Photosynthesis and Transpiration in Dicotyledonous Plants. I. Expanding Leaves of Tobacco and Sunflower.

Abstract

The aims of this paper were (1) to determine if irradiance and nutrition influence the timing of peak photosynthesis (F_max) in expanding tobacco Ieaves and (2) to assess the relative importance of stomatal (r_s) and mesophyll (r_m) resistances to CO₂ uptake as leaves expand.

Growth conditions affected rates of leaf expansion and final leaf areas (A_max) but not the time from leaf emergence to A_max (25 days). Patterns of photosynthesis with time were similar in all treatments-F rose rapidly to F_max and then declined, and F_max occurred on day 13 when the areas of leaves in different treatments were from 65-80 % A_max. We suggest that temperature may determine the timing of F_max. Changes in both r_s and r_m were associated with changes in F prior to F_max. Although changes in r_m were up to five times greater than in r_s during this period, the relative reduction in the two resistances was similar. Absolute changes in r_s and r_m were similar immediately after F_max, and during this period of 7 days F declined by almost 50%. Thereafter, relative changes were closely matched. Possible mechanisms for the control of the resistances by the plant are discussed. We discuss the hypothesis that F_max was maintained for only a limited period in expanding tobacco leaves because of the lack of a sink for assimilate. Expanding sunflower leaves maintained F_max for a much longer period and reasons are presented for this different behaviour. Water use efficiency (ω) of tobacco leaves changed as they expanded and was greatest under good nutrition and poorest under low irradiance. Maximum ω for tobacco and sunflower was similar.