Is stem extension determined by DIF or by absolute day and night temperatures?

Abstract

Plants of tomato and chrysanthemum were grown in 24 combinations of day temperature (DT) and night temperature (NT) ranging from 12 to 32°C. Internodes about 1 mm in length were identified and their length recorded after 10 days of growth. For neither species could the extension growth in 10 days be related to the difference between DT and NT (DIF). Extension growth in tomato could largely be accounted for by linear DT effects, with linear NT and quadratic DT and NT effects having a smaller influence; the quadratic model accounted for over 93% of the total variance. The optimum temperature for extension growth was rather higher for DT than for NT so that increase in DT over the full temperature range, and increase in NT up to about 24°C, resulted in increased extension growth. DT had a similarly large influence on internode extension in chrysanthemum, but NT had relatively less effect than in tomato; the quadratic model accounted for over 87% of variance. It is concluded that extension growth in these two species responded to the absolute day and night temperatures rather than to DIF.