Monitoring physiological and biochemical responses of two apple cultivars to water supply regimes with non-destructive fluorescence sensors

Worldwide, increased drought stress induced by water shortages will be especially hazardous for maintaining high yields in agricultural and horticultural crops. It is known that not only do different crops have different water requirements, but that these can also vary between cultivars. Most commonly, plant responses to drought stress are monitored on the physiological level, for example via plant water status (leaf water potential, relative water content) or stomatal behavior, photosynthesis and osmotic adjustment. However, analyses of these parameters are time-consuming and destructive. In contrast, novel approaches such as fluorescence sensors seem promising as they are easy to embark in the field, non-destructive and quick to analyze. Therefore, the aim of this study was to investigate whether two apple (Malus domestica) cultivars, 'Pinova 10' and 'Gala Galaxy', show different physiological reactions, for example in leaf water potential, relative water content as well as chlorophyll and proline concentration, in response to water stress. In addition, chlorophyll fluorescence parameters were analyzed as a promising stress symptom detection technique. 'Gala Galaxy' showed higher tolerance to water deprivation conditions than 'Pinova 10', indicated especially by increased chlorophyll fluorescence indices on certain measurement days throughout the experimental course. Fluorescence indices related to chlorophyll content (Chl_Index and SFR_R) and nitrogen balance (NBI and NBI_R) showed similar curves. However, leaf chlorophyll analysis performed wet-chemically was not a reliable indicator of water stress in apple trees. Leaf water potential was affected on DAT 38, during complete watering withholding, without significant differences between the cultivars. In summary, fluorescence-based indices, related to chlorophyll content and nitrogen balance, promise to be a useful non-destructive tool to estimate physiological status of young apple trees submitted to water restriction regimes.