Carbon Production of Sunflower Cultivars in Field and Controlled Environment. II. Leaf Growth

Abstract

In field studies of several cultivars of sunflower grown on stored soil moisture or with irrigation, yield was positively related to leaf area at anthesis. The regression which described this relationship stated that 1370 kg seed ha^-1 were associated with a unit increment in leaf area index.

Cultivars differed in final leaf number, rate of leaf appearance, and in the vertical distribution of leaf area in the profiles. Final leaf number of plants grown on stored moisture remained the same as in irrigated plants, but the period for which each leaf grew was reduced from 19 to 16 days while the average growth rate was reduced from 13 to 6 cm² day^-1. These changes reduced the final leaf area from 5700 to 1900 cm² per plant. As water stress increased, the period when leaves grew fastest became progressively earlier, from approximately 35% A_max to when leaves were less than 5 cm₂.

Water stress increased stomatal frequencies but reduced the area of individual stomata so that the area of the stomatal apparatus per unit leaf area was unchanged. This may partially explain the constancy of peak gas exchange per unit leaf area of sunflower grown under different water regimes. There was evidence that leaves could recommence growth when the stress was alleviated.