UV-B-INDUCED INCREASE IN SPECIFIC LEAF WEIGHT OF CUCUMBER AS A CONSEQUENCE OF INCREASED STARCH CONTENT

Specific leaf weight (SLW), the ratio of leaf dry matter to area, ofte n increases in plants exposed to elevated UV-B radiation (280-315 nm). Increased SLW can result from greater leaf thickness or increased lea f density (e.g. accumulation of high density substances in cells). The basis for large increases in SLW was examined in the first and third leaves of cucumber differing in developmental stage at the start of UV treatment. Leaf 1 was approximately 50% fully expanded, while leaf 3 had just unfolded. It is shown here that up to 80% of the W-generated change in SLW in leaf 1 was caused by accumulation of nonstructural ca rbohydrates, especially starch (increasing from 13 to 23% of total dry weight). Leaf 3 contained a much smaller proportion of nonstructural carbohydrates (less than 8%) and the effect on SLW was correspondingly less. As shown in the previous paper, UV-B inhibition of growth in le af 3 was reversed by supplemental blue light (BL) in a fluence-depende nt manner between 0.23 to 2.68 mol m(-2) per day. Fluence-response cur ves revealed that supplemental BL reversed both the UV-induced accumul ation of starch and increase in SLW in leaf 1 over the same range. The data are consistent with a back-up of photosynthate into leaf 1 as a result of UV-B inhibition of growth in leaf 3. The data also demonstra te that increases in SLW cannot be assumed to represent increases in l eaf thickness.