Mixed plantings made more efficient use of land than pure stands in 2002, but not in 2001. Tomato and Brussels sprout plants were transplanted into plots later in 2002 than in 2001 (Figure 2), and 2002 was drier than 2001 (Figure 3). Both of these differences likely contributed to smaller and less competitive tomato and Brussels sprout plants in 2002. Basil seems to have benefited from the reduced size of its companions: With more access to light it accumulated more chlorophyll and produced more aboveground biomass. Tomato and Brussels sprout also used land more efficiently when mixed together in 2002, but not in 2001, suggesting complementary use of resources when plants were smaller and moisture was more limiting. The observation supports the contention of Rao and Willey (1980) and Natarajan and Willey (1986) that mixtures offer a greater yield advantage over pure stands when moisture is limited, but does not agree with a recent model suggesting no effect of moisture availability on LER (Tsubo et al. 2005) or with an earlier report of a mixture yield advantage in a wet year but not in a dry one (Fisher 1976).

This study showed a relationship between SPAD readings and basil yield, but no relationship between SPAD readings and tomato or Brussels sprout yield. SPAD readings have previously been correlated with leaf chlorophyll content in a variety of crops including rice (Jiang and Vergara 1986), corn (Dwyer et al. 1991), wheat, barley, and triticale (Giunta et al. 2002). SPAD readings have also been correlated with yields of crops including corn (Blackmer and Schepers 1994), potato (Gianquinto et al. 2003) and cabbage (Westerveld et al. 2003). This effect is not consistent, however: Westerveld et al. (2003) report correlations between SPAD readings and cabbage in one year, but not another; and Martini et al. (2004) report higher SPAD readings in conventional tomato