Light Mediated Changes in the Chloroplasts of the Liverwort, Sphaerocarpos donnellii Aust.

Abstract

Abstract. Dark-grown plants of Sphaerocarpos, incubated in a liquid medium containing sucrose and mineral salts, have a much lower chlorophyll and nitrogen content than do light-grown plants. Two minutes of red 'light per 12 hours is about two-thirds as effective in inoreasing chlorophyll and nitrogen con,tent as is oontinuous white light. These red lightinduced increases are mediated by phytochrome, as they are reversible by alternating exposures to red and far-red light. They appear to be related to differences in the ultrastructure of the chiloroplasts. Plastids from dark-grown plants aire full of starch and develop few lamellae, while light-grown plastids contain little starch and have many lamell'ae. The ultrastructural studies are supported by starch determinations which revealed a phytochrome-mediated decrease in starch oontent. The The nitrogen content of the p1lants was also examined lbecaulse of differences in the pH changes of the growth medium of light-ancd dark-grown plants. The more rapid rise of the pH of the medium of light-grown plants was presnlmedl du-le to their greater tuptake of nitrate. The Kjehildahl nitrogen determinations revealed that both red and white light ca,u1ise aln increase in the nitrogen conten(t olf the plants over dark controls. Independence of Chlorophyll and Nitrogen Content from Photosynthesis a1nd Light-Mediated Grozwth Responses. Both continuous white light and 'intermittent red light in,crease the growt'h rate of Sphaerocarpos. The,se increases are mediated by the photosynthetic pigments and phytochrome, respectively (16). Experiments were done to see if 'the light-mediated increases in chloro-phylll and niitrogen contengt might be the direct resullt of these growth responses. DCMU, at a concentration shown to comtpletely inhibit light-mediated CO2 fixation (16), lhas no effect on the chliorophyll or nitrogen content o'f the plants. Hence the whiite light-induced increases in chlorophyll. and nitrogen content are iiot caused by photosynthetic activity.It seemed possible that the red light-indudcedl increases in chlorophyll and nitrogen content were simply the resuilt of the increased growth rate of these plants sin,ce intermittent red light indtles bot,h of these responses. However, it can be shown that these 2 responses are independent because they have different sensitivities to 'low intensity short wavelength far-red light (