In Holland, the primary effect on greenhouse tomato of a +4 [degrees] C air DIF compared with a 0 to +1 [degrees] C DIF was increased stem elongation (de Koning, 1988).
The two air temperature regimes are referred to as + 14 [degrees] C DIF and +5 [degrees] C DIF, respectively.
Relative humidity was higher in the day and lower at night under +5 [degrees] C compared with +14 [degrees] C DIF. Averaged over all plantings, relative humidity was 0.77 and 0.72 in the day, and 0.86 and 0.97 at night, under +5 [degrees] C and +14 [degrees] C DIF, respectively.
Averaged over root heating regimes, the relative growth rate under +5 [degrees] C DIF was faster than under +14 [degrees] C DIF, 0.082 compared with 0.077 g [g.sup.-1] [d.sup.-1] (P [is less than] 0.05).
Overall, the concentrations of nitrogen and other elements, including minor elements (not shown), were higher in seedlings grown under +5 [degrees] C compared with +14 [degrees] C DIF. Each root-zone heating regime increased nutrient concentrations significantly compared with the unheated regime.