Effects of Temperature on Leaf Expansion in Sunflower

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Abstract
Five commercial cultivars of sunflower [cultivars Suncross 51, Suncross 52, Hysun 30, sunfola 68/2 and Vniimk] were grown in cabinets at 3 temperature regimes, 32.degree./22.degree., 27.degree./17.degree. and 22.degree./12.degree. C, and with 15-h and 11-h photoperiods, and expansion of leaves 5-15 was followed. Leaves appeared faster with increasing temperature (0.022 leaves day-1 .degree.C-1) and with increasing daylength. Areas of individual leaves increased linearly up the plant profile and, although final area per leaf (Amax) decreased with increasing temperature, the relative change was similar for each leaf position. Cultivars maintained their ranking for Amax across temperatures and these rankings agreed with those in previous field studies. Within each temperature regime, the expansion rate of leaves and the duration of expansion increased with leaf position. As temperature increased, leaves grew for shorter periods with a change of 1.04 days .degree. C-1, but under the photon flux density used (500 .mu.mol m-2 s-1, or .apprx. 25% full sunlight) expansion rates were greatest at the lowest temperature. Expansion rates were only 1/3 of those in field studies at comparable temperatures, but durations were similar. Cultivars that achieved the largest Amax did so via faster rates of expansion and not via longer durations: only 1 cultivar differed from the mean (20 days) duration of leaf expansion. All cultivars reached floral initiation progressively earlier with extension of photoperiod from 10 to 15 h, with the change for the most sensitive cultivars being 8 days and for the least sensitive 5 days. Rates of leaf emergence were linked with this sensitivity.