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A. Naor, I. Klein, H. Hupert, Y. Grinblat, M. Peres and A. Kaufman

The interactions between irrigation and crop level with respect to fruit size distribution and soil and stem water potentials were investigated in a nectarine (Prunus persica (L.) Batsch. `Fairlane') orchard located in a semiarid zone. Irrigation treatments during stage III of fruit growth ranged from 0.62 to 1.29 of potential evapotranspiration (ETp). Fruit were hand thinned to a wide range of fruit levels (200 to 1200 fruit/tree in the 555-tree/ha orchard). Total yield did not increase with increasing irrigation rate above 0.92 ETp in 1996 and maximum yield was found at 1.06 ETp in 1997. Fruit size distribution was shifted towards larger fruit with increasing irrigation level and with decreasing crop level. The two highest irrigation treatments had similar midday stem water potentials. Our findings indicate that highest yields and highest water use efficiency (yield/water consumption) are not always related to minimum water stress. Total yield and large fruit yield were highly and better correlated with midday stem water potential than with soil water potential. This confirms other reports that midday stem water potential is an accurate indicator of tree water stress and may have utility in irrigation scheduling.

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A. Naor, H. Hupert, Y. Greenblat, M. Peres, A. Kaufman and I. Klein

The interactions between irrigation and crop level with respect to fruit size distribution and midday stem water potential were investigated for 3 years in a nectarine (Prunus persica L. `Fairlane') orchard located in a semi-arid zone. Wide ranges of crop loads and irrigation rates in stage III were employed, extending from practically nonlimiting to severely limiting levels. Irrigation during stage III of fruit growth ranged from 0.63 to 1.29 of potential evapotranspiration (ETp). Fruit were hand thinned to a wide range of fruit levels (300 to 2000) fruit/tree in the 555-tree/ha orchard. The yields and stem water potentials from 1996, 1997 and 1998 were combined together and the interrelations among yield, crop load and stem water potential were examined. Fruit <55 mm in diameter growing at 400 fruit per tree were the only ones not affected by irrigation level. The yield of fruit of 60 to 75 mm in diameter increased with irrigation level, but only a slight increase was observed when the irrigation rate rose above 1.01 ETp. A significant decrease in the yields of 60 to 65, 65 to 70, and 70 to 75-mm size grades occurred at crop levels greater than 1000, 800, and 400 fruit per tree, respectively. Midday stem water potential decreased with increasing crop level, and it is suggested that midday stem water potential responds to crop load rather than crop level. Relative yields of the various size grades were highly correlated with midday stem water potential. It was suggested that the midday stem water potential integrates the combined effects of water stress and crop load on nectarine fruit size.