Radiation and water use efficiencies of greenhouse zucchini squash in relation to different climate parameters

In the Mediterranean region, zucchini squash (Cucurbita pepo L.) is widely grown under unheated greenhouse conditions for off-season production. Climatic conditions affecting both radiation and water use are mainly determined by the planting dates. Elucidating the relationship between the radiation and the water use with climate variables is important for the quantification of crop productivity and for improving water use efficiency (WUE) and radiation use efficiency (RUE). The influence of seasonal fluctuations (spring–summer and summer–fall) resulting from two planting dates (28 March and 6 September) on growth, radiation and water use efficiencies was investigated in zucchini plants cultivated hydroponically in a unheated greenhouse located at Viterbo, central Italy. Plants grown in the spring–summer season exhibited a higher maximum value of total, leaf, stem, fruit dry matter and leaf area index (971, 460, 91, 422 g m−2 and 5.1, respectively) than those grown during the summer–fall season (672, 306, 58, 311 g m−2 and 2.7, respectively). The marketable fresh yield in the spring–summer was higher than the summer–fall (6.2 versus 4.7 kg m−2). The canopy extinction coefficient was empirically determined as 0.72 for the light interception. The crop coefficient (K(c)) during the spring–summer period ranged from 0.10 to 1.15, while for the summer–fall season the K(c) ranged from 0.12 to 0.80. Before harvest initiation, the transpiration rates, RUE and water use efficiency calculated on above dry weight basis (WUE(b)) were similar between growing seasons (average 0.51 L m−2 day−1, 2.07 g MJ−1 and 2.92 g kg−1, respectively), while after harvest initiation, summer–fall season showed a lower transpiration rate (0.97 versus 2.60 L m−2 day−1) and a higher RUE (4.17 versus 2.46 g MJ−1) and WUE(b) (4.02 versus 2.73 g kg−1) in comparison to the spring–summer season. Water use efficiency on fresh fruit yield basis (WUE(y)) was also higher in summer–fall season compared to spring–summer season (50.5 versus 31.5 g kg−1). The variability in RUE and WUE(b) was associated with, daily mean solar radiation (R(s)), daily mean air temperature (T(a)) and daily mean vapour pressure deficit (VPD). In both growing phases, negative exponential relationships were evident by plotting the RUE, and WUE(b) against climatic variables (R(s), T(a) and VPD). R(s), T(a) and VPD were greatly higher in the spring–summer season, which influences RUE and WUE(b) negatively.