Photoacclimation and photoinhibition in Ulva rotundata as influenced by nitrogen availability

Clonal tissue of the marine chlorophyte macroalga, Ulva rotundata Blid., was transferred from 100 to 1700 μmol photons · m(-2) · s(-1) under limiting (1.5 μM NH 4 (+) maximum, N/P=2) and sufficient (15 μM NH 4 (+) maximum, N/P=20) nitrogen supply at 18° C and 11 h light-13 h darkness daily. Photoinhibition was assayed by light-response curves (photosynthetic O2 exchange), and chlorophyll fluorescence at 77 K and room temperature. Daily surface-area growth rate (μSA) in N-sufficient plants increased sixfold over 3 d and was sustained at that level. During this period, respiration (R d) doubled and light-saturated net photosynthesis capacity (P m) increased by nearly 50%, indicating acclimation to high light. Quantum yield (ϕ) decreased by 25% on the first day, but recovered completely within one week. The ratio of variable to maximum fluorescence (F v/F m) also decreased markedly on the first day, because of an increase in initial fluorescence (F o) and a decrease in F m, and partially recovered over several days. Under the added stress of N deficiency, μSA accelerated fivefold over 4 d, despite chronic photoinhibition, then declined along with tissue-N. Respiration doubled, but P m decreased by 50% over one week, indicating inability to acclimate to high light. Both ϕ and F v/F m decreased markedly on the first day and did not significantly recover. Changes in F o, F m and xanthophyll-cycle components indicate concurrent photodamage to photosystem II (PSII) and photoprotection by thermal deexcitation in the antenna pigments. Increasing μSA coincided with photoinhibition of PSII. Insufficient diel-carbon balance because of elevated R d and declining P m and tissue-N, rather than photochemical damage per se, was the apparent proximate cause of decelerating growth rate and subsequent tissue degeneration under N deficiency in U. rotundata.