INFLUENCE OF WINDBREAK-SHELTER ON SOYBEANS (GLYCINE MAX (L.) MERRILL)

The influence of windbreak-shelter on soybeans {Glycine max (L.) Merrill} was studied during the 1978 and 1979 growing seasons at the University of Nebraska-Lincoln Field Laboratory, Mead, Nebraska (41(DEGREES), 29\u27 N; 96(DEGREES) 36\u27 W; 354 m above mean sea level). Crop growth as indicated by plant height, number of nodes, internode length, leaf number, leaf area index and dry matter production was significantly increased (P = 0.05) in shelter during both years. Crop growth rate as well as the growth rate of leaves, petioles and stems was higher for sheltered soybeans. Each of these growth rates declined during pod filling in both sheltered and exposed plants probably because of assimilate redistribution. Because of environmental and seasonal variations in number of leaves per plant and specific leaf area, prediction regression equations for leaf area developed with leaf number and leaf dry weight as independent variables were found to be very inaccurate in predicting the leaf area of soybeans grown in a different environment or season. Greater leaf area development in shelter resulted in greater interception of incident photosynthetically active radiation. Longer internodes of sheltered soybean plants meant greater spatial separation of leaves, lower canopy leaf area density and deeper penetration of light to lower canopy strata. Regression of net assimilation rate on leaf area index also indicated better light climate within the canopy of the sheltered plants. The slopes of the regression lines were greater for exposed soybeans, indicating that for each unit increase in leaf area index there was a greater decline in the net assimilation rate of the exposed plants due to a greater mutual shading of leaves. A less severe plant water stress indicated by a less negative leaf water potential, higher stomatal conductance and lower leaf temperature was observed in the sheltered plants. This was a consequence of lower atmospheric evaporative demand. Because of a lower plant water stress and a better canopy light climate, sheltered plants had higher stomatal conductance and higher CO(,2)-exchange rate at equivalent relative heights within the canopy, when compared to exposed plants. Since no vertical gradient of leaf water potential was found in either environment, the observed gradients of stomatal conductance and CO(,2)-exchange rate were solely a response to light. No differences in soil water content and soil water-use were observed between the environments, but water-use efficiency was higher in shelter because of its significantly greater grain yield. Grain yield in shelter was 20 and 26% higher in 1978 and 1979, respectively. Results indicated the operation of 5 factors of differential yield in sheltered and exposed soybeans. The vegetative size of the plant at flowering, number of pods per plant, pod filling period, number of seeds per pod and harvest index were all increased in shelter and proved to be important yield determinants. The close relationship between grain yield and plant height, leaf area index and vegetative dry weight implied that accurate prediction of soybean yield response to shelter can be made using any of these growth indices