ALBERT

Notes: Six genotypes of Zea mays L. were grown in pots inside a glasshouse at a mean temperature of 22±2°C and a minimum photosynthetic photon flux density (Q) during the daylight period of 400 μmol m−2 s−1. Chilling-dependent photoinhibition was induced by exposing plants to a temperature of 7°C and a Q of 1 000 μmol m−2 s−1 for 6 h. Recovery from photoinhibition was then followed at a temperature of 25°C and a Q of 200 μmol m−2 s−1. Leaf gas exchange and chlorophyll fluorescence were measured on attached leaves at room temperature prior to the photoinhibitory treatments and at 6 sampling intervals from 0 to 24 h during the recovery period. The relative water content (RWC) was also measured during the recovery period. The results showed a significant genotypic variation in the susceptibility to and rate of recovery from chilling-dependent photoinhibition of photosynthesis in Zea mays seedlings. The Highland Pool 1a from highland sites in Mexico was the least susceptible to chill-induced photoinhibition, but had the slowest rate of recovery. The hybrid variety LG11 showed the highest rate of recovery, whilst the inbred line ZPF307 was the most susceptible to chill-induced photoinhibition. Susceptibility to photoinhibition and subsequent recovery were at least partially independent, suggesting that selection for improved genotypes will require independent selection for both tolerance and capacity for recovery. Although chlorophyll fluorescence provided a more rapid method of assessing the occurrence of photoinhibition, it was not as effective as direct gas-exchange measurements of the maximum quantum yield of photosynthesis (φ) in separating genotypes with respect to their susceptibility to photoinhibition, especially in the most vulnerable genotypes such as ZPF307. Water stress induced by chilling and high Q treatments appeared to impair the recovery processes. Decreases in stomatal conductance (gs) produce a significant decrease in intercellular CO2 concentration (Ci), although this decrease was never so extreme that it limited photosynthetic rates at the light intensities used to determine φ. Nevertheless, closure of stomata in patches, producing local restriction of CO2 supply, would explain the poor correlation between chlorophyll fluorescence and quantum yield measurements in some genotypes immediately after photoinhibitory treatments.