Effects of two photoperiod treatments on leaf photosynthesis throughout ontogeny in 'Fergus' barley

Abstract

Leaf photosynthesis was measured throughout ontogeny for barley plants (Hordeum distichum L. emend. Lam. cv. Fergus) grown under 12- and 24-h photoperiods. Plants were grown in silica and irrigated with one-half strength of a modified Hoagland solution at 700 μE m−2 s−1. The 12-h photoperiod was extended to 24 h with low-intensity incandescent lamps (10 μE m−2 s−1). The ontogenetic pattern of change in the leaf carbon dioxide exchange rate (CER) differed between photoperiodic treatments. Under the 12-h photoperiod CER increased from leaf 1 to leaf 5 and declined to a minimum for leaf 9; whereas CER under the 24-h photoperiod continued to increase with ascending leaf position. Both stomatal and residual, which was six times larger, resistance components were responsible for the changes in CER. The pattern of change in CER with individual leaf age differed between stages of plant development but was similar at a given stage of development between photoperiodic treatments. During the vegetative phase, CER decreased slowly for 5 days from full expansion and rapidly thereafter, while during the stem elongation phase CER decreased rapidly soon after full expansion. During the grain development phase in the 24-h treatment, two plateaus were observed in CER of the flag leaf. It is clear that photoperiodic treatments affect leaf photosynthesis not only directly but also indirectly through changes in the developmental pattern.