2017 ASHS Annual Conference

The relative growth rate of hydroponic lettuce and spinach in greenhouses was predicted from contributions due to various environmental parameters. The growth experiments for lettuce in 2006-2008 including all times of year, high vs low temperature, and effect of withdrawal or resupply of nitrogen. Four of the environmental parameters were significant; irradiance divided by leaf area index if it was greater than one, solution electrical conductivity, solution temperature, and solution nitrate that was a linear function of concentration between 6 and 65 mg L^-1, and 1.0 when above this. The regression for RGR had R² = 0.606 for 114 growth experiments. After eliminating three values with a high residual, the response increased to R² = 0.640. The coefficient for regression vs irradiance divided by leaf area index was most significant, but all coefficients had a value that was more than three time standard error.

The relative growth rate of spinach in 2009-2010 was predicted from the four parameters defined above. The regression for 30 growth experiments had an R² = 0.200. Solution electrical conductivity did not differ among these experiments, and regressions vs this parameter were not significant. After three outliers were deleted, regression vs the other three parameters gave R² = 0.758. All coefficients had a value that was more than three time standard error. The coefficient for irradiance divided by LAI was about half the value for lettuce. The coefficient for solution temperature had a value two times that for lettuce, and the coefficient for low nitrate was more than twice that for lettuce.

When these parameters were used to predict relative growth rate for lettuce in 1996-1998, they showed many differences. Using the values from 2006-2008 to predict results for 52 growth experiments in 1996-1998, showed that solution temperature was the only significant parameter. The constant in regression was positive not negative, as in the regressions above. The coefficient for solution temperature was about the same as that for regression of lettuce in 2006-2008. Because only one of the four parameters used for regression in 1996-1998 was significant, we could not show any relationship for the other parameters in the two sets of lettuce data.