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Effects of water deficit during vegetative growth on leaf expansion of legumes

K.C. Kawonga, F.P.C. Blarney and H.M. Shelton

Department of Agriculture, University of Queensland, QLD 4072

Water deficit during early growth has detrimental effects on production through reduced leaf expansion and reduced photosynthesis per unit leaf area. Subsequent crop production is reduced also, because of decreased leaf area (1,2), and the consequent reduction in light interception. Crops have been found to differ in tolerance to water deficit as well as in water use, possibly through differences in leaf area index. Thus, the effects of inadequate water supply on leaf expansion would be expected to differ among species.

Methods

A glasshouse experiment was conducted with six legumes, pigeonpea (Cajanus cajan), cowpea (Vigna unguiculata), blackgram (Vigna mungo), mungbean (Vigna radiata), lablab (Dolichos lablab) and soybean (Glycine max). Two plants were grown per pot containing a potting mix, there being c. 1.6 kg plant available water (PAW) per pot. Pots were watered daily to field capacity for 21 days after which one half of the plants were partially defoliated to a common leaf area. From that time also, two watering treatments were imposed for a further 19 days period. Plants were grown either without water stress, being watered daily to field capacity, or were subjected to water stress through not replacing all of the water used by the plants on the previous day. Leaf area was estimated non-destructively every second day by measuring the length and maximum width of the terminal leaflet of every trifoliolate leaf. Plant dry matter (DM) was measured before and after the period of water stress.

Results and discussion

Daily water use and leaf expansion differed significantly among species, and between plants of the same species with different leaf area. Cowpea and soybean were most sensitive to water stress, relative leaf expansion being reduced after four days of reduced water supply. On the other hand, relative leaf expansion of blackgram and lablab was reduced only after 10 and 12 days of reduced water supply, respectively. No clear reduction in leaf expansion was evident in mungbean or pigeonpea up until 19 days after the imposition of water stress.

On average, the species differed significantly in DM production which, over the water stress period, ranged from 2.0 in pigeonpea to 20.8 g pot in cowpea. Partial defoliation reduced DM production by an average of 25%. The species differed significantly in response to limited water supply, the greatest reduction being evident in those species producing the greatest DM under non-limiting conditions. For example, the DM of cowpea was reduced by 17.0 g pot while that of mungbean was reduced by only 1.7 g pot. Water use efficiency was not affected by partial defoliation but was significantly reduced by water stress. These results suggest that low leaf area through partial defoliation or water stress reduces DM production in the legumes studied.

References

Passioura, J.B. and Gardner, P.A. 1990. Aust. J. Plant. Physiol. 17, 149-157.

Sinclair, T.R. 1990. Field Crops Res. 15, 125-141.

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