LONG-TERM CHILLING OF YOUNG TOMATO PLANTS UNDER LOW-LIGHT .3. LEAF DEVELOPMENT AS REFLECTED BY PHOTOSYNTHESIS PARAMETERS

Young tomato plants were exposed to two weeks of chilling under non-ph otoinhibiting or mild photoinhibiting conditions. The development of t he leaves was studied under chilling and control conditions by measuri ng several physiological parameters. A gradual decrease of the efficie ncy of the photosynthetic apparatus with maturation and ageing occurre d in unchilled plants. This was reflected by gradual changes in CO2-sa turated photosynthesis and protein and rubisco contents. Except for se nescing leaves, a correlation close to 1:1 was observed between maximu m rubisco activity and CO2-saturated photosynthesis. Chlorophyll (Chl) contents and photochemical chlorophyll fluorescence quenching showed strong decreases only in the last phase of senescence in the oldest le aves. In plants chilled under non-photoinhibiting conditions (10-degre es-C, 100-150 muE m-2 s-1 or 6-degrees-C, 30-50 muE m-2 s-1), a simila r pattern of ageing was observed, and no indications were found for an induction of protein or rubisco degradation by chilling. Since these plants stopped growing in the cold, they revealed lower total photosyn thetic capacities than unchilled plants of the same size. When the chi lling conditions were mildly photoinhibitory (6-degrees-C, 100-150 muE m-2 s-1), a much stronger depression of rubisco activity and photosyn thetic capacity was found in all leaves, which was partly reversible i n the young ones. This decrease in CO2 fixation capacity, in turn, led to a higher susceptibility of the chilled plants to photoinhibition a t 20-degrees-C. It is concluded that the decrease of both photosynthet ic capacity and growth after long-term chilling in tomato is a consequ ence of the preceeding ageing and senescing of the leaves during chill ing, in contrast to chilling-tolerant species with the ability for acc limation to low temperatures.