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-photoinhibiting or mild photoinhibiting conditions. The development of the leaves was studied under chilling and control conditions by measuring several physiological parameters. A gradual decrease of the efficiency of the photosynthetic apparatus with maturation and ageing occurred in unchilled plants. This was reflected by gradual changes in CO2-saturated photosynthesis and protein and rubisco contents. Except for senescing leaves, a correlation close to 1:1 was observed between maximum 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 leaves. In plants chilled under non-photoinhibiting conditions (10-degrees-C, 100-150 muE m-2 s-1 or 6-degrees-C, 30-50 muE m-2 s-1), a similar 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 photosynthetic capacities than unchilled plants of the same size. When the chilling conditions were mildly photoinhibitory (6-degrees-C, 100-150 muE m-2 s-1), a much stronger depression of rubisco activity and photosynthetic capacity was found in all leaves, which was partly reversible in the young ones. This decrease in CO2 fixation capacity, in turn, led to a higher susceptibility of the chilled plants to photoinhibition at 20-degrees-C. It is concluded that the decrease of both photosynthetic capacity and growth after long-term chilling in tomato is a consequence of the preceeding ageing and senescing of the leaves during chilling, in contrast to chilling-tolerant species with the ability for acclimation to low temperatures.