Radiation interception and growth in an intercrop of pearl millet/groundnut

Abstract

Abstract Quantum flux measurements of the transmission of photosynthetically active radiation (PAR) are presented for the monocrops and an intercrop of an 82-day millet and 105-day groundnut. The intercrop row arrangement was 1 millet : 3 groundnut and intra-row spacing of each species was the same in monocrop and intercrop. The results for PAR were compared with similar measurements of total solar radiation. A linear relation was found between the logarithms of the transmission coefficients in the two wavebands. The relation was independent of both age and structure of the canopies and was used to convert measurements of total solar radiation into quantities of PAR intercepted by the crops. Dry weight of monocropped millet increased linearly with intercepted PAR during the vegetative and much of the reproductive phases. In contrast, dry weight of monocropped groundnut only increased linearly in the vegetative phase. During the first half of pod filling, there was no increase in dry weight despite a substantial quantity of PAR interception. In the second half, dry weight of the groundnut increased by a further 30%. Similar relations were observed for the two components of the intercrop. On the basis of a Land Equivalent Ratio (LER) intercropping gave 28% more total dry matter (LER = 1.28) than growing the two crops separately. The processes producing the intercropping advantage are separated by defining two ratios; the Resource Capture Ratio (RCR) and the Conversion Efficiency Ratio (CER). These ratios compare, on a per plant basis, the performance of the component species in the intercrop relative to their respective monocrops in terms of the interception of radiation and the production of dry matter/unit of radiation intercepted, respectively. Per row, the millet intercepted 2.1 (RCR = 2.1) times more PAR in the intercrop than in the monocrop and used it with a similar efficiency (CER = 0.97) to produce twice as much dry matter. Per row in the intercrop, the groundnut intercepted 27% less PAR than in the monocrop (RCR = 0.73) but used it with 46% (CER = 1.46) greater efficiency to yield the same.