Non-destructive portable fluorescence-based sensors to detect physiological responses of young apple trees cultivated under water restriction regimes

Worldwide, drought stress induced by water shortages is on the rise and 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, analysis 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 moderate and severe water restriction regimes. In addition, chlorophyll fluorescence parameters were analyzed as a potential early stress symptom detection technique. ‘Gala Galaxy’ showed higher tolerance to moderate and severe water deprivation conditions than ‘Pinova 10’. This better performance from ‘Gala Galaxy’ was especially visible during the recovery period, when water was re-supplied to the apple trees. Fluorescence indices related to chlorophyll content (Chl_Index and SFR_R) and nitrogen balance (NBI and NBI_R) showed similar trends when compared to leaf chlorophyll analysis performed wet-chemically, suggesting that these sensors provide a sensible tool to detect plant physiological status on time and non-destructively. Key words: drought stress – plant water status – fluorescence sensors – Malus domestica – leaf chlorophyll content