vignaunguiculata

Control drought-stressed Control drought-stressed FW [g per plant ] 5.881 ±0.34 2.688 ±0.22** 3.786 ±0.20 2.711 ±0.41* DW [g per plant] 0.484 + 0.05 0.272 ± 0.04* 0.500 ±0.05 0.328 ±0.02* LA [cm^] 12914.0 89±5.2** 66 ±4.8 65 ±3.5 RWR[mg (root) 0.250 ± 0.02 0.187±0.01* 0.178 ±0.02 0.213 ±0.04 mg'' (plant)] SWR[mg (stem) 0.283 + 0.03 0.349 ±0.02 0.290 ±0.04 0.361 ±0.02 mg'(plant)] LAR [cm^ mg'' 0.266 ±0.02 0.327 ±0.05 0.132 ±0.01 0.198±0.02* (plant)] LWR [mg (leaf) 0.467 ±0.05 0.463 ±0.04 0.532 ±0.03 0.488 ± 0.05 mg'' (plant)] SLA [cm^ mg"' 0.571 ±0.03 0.706±0.02* 0.248 ±0.01 0.406 ±0.02** (leaf)] NAR [mgcm'^d" 0.188 ±0.01 0.118±0.01** 0.379 ± 0.03 0.253 ± 0.02* '1 *P< 0.05; **P< 0.01

The changes in the net assimilation rate (NAR) were similar for both genera tested. The NAR inhibition (more strongly expressed in beans) allowed our assumption that under water-deficit conditions, the processes of photosynthesis and dark respiration were disturbed. On the basis of the results obtained it can be assumed that the plants of genus Vigna are more drought-resistant than those of genus Phaseolus.

References Beadle, C. 1993. Growth analysis. In: Hall, D., J. Scurlock, H. Bolhar- Nordenkampf, R, Leegood, S. Long (ed.): Photosynthesis and production in a changing environment. A field and laboratory manual. Chapman & Hall. London, pp. 36-45. Bewley, J. 1979. Physiological aspects of desiccation tolerance. Annu. Rev. Plant Physiol.30: 195-238. Blum, A. 1996. Crop responses of drought and the interpretation of adaptation. Plant Growth ReguL 20: 135-148. Gyurov, G. 1959. The complex of saline soils, located to the East of Plovdiv. Scientific Reports of Agricultural University - Plovdiv. 6: 431 -461. (Bg). Yordanov, L, V. Velikova, T. Tsonev. 2000. Plant responses to drought, acclimation, and stress tolerance. Photosynthetica. 38 (1): 171-186.