Soil Temperature and Moisture as Key Determinants of SPAD Values in Greenhouse-Grown Cucumber in Qatar

This study aimed to explore the relationship between Soil-Plant Analysis Development (SPAD) values and key environmental factors in cucumber (Cucumis sativus L.) cultivation in a greenhouse. SPAD values, indicative of chlorophyll content, reflect plant health and productivity. The analysis revealed strong positive correlations between SPAD values and both indoor light intensity (ILI, r = 0.59, p < 0.001) and outdoor light intensity (OLI, r = 0.62, p < 0.001), suggesting that higher light intensities were associated with enhanced SPAD values. In contrast, significant negative correlations were found between SPAD values and soil temperature at 15–30 cm depth (ST1530, r = −0.47, p < 0.001) and volumetric soil moisture content at the same depth (SM1530, r = −0.52, p < 0.001), with higher soil temperatures (e.g., 28°C) and excessive moisture (e.g., 25%) leading to reduced SPAD values. Multiple regression analysis identified ST1530 and SM1530 as significant negative predictors of SPAD, with coefficients of −0.97 (p = 0.05) and −0.34 (p = 0.05), respectively, suggesting that increases in soil temperature and moisture result in lower SPAD values. Indoor light intensity (e.g., 600–800 μmol/m2/s) emerged as a significant positive contributor, with a coefficient of 0.01 (p < 0.001), highlighting its role in promoting chlorophyll synthesis. Additionally, relative humidity (r = 0.27, p < 0.01) showed a positive, although less pronounced, association with SPAD. These results underscore the importance of both direct and indirect environmental factors in influencing SPAD variability and, by extension, plant health and productivity in cucumber cultivation.