Physiological Characterization of Broccoli Responses to Irrigation Type and Amount and Soil Amendment in Desert Production

Arizona ranks as the second largest producer of broccoli ( Brassica oleracea var. italica ) in the United States, largely supported by irrigation water from the Colorado River. Reduced water allocations and rising salinity in the Colorado River are growing challenges facing broccoli production in Arizona. Key physiological processes sensitive to these production challenges need to be identified to manage irrigation water. This study examined the stomatal and nonstomatal limitations to photosynthesis in broccoli ( B. oleracea L.). Specifically, broccoli plants grown in the field were subjected to three irrigation types and two irrigation amounts, with or without the application of the soil amendment [liquid natural clay (LNC)]. Evaluated parameters included gas exchange measurements, chlorophyll fluorescence, and pigment content at both the seedling and reproductive stages. The concentration of photoprotective pigments served as a reliable indicator for possible osmotic stress caused by the overhead irrigation of the saline water with the center pivot system during the seedling phase. Efficient water application by the drip irrigation system led to improved stomatal conductance and increased pigment levels. Conversely, excessive water from full irrigation restricted both stomatal and nonstomatal processes involved in photosynthesis. At the same time, the application of LNC resulted in a 24% increase in carotenoids, a 42% rise in chlorophyll a, and a 23% growth in chlorophyll b under full irrigation conditions. LNC preserved high levels of both gas exchange and pigment concentration. We infer that improved broccoli stomatal and pigment performance subsequently led to an enhanced photosynthetic rate in the presence of LNC, regardless of whether irrigation was full or deficit. These findings offer insights into improving irrigation water productivity and resilience of leafy vegetable production systems under arid Arizona conditions.