Light absorption and partitioning in relation to nitrogen content in 'Fuji' apple leaves

Bench-grafted 'Fuji' apple [Malus sylvestris (L,) Mill, var. domestica (Bor kh,) Mansf,] trees on Malling 26 (M.26) rootstocks were fertigated for 6 we eks with N concentrations ranging from 0 to 20 mM, These treatments produce d levels of leaf N ranging from 0.9 to 4.3 g.m(-2). Over this range, leaf a bsorptance increased curvilinearly from 74.8% to 92.5%. The light saturatio n point for CO2 assimilation expressed on the basis of absorbed light incre ased linearly at first,vith increasing leaf N, then reached a plateau at a leaf N content of approximate to 3 g.m(-2). Under high light conditions (ph otosynthetic photon flux of 1500 mu mol.m(-2).s(-1)), the amount of absorbe d light in excess of that required to saturate CO2 assimilation decreased w ith increasing leaf N. Chlorophyll fluorescence measurements revealed that the maximum photosystem II (PSII) efficiency of dark-adapted leaves was rel atively constant over the leaf N range, except for a slight decrease at the lower end, As leaf N increased, nonphotochemical quenching declined under high light, and there was an increase in the efficiency with which the abso rbed photons were delivered to open PSII centers. The photochemical quenchi ng coefficient remained high except for a decrease at the lower end of the leaf IV range. Actual PSII efficiency increased curvilinearly with increasi ng leaf N, and was highly correlated with light-saturated CO2 assimilation. The fraction of absorbed light potentially going into singlet oxygen forma tion was estimated to be approximate to 10%, regardless of leaf N status. I t was concluded that there was more excess absorbed light in low N leaves t han in high N leaves under high light conditions, Nonphotochemical quenchin g was enhanced with decreasing leaf N to reduce both the PSII efficiency an d the probability of damage from photooxidation by excess absorbed light.