A Carica papaya L. genotype with low leaf chlorophyll concentration copes successfully with soil water stress in the field

In the context of increasing drought frequencies, water sensitive plant species need to have defined irrigation thresholds. One such species is papaya (Carica papaya L.), with a current demand for the selection of drought tolerance genotypes. The aim of this study was to evaluate the growth, leaf gas exchanges, sap flow and chlorophyll (Chl) fluorescence in two papaya genotypes (pale-green leaf 'Golden' - G and dark-green leaf 'Alianca' - AL) under soil water deficit (psi soil) conditions. The experiments were carried out in the field, in an unconsolidated alluvial sediment soil type. The two papaya genotypes, contrasting in their leaf Chl contents, were grown under irrigated (I, psi soil of -12 kPa), and non-irrigated (NI, psi soil gradually diminished from - 23 to -311 kPa) conditions for 41 days. Leaf gas exchanges, xylem sap-flow, green intensity (SPAD index), plant height, trunk diameter and leaf area were measured five times, following psi soil decreases. At the lowest psi soil (-311 kPa), the Chl photochemistry and fruit number measurements were performed. Intrinsic water use efficiency was higher in the GNI than in the ALNI genotype. Leaf temperature and photochemical efficiency was not affected by soil water restrictions in GNI and ALNI. The water deficit effect on growth traits was linked to whole canopy assimilation, due to the higher leaf area in ALI than GI, or in ALNI than in GNI, once the leaf assimilation rate was similar between those treatments. The psi soil of ca. -30 kPa can be considered as a threshold for papaya irrigation for the soil type studied here. Soil water stress did not affect the number of fruits in GNI, whilst this was reduced by 83% in ALNI compared to irrigated plants, indicating that pale-green leaf papaya genotypes are an alternative to cope with water stress.